RFC patch, 3 partstest-lib: add the '--stress' option to help reproduce occasional failures in flaky tests

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 0f1faa24b2..49e4563405 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -71,6 +71,9 @@ then
 	exit 1
 fi
 
+TEST_NAME="$(basename "$0" .sh)"
+TEST_RESULTS_BASE="$TEST_OUTPUT_DIRECTORY/test-results/$TEST_NAME"
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
 case "$GIT_TEST_TEE_STARTED, $* " in
@@ -78,12 +81,11 @@ done,*)
 	# do not redirect again
 	;;
 *' --tee '*|*' --va'*|*' -V '*|*' --verbose-log '*)
-	mkdir -p "$TEST_OUTPUT_DIRECTORY/test-results"
-	BASE="$TEST_OUTPUT_DIRECTORY/test-results/$(basename "$0" .sh)"
+	mkdir -p "$(dirname "$TEST_RESULTS_BASE")"
 
 	# Make this filename available to the sub-process in case it is using
 	# --verbose-log.
-	GIT_TEST_TEE_OUTPUT_FILE=$BASE.out
+	GIT_TEST_TEE_OUTPUT_FILE=$TEST_RESULTS_BASE.out
 	export GIT_TEST_TEE_OUTPUT_FILE
 
 	# Truncate before calling "tee -a" to get rid of the results
@@ -91,8 +93,8 @@ done,*)
 	>"$GIT_TEST_TEE_OUTPUT_FILE"
 
 	(GIT_TEST_TEE_STARTED=done ${TEST_SHELL_PATH} "$0" "$@" 2>&1;
-	 echo $? >"$BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
-	test "$(cat "$BASE.exit")" = 0
+	 echo $? >"$TEST_RESULTS_BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
+	test "$(cat "$TEST_RESULTS_BASE.exit")" = 0
 	exit
 	;;
 esac
@@ -818,12 +820,9 @@ test_done () {
 
 	if test -z "$HARNESS_ACTIVE"
 	then
-		test_results_dir="$TEST_OUTPUT_DIRECTORY/test-results"
-		mkdir -p "$test_results_dir"
-		base=${0##*/}
-		test_results_path="$test_results_dir/${base%.sh}.counts"
+		mkdir -p "$(dirname "$TEST_RESULTS_BASE")"
 
-		cat >"$test_results_path" <<-EOF
+		cat >"$TEST_RESULTS_BASE.counts" <<-EOF
 		total $test_count
 		success $test_success
 		fixed $test_fixed
@@ -1029,7 +1028,7 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$(basename "$0" .sh)"
+TRASH_DIRECTORY="trash directory.$TEST_NAME"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good

t/lib-git-daemon.sh, t/lib-git-p4.sh, t/lib-git-svn.sh, t/lib-httpd.sh, t/t0410-partial-clone.sh, t/t5512-ls-remote.sh, t/test-lib-functions.sh

diff --git a/t/lib-git-daemon.sh b/t/lib-git-daemon.sh
index f98de95c15..41eb1e3ae8 100644
--- a/t/lib-git-daemon.sh
+++ b/t/lib-git-daemon.sh
@@ -28,7 +28,7 @@ then
 	test_skip_or_die $GIT_TEST_GIT_DAEMON "file system does not support FIFOs"
 fi
 
-LIB_GIT_DAEMON_PORT=${LIB_GIT_DAEMON_PORT-${this_test#t}}
+test_set_port LIB_GIT_DAEMON_PORT
 
 GIT_DAEMON_PID=
 GIT_DAEMON_DOCUMENT_ROOT_PATH="$PWD"/repo
diff --git a/t/lib-git-p4.sh b/t/lib-git-p4.sh
index c27599474c..b3be3ba011 100644
--- a/t/lib-git-p4.sh
+++ b/t/lib-git-p4.sh
@@ -53,14 +53,7 @@ time_in_seconds () {
 	(cd / && "$PYTHON_PATH" -c 'import time; print(int(time.time()))')
 }
 
-# Try to pick a unique port: guess a large number, then hope
-# no more than one of each test is running.
-#
-# This does not handle the case where somebody else is running the
-# same tests and has chosen the same ports.
-testid=${this_test#t}
-git_p4_test_start=9800
-P4DPORT=$((10669 + ($testid - $git_p4_test_start)))
+test_set_port P4DPORT
 
 P4PORT=localhost:$P4DPORT
 P4CLIENT=client
diff --git a/t/lib-git-svn.sh b/t/lib-git-svn.sh
index a8130f9119..f3b478c307 100644
--- a/t/lib-git-svn.sh
+++ b/t/lib-git-svn.sh
@@ -13,6 +13,7 @@ fi
 GIT_DIR=$PWD/.git
 GIT_SVN_DIR=$GIT_DIR/svn/refs/remotes/git-svn
 SVN_TREE=$GIT_SVN_DIR/svn-tree
+test_set_port SVNSERVE_PORT
 
 svn >/dev/null 2>&1
 if test $? -ne 1
@@ -119,7 +120,6 @@ require_svnserve () {
 }
 
 start_svnserve () {
-	SVNSERVE_PORT=${SVNSERVE_PORT-${this_test#t}}
 	svnserve --listen-port $SVNSERVE_PORT \
 		 --root "$rawsvnrepo" \
 		 --listen-once \
diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index a8729f8232..e465116ef9 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -82,7 +82,7 @@ case $(uname) in
 esac
 
 LIB_HTTPD_PATH=${LIB_HTTPD_PATH-"$DEFAULT_HTTPD_PATH"}
-LIB_HTTPD_PORT=${LIB_HTTPD_PORT-${this_test#t}}
+test_set_port LIB_HTTPD_PORT
 
 TEST_PATH="$TEST_DIRECTORY"/lib-httpd
 HTTPD_ROOT_PATH="$PWD"/httpd
diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index ba3887f178..0aca8d7588 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh
@@ -480,7 +480,6 @@ test_expect_success 'gc stops traversal when a missing but promised object is re
 	! grep "$TREE_HASH" out
 '
 
-LIB_HTTPD_PORT=12345  # default port, 410, cannot be used as non-root
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
diff --git a/t/t5512-ls-remote.sh b/t/t5512-ls-remote.sh
index 32e722db2e..cd9e60632d 100755
--- a/t/t5512-ls-remote.sh
+++ b/t/t5512-ls-remote.sh
@@ -260,7 +260,7 @@ test_lazy_prereq GIT_DAEMON '
 # This test spawns a daemon, so run it only if the user would be OK with
 # testing with git-daemon.
 test_expect_success PIPE,JGIT,GIT_DAEMON 'indicate no refs in standards-compliant empty remote' '
-	JGIT_DAEMON_PORT=${JGIT_DAEMON_PORT-${this_test#t}} &&
+	test_set_port JGIT_DAEMON_PORT &&
 	JGIT_DAEMON_PID= &&
 	git init --bare empty.git &&
 	>empty.git/git-daemon-export-ok &&
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 6b3bbf99e4..d9a602cd0f 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1263,3 +1263,39 @@ test_oid () {
 	fi &&
 	eval "printf '%s' \"\${$var}\""
 }
+
+# Choose a port number based on the test script's number and store it in
+# the given variable name, unless that variable already contains a number.
+test_set_port () {
+	local var=$1 port
+
+	if test $# -ne 1 || test -z "$var"
+	then
+		BUG "test_set_port requires a variable name"
+	fi
+
+	eval port=\"\${$var}\"
+	case "$port" in
+	"")
+		# No port is set in the given env var, use the test
+		# number as port number instead.
+		# Remove not only the leading 't', but all leading zeros
+		# as well, so the arithmetic below won't (mis)interpret
+		# a test number like '0123' as an octal value.
+		port=${this_test#${this_test%%[1-9]*}}
+		if test "${port:-0}" -lt 1024
+		then
+			# root-only port, use a larger one instead.
+			port=$(($port + 10000))
+		fi
+
+		eval $var=$port
+		;;
+	*[^0-9]*)
+		error >&7 "invalid port number: $port"
+		;;
+	*)
+		# The user has specified the port.
+		;;
+	esac
+}

t/README, t/test-lib-functions.sh, t/test-lib.sh

diff --git a/t/README b/t/README
index 28711cc508..9851de25c2 100644
--- a/t/README
+++ b/t/README
@@ -186,6 +186,16 @@ appropriately before running "make".
 	this feature by setting the GIT_TEST_CHAIN_LINT environment
 	variable to "1" or "0", respectively.
 
+--stress::
+--stress=<N>::
+	Run the test script repeatedly in multiple parallel
+	invocations until one of them fails.  Useful for reproducing
+	rare failures in flaky tests.  The number of parallel
+	invocations is, in order of precedence: <N>, or the value of
+	the GIT_TEST_STRESS_LOAD environment variable, or twice the
+	number of available processors in '/proc/cpuinfo', or 8.
+	Implies `--verbose-log`.
+
 You can also set the GIT_TEST_INSTALLED environment variable to
 the bindir of an existing git installation to test that installation.
 You still need to have built this git sandbox, from which various
@@ -425,7 +435,8 @@ This test harness library does the following things:
  - Creates an empty test directory with an empty .git/objects database
    and chdir(2) into it.  This directory is 't/trash
    directory.$test_name_without_dotsh', with t/ subject to change by
-   the --root option documented above.
+   the --root option documented above, and a '.stress-<N>' suffix
+   appended by the --stress option.
 
  - Defines standard test helper functions for your scripts to
    use.  These functions are designed to make all scripts behave
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index d9a602cd0f..9af11e3eed 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1288,8 +1288,6 @@ test_set_port () {
 			# root-only port, use a larger one instead.
 			port=$(($port + 10000))
 		fi
-
-		eval $var=$port
 		;;
 	*[^0-9]*)
 		error >&7 "invalid port number: $port"
@@ -1298,4 +1296,9 @@ test_set_port () {
 		# The user has specified the port.
 		;;
 	esac
+
+	# Make sure that parallel '--stress' test jobs get different
+	# ports.
+	port=$(($port + ${GIT_TEST_STRESS_JOB_NR:-0}))
+	eval $var=$port
 }
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 49e4563405..9b7f687396 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -71,8 +71,81 @@ then
 	exit 1
 fi
 
+TEST_STRESS_SFX="${GIT_TEST_STRESS_JOB_NR:+.stress-$GIT_TEST_STRESS_JOB_NR}"
 TEST_NAME="$(basename "$0" .sh)"
-TEST_RESULTS_BASE="$TEST_OUTPUT_DIRECTORY/test-results/$TEST_NAME"
+TEST_RESULTS_BASE="$TEST_OUTPUT_DIRECTORY/test-results/$TEST_NAME$TEST_STRESS_SFX"
+
+# If --stress was passed, run this test repeatedly in several parallel loops.
+case "$GIT_TEST_STRESS_STARTED, $* " in
+done,*)
+	# Don't stress test again.
+	;;
+*' --stress '*|*' '--stress=*' '*)
+	job_count=${*##*--stress=}
+	if test "$job_count" != "$*"
+	then
+		job_count=${job_count%% *}
+	elif test -n "$GIT_TEST_STRESS_LOAD"
+	then
+		job_count="$GIT_TEST_STRESS_LOAD"
+	elif test -r /proc/cpuinfo
+	then
+		job_count=$((2 * $(grep -c ^processor /proc/cpuinfo)))
+	else
+		job_count=8
+	fi
+
+	mkdir -p "$(dirname "$TEST_RESULTS_BASE")"
+	stressfail="$TEST_RESULTS_BASE.stress-failed"
+	rm -f "$stressfail"
+	trap 'echo aborted >"$stressfail"' TERM INT HUP
+
+	job_nr=0
+	while test $job_nr -lt "$job_count"
+	do
+		(
+			GIT_TEST_STRESS_STARTED=done
+			GIT_TEST_STRESS_JOB_NR=$job_nr
+			export GIT_TEST_STRESS_STARTED GIT_TEST_STRESS_JOB_NR
+
+			cnt=0
+			while ! test -e "$stressfail"
+			do
+				if $TEST_SHELL_PATH "$0" "$@" >/dev/null 2>&1
+				then
+					printf >&2 "OK   %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				elif test -f "$stressfail" &&
+				     test "$(cat "$stressfail")" = "aborted"
+				then
+					printf >&2 "ABORTED %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				else
+					printf >&2 "FAIL %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+					echo $GIT_TEST_STRESS_JOB_NR >>"$stressfail"
+				fi
+				cnt=$(($cnt + 1))
+			done
+		) &
+		job_nr=$(($job_nr + 1))
+	done
+
+	job_nr=0
+	while test $job_nr -lt "$job_count"
+	do
+		wait
+		job_nr=$(($job_nr + 1))
+	done
+
+	if test -f "$stressfail" && test "$(cat "$stressfail")" != "aborted"
+	then
+		echo "Log(s) of failed test run(s) be found in:"
+		for f in $(cat "$stressfail")
+		do
+			echo "  $TEST_RESULTS_BASE.stress-$f.out"
+		done
+	fi
+	exit
+	;;
+esac
 
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
@@ -80,7 +153,7 @@ case "$GIT_TEST_TEE_STARTED, $* " in
 done,*)
 	# do not redirect again
 	;;
-*' --tee '*|*' --va'*|*' -V '*|*' --verbose-log '*)
+*' --tee '*|*' --va'*|*' -V '*|*' --verbose-log '*|*' --stress '*|*' '--stress=*' '*)
 	mkdir -p "$(dirname "$TEST_RESULTS_BASE")"
 
 	# Make this filename available to the sub-process in case it is using
@@ -341,6 +414,9 @@ do
 	-V|--verbose-log)
 		verbose_log=t
 		shift ;;
+	--stress|--stress=*)
+		verbose_log=t
+		shift ;;
 	*)
 		echo "error: unknown test option '$1'" >&2; exit 1 ;;
 	esac
@@ -1028,7 +1104,7 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$TEST_NAME"
+TRASH_DIRECTORY="trash directory.$TEST_NAME$TEST_STRESS_SFX"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good

t/test-lib.sh, t/t0000-basic.sh

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 9b7f687396..5e0c41626f 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -98,7 +98,7 @@ done,*)
 	mkdir -p "$(dirname "$TEST_RESULTS_BASE")"
 	stressfail="$TEST_RESULTS_BASE.stress-failed"
 	rm -f "$stressfail"
-	trap 'echo aborted >"$stressfail"' TERM INT HUP
+	trap 'echo aborted >"$stressfail"; wait' TERM INT HUP
 
 	job_nr=0
 	while test $job_nr -lt "$job_count"
@@ -128,12 +128,7 @@ done,*)
 		job_nr=$(($job_nr + 1))
 	done
 
-	job_nr=0
-	while test $job_nr -lt "$job_count"
-	do
-		wait
-		job_nr=$(($job_nr + 1))
-	done
+	wait
 
 	if test -f "$stressfail" && test "$(cat "$stressfail")" != "aborted"
 	then

which behaves well for me in all cases.

> Bash 4.3 or later are strange: I get back the shell prompt immediately
> after ctrl-C as well, so it doesn't appear to be waiting for all
> remaining jobs to finish either, but! I don't get any of the progress
> output from those jobs to mess up my next command.

Interesting. My bash 4.4 seems to behave the same as dash. It almost
sounds like the SIGINT is getting passed to the subshells for you.
Probably not really worth digging into, though.

In case anybody is playing with this and needed to simulate a stress
failure, here's what I used:

diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index b6566003dd..a370cd9977 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -1171,4 +1171,11 @@ test_expect_success 'very long name in the index handled sanely' '
 	test $len = 4098
 '
 
+test_expect_success 'roll those dice' '
+	case "$(openssl rand -base64 1)" in
+	z*)
+		return 1
+	esac
+'
+
 test_done

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 9b7f687396..357dead3ff 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -98,8 +98,9 @@ done,*)
 	mkdir -p "$(dirname "$TEST_RESULTS_BASE")"
 	stressfail="$TEST_RESULTS_BASE.stress-failed"
 	rm -f "$stressfail"
-	trap 'echo aborted >"$stressfail"' TERM INT HUP
+	trap 'echo aborted >"$stressfail"; kill $job_pids 2>/dev/null; wait' TERM INT HUP
 
+	job_pids=
 	job_nr=0
 	while test $job_nr -lt "$job_count"
 	do
@@ -108,10 +109,13 @@ done,*)
 			GIT_TEST_STRESS_JOB_NR=$job_nr
 			export GIT_TEST_STRESS_STARTED GIT_TEST_STRESS_JOB_NR
 
+			trap 'kill $test_pid 2>/dev/null' TERM
+
 			cnt=0
 			while ! test -e "$stressfail"
 			do
-				if $TEST_SHELL_PATH "$0" "$@" >/dev/null 2>&1
+				$TEST_SHELL_PATH "$0" "$@" >/dev/null 2>&1 & test_pid=$!
+				if wait
 				then
 					printf >&2 "OK   %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
 				elif test -f "$stressfail" &&
@@ -124,16 +128,11 @@ done,*)
 				fi
 				cnt=$(($cnt + 1))
 			done
-		) &
+		) & job_pids="$job_pids $!"
 		job_nr=$(($job_nr + 1))
 	done
 
-	job_nr=0
-	while test $job_nr -lt "$job_count"
-	do
-		wait
-		job_nr=$(($job_nr + 1))
-	done
+	wait
 
 	if test -f "$stressfail" && test "$(cat "$stressfail")" != "aborted"
 	then

> > > +				elif test -f "$stressfail" &&
> > > +				     test "$(cat "$stressfail")" = "aborted"
> > > +				then
> > > +					printf >&2 "ABORTED %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
> > > +				else
> > > +					printf >&2 "FAIL %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
> > > +					echo $GIT_TEST_STRESS_JOB_NR >>"$stressfail"
> > > +				fi
> > 
> > Hmm. What happens if we see a failure _and_ there's an abort? That's
> > actually pretty plausible if you see a failure whiz by, and you want to
> > abort the remaining scripts because they take a long time to run.
> > 
> > If the abort happens first, then the goal is to assume any errors are
> > due to the abort.
> 
> Yeah, that's why I added this ABORTED case.  When I hit ctrl-C, a
> couple of the tests tend to fail, but that's not the rare failure we
> are hoping to find when stress testing, so printing FAIL would be
> misleading.  The test script exits with 1 all the same, so we can't
> tell the difference, but since rare failures are, well, rare, this
> failure is most likely due to the abort.

With the patch as posted I actually see most of them still say "OK",
because the SIGINT does not make it to them (it would be a different
story if I killed the whole process group).

But with the modification above, I get "ABORTED" for most, which is what
I'd want. And it should be pretty race-free, since the "aborted" file is
put into place before triggering the signal cascade.

> > If the fail happens first (which I think is the more likely case), then
> > the abort handler will overwrite the file with "aborted", and we'll
> > forget that there was a real failure. This works in my script because of
> > the symlinking (if a real failure symlinked $fail to a directory, then
> > the attempt to write "aborted" will just be a noop).
> 
> OK, I think the abort handler should append, not overwrite, and then
> the file's contents should be checked e.g. with a case statement
> looking for *aborted*.
> 
> Or use two separate files for signaling abort and test failure.

Two separate files seems a lot simpler to me.

> > In my experience, outputting the failed log saves a step (especially
> > with "-i"), since seeing the failed test and its output is often
> > sufficient.
> 
> I just don't like when test output, especially with '-x', is dumped on
> my terminal :)

Don't you then use your terminal to display the file? ;) (I'm kidding, I
assume you load it in "less" or whatever, which is distinct).

I wish this could be optional somehow, though. I really prefer the dump.
I guess if I continue to use my script as a wrapper, it can dump for me
(after digging in the $stressfail file, I guess).

> > I'm also sad to lose the symlink to the failed trash directory, which
> > saves cutting and pasting when you have to inspect it. But I guess we
> > can't rely on symlinks everywhere.
> 
> You can tab-complete most of the trash directory, and then there are
> only those 1-2-3 digits of the job number that you have to type.  That
> worked out well enough for me so far (but I've only used it for a few
> days, so...).
> 
> We could rename the trash directory of the failed test to something
> short, sweet, and common, but that could be racy in the unlikely case
> that two tests fail at the same time.  And I like the 'trash
> directory.' prefix, because then 'make clean' will delete that one,
> too, without modifying 't/Makefile' (which is also the reason for
> adding the 'stress-N' suffix instead of a prefix, and putting the fail
> file into the 'test-results' directory).

We could rename it to "trash directory.xxx.failed", which is at least
predictable. That can even be done atomically, though I think it's a
little tricky to do portably with shell tools. I guess I can continue to
do the symlink thing in my wrapper script.

-Peff

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 9a3f7930a3..5577d9dc5a 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -71,13 +71,33 @@ then
 	exit 1
 fi
 
+# Parse some options early, taking care to leave $@ intact.
+for opt
+do
+	case "$opt" in
+	--tee)
+		tee=t ;;
+	-V|--verbose-log)
+		verbose_log=t ;;
+	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
+		valgrind=memcheck ;;
+	--valgrind=*)
+		valgrind=${opt#--*=} ;;
+	--valgrind-only=*)
+		valgrind_only=${opt#--*=} ;;
+	*)
+		# Other options will be handled later.
+	esac
+done
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
-case "$GIT_TEST_TEE_STARTED, $* " in
-done,*)
-	# do not redirect again
-	;;
-*' --tee '*|*' --va'*|*' -V '*|*' --verbose-log '*)
+if test "$GIT_TEST_TEE_STARTED" = "done"
+then
+	: # do not redirect again
+elif test -n "$tee" || test -n "$verbose_log" ||
+     test -n "$valgrind" || test -n "$valgrind_only"
+then
 	mkdir -p "$TEST_OUTPUT_DIRECTORY/test-results"
 	BASE="$TEST_OUTPUT_DIRECTORY/test-results/$(basename "$0" .sh)"
 
@@ -94,8 +114,7 @@ done,*)
 	 echo $? >"$BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
 	test "$(cat "$BASE.exit")" = 0
 	exit
-	;;
-esac
+fi
 
 # For repeatability, reset the environment to known value.
 # TERM is sanitized below, after saving color control sequences.
@@ -296,17 +315,6 @@ do
 		with_dashes=t; shift ;;
 	--no-color)
 		color=; shift ;;
-	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
-		valgrind=memcheck
-		shift ;;
-	--valgrind=*)
-		valgrind=${1#--*=}
-		shift ;;
-	--valgrind-only=*)
-		valgrind_only=${1#--*=}
-		shift ;;
-	--tee)
-		shift ;; # was handled already
 	--root=*)
 		root=${1#--*=}
 		shift ;;
@@ -336,9 +344,12 @@ do
 			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
 		fi
 		shift ;;
-	-V|--verbose-log)
-		verbose_log=t
-		shift ;;
+	--tee|\
+	-V|--verbose-log|\
+	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind|\
+	--valgrind=*|\
+	--valgrind-only=*)
+		shift ;; # These options were handled already.
 	*)
 		echo "error: unknown test option '$1'" >&2; exit 1 ;;
 	esac

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 9a3f7930a3..efdb6be3c8 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -264,58 +264,65 @@ test "x$TERM" != "xdumb" && (
 	) &&
 	color=t
 
-while test "$#" -ne 0
+store_arg_to=
+prev_opt=
+for opt
 do
-	case "$1" in
+	if test -n "$store_arg_to"
+	then
+		eval $store_arg_to=\$opt
+		store_arg_to=
+		prev_opt=
+		continue
+	fi
+	case "$opt" in
 	-d|--d|--de|--deb|--debu|--debug)
-		debug=t; shift ;;
+		debug=t ;;
 	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
-		immediate=t; shift ;;
+		immediate=t ;;
 	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
-		GIT_TEST_LONG=t; export GIT_TEST_LONG; shift ;;
+		GIT_TEST_LONG=t; export GIT_TEST_LONG ;;
 	-r)
-		shift; test "$#" -ne 0 || {
-			echo 'error: -r requires an argument' >&2;
-			exit 1;
-		}
-		run_list=$1; shift ;;
+		store_arg_to=run_list
+		prev_opt=-r
+		;;
 	--run=*)
-		run_list=${1#--*=}; shift ;;
+		run_list=${opt#--*=} ;;
 	-h|--h|--he|--hel|--help)
-		help=t; shift ;;
+		help=t ;;
 	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
-		verbose=t; shift ;;
+		verbose=t ;;
 	--verbose-only=*)
-		verbose_only=${1#--*=}
-		shift ;;
+		verbose_only=${opt#--*=}
+		;;
 	-q|--q|--qu|--qui|--quie|--quiet)
 		# Ignore --quiet under a TAP::Harness. Saying how many tests
 		# passed without the ok/not ok details is always an error.
-		test -z "$HARNESS_ACTIVE" && quiet=t; shift ;;
+		test -z "$HARNESS_ACTIVE" && quiet=t ;;
 	--with-dashes)
-		with_dashes=t; shift ;;
+		with_dashes=t ;;
 	--no-color)
-		color=; shift ;;
+		color= ;;
 	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
 		valgrind=memcheck
-		shift ;;
+		;;
 	--valgrind=*)
-		valgrind=${1#--*=}
-		shift ;;
+		valgrind=${opt#--*=}
+		;;
 	--valgrind-only=*)
-		valgrind_only=${1#--*=}
-		shift ;;
+		valgrind_only=${opt#--*=}
+		;;
 	--tee)
-		shift ;; # was handled already
+		;; # was handled already
 	--root=*)
-		root=${1#--*=}
-		shift ;;
+		root=${opt#--*=}
+		;;
 	--chain-lint)
 		GIT_TEST_CHAIN_LINT=1
-		shift ;;
+		;;
 	--no-chain-lint)
 		GIT_TEST_CHAIN_LINT=0
-		shift ;;
+		;;
 	-x)
 		# Some test scripts can't be reliably traced  with '-x',
 		# unless the test is run with a Bash version supporting
@@ -335,15 +342,21 @@ do
 		else
 			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
 		fi
-		shift ;;
+		;;
 	-V|--verbose-log)
 		verbose_log=t
-		shift ;;
+		;;
 	*)
-		echo "error: unknown test option '$1'" >&2; exit 1 ;;
+		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
 	esac
 done
 
+if test -n "$store_arg_to"
+then
+	echo "error: $prev_opt requires an argument" >&2
+	exit 1
+fi
+
 if test -n "$valgrind_only"
 then
 	test -z "$valgrind" && valgrind=memcheck

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 0f1faa24b2..9a3f7930a3 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -476,7 +476,7 @@ die () {
 
 GIT_EXIT_OK=
 trap 'die' EXIT
-trap 'exit $?' INT
+trap 'exit $?' INT TERM HUP
 
 # The user-facing functions are loaded from a separate file so that
 # test_perf subshells can have them too

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 09c77cbd1b..ea1cd34013 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -85,6 +85,8 @@ do
 		valgrind=${opt#--*=} ;;
 	--valgrind-only=*)
 		valgrind_only=${opt#--*=} ;;
+	--root=*)
+		root=${opt#--*=} ;;
 	*)
 		# Other options will be handled later.
 	esac
@@ -93,6 +95,12 @@ done
 TEST_NAME="$(basename "$0" .sh)"
 TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
 TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
+TRASH_DIRECTORY="trash directory.$TEST_NAME"
+test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
+case "$TRASH_DIRECTORY" in
+/*) ;; # absolute path is good
+ *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
+esac
 
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
@@ -318,9 +326,6 @@ do
 		with_dashes=t; shift ;;
 	--no-color)
 		color=; shift ;;
-	--root=*)
-		root=${1#--*=}
-		shift ;;
 	--chain-lint)
 		GIT_TEST_CHAIN_LINT=1
 		shift ;;
@@ -351,7 +356,8 @@ do
 	-V|--verbose-log|\
 	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind|\
 	--valgrind=*|\
-	--valgrind-only=*)
+	--valgrind-only=*|\
+	--root=*)
 		shift ;; # These options were handled already.
 	*)
 		echo "error: unknown test option '$1'" >&2; exit 1 ;;
@@ -1040,12 +1046,6 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$TEST_NAME"
-test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
-case "$TRASH_DIRECTORY" in
-/*) ;; # absolute path is good
- *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
-esac
 rm -fr "$TRASH_DIRECTORY" || {
 	GIT_EXIT_OK=t
 	echo >&5 "FATAL: Cannot prepare test area"

diff --git a/t/test-lib.sh b/t/test-lib.sh
index ea1cd34013..d55d158580 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -333,24 +333,7 @@ do
 		GIT_TEST_CHAIN_LINT=0
 		shift ;;
 	-x)
-		# Some test scripts can't be reliably traced  with '-x',
-		# unless the test is run with a Bash version supporting
-		# BASH_XTRACEFD (introduced in Bash v4.1).  Check whether
-		# this test is marked as such, and ignore '-x' if it
-		# isn't executed with a suitable Bash version.
-		if test -z "$test_untraceable" || {
-		     test -n "$BASH_VERSION" && {
-		       test ${BASH_VERSINFO[0]} -gt 4 || {
-			 test ${BASH_VERSINFO[0]} -eq 4 &&
-			 test ${BASH_VERSINFO[1]} -ge 1
-		       }
-		     }
-		   }
-		then
-			trace=t
-		else
-			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
-		fi
+		trace=t
 		shift ;;
 	--tee|\
 	-V|--verbose-log|\
@@ -373,6 +356,24 @@ then
 	test -z "$verbose_log" && verbose=t
 fi
 
+if test -n "$trace" && test -n "$test_untraceable"
+then
+	# '-x' tracing requested, but this test script can't be reliably
+	# traced, unless it is run with a Bash version supporting
+	# BASH_XTRACEFD (introduced in Bash v4.1).
+	if test -n "$BASH_VERSION" && {
+	     test ${BASH_VERSINFO[0]} -gt 4 || {
+	       test ${BASH_VERSINFO[0]} -eq 4 &&
+	       test ${BASH_VERSINFO[1]} -ge 1
+	     }
+	   }
+	then
+		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
+	else
+		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		trace=
+	fi
+fi
 if test -n "$trace" && test -z "$verbose_log"
 then
 	verbose=t

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 5577d9dc5a..09c77cbd1b 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -90,6 +90,10 @@ do
 	esac
 done
 
+TEST_NAME="$(basename "$0" .sh)"
+TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
+TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
 if test "$GIT_TEST_TEE_STARTED" = "done"
@@ -98,12 +102,11 @@ then
 elif test -n "$tee" || test -n "$verbose_log" ||
      test -n "$valgrind" || test -n "$valgrind_only"
 then
-	mkdir -p "$TEST_OUTPUT_DIRECTORY/test-results"
-	BASE="$TEST_OUTPUT_DIRECTORY/test-results/$(basename "$0" .sh)"
+	mkdir -p "$TEST_RESULTS_DIR"
 
 	# Make this filename available to the sub-process in case it is using
 	# --verbose-log.
-	GIT_TEST_TEE_OUTPUT_FILE=$BASE.out
+	GIT_TEST_TEE_OUTPUT_FILE=$TEST_RESULTS_BASE.out
 	export GIT_TEST_TEE_OUTPUT_FILE
 
 	# Truncate before calling "tee -a" to get rid of the results
@@ -111,8 +114,8 @@ then
 	>"$GIT_TEST_TEE_OUTPUT_FILE"
 
 	(GIT_TEST_TEE_STARTED=done ${TEST_SHELL_PATH} "$0" "$@" 2>&1;
-	 echo $? >"$BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
-	test "$(cat "$BASE.exit")" = 0
+	 echo $? >"$TEST_RESULTS_BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
+	test "$(cat "$TEST_RESULTS_BASE.exit")" = 0
 	exit
 fi
 
@@ -829,12 +832,9 @@ test_done () {
 
 	if test -z "$HARNESS_ACTIVE"
 	then
-		test_results_dir="$TEST_OUTPUT_DIRECTORY/test-results"
-		mkdir -p "$test_results_dir"
-		base=${0##*/}
-		test_results_path="$test_results_dir/${base%.sh}.counts"
+		mkdir -p "$TEST_RESULTS_DIR"
 
-		cat >"$test_results_path" <<-EOF
+		cat >"$TEST_RESULTS_BASE.counts" <<-EOF
 		total $test_count
 		success $test_success
 		fixed $test_fixed
@@ -1040,7 +1040,7 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$(basename "$0" .sh)"
+TRASH_DIRECTORY="trash directory.$TEST_NAME"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good

t/lib-git-daemon.sh, t/lib-git-p4.sh, t/lib-git-svn.sh, t/lib-httpd.sh, t/t0410-partial-clone.sh, t/t5512-ls-remote.sh, t/test-lib-functions.sh

diff --git a/t/lib-git-daemon.sh b/t/lib-git-daemon.sh
index f98de95c15..41eb1e3ae8 100644
--- a/t/lib-git-daemon.sh
+++ b/t/lib-git-daemon.sh
@@ -28,7 +28,7 @@ then
 	test_skip_or_die $GIT_TEST_GIT_DAEMON "file system does not support FIFOs"
 fi
 
-LIB_GIT_DAEMON_PORT=${LIB_GIT_DAEMON_PORT-${this_test#t}}
+test_set_port LIB_GIT_DAEMON_PORT
 
 GIT_DAEMON_PID=
 GIT_DAEMON_DOCUMENT_ROOT_PATH="$PWD"/repo
diff --git a/t/lib-git-p4.sh b/t/lib-git-p4.sh
index c27599474c..b3be3ba011 100644
--- a/t/lib-git-p4.sh
+++ b/t/lib-git-p4.sh
@@ -53,14 +53,7 @@ time_in_seconds () {
 	(cd / && "$PYTHON_PATH" -c 'import time; print(int(time.time()))')
 }
 
-# Try to pick a unique port: guess a large number, then hope
-# no more than one of each test is running.
-#
-# This does not handle the case where somebody else is running the
-# same tests and has chosen the same ports.
-testid=${this_test#t}
-git_p4_test_start=9800
-P4DPORT=$((10669 + ($testid - $git_p4_test_start)))
+test_set_port P4DPORT
 
 P4PORT=localhost:$P4DPORT
 P4CLIENT=client
diff --git a/t/lib-git-svn.sh b/t/lib-git-svn.sh
index a8130f9119..f3b478c307 100644
--- a/t/lib-git-svn.sh
+++ b/t/lib-git-svn.sh
@@ -13,6 +13,7 @@ fi
 GIT_DIR=$PWD/.git
 GIT_SVN_DIR=$GIT_DIR/svn/refs/remotes/git-svn
 SVN_TREE=$GIT_SVN_DIR/svn-tree
+test_set_port SVNSERVE_PORT
 
 svn >/dev/null 2>&1
 if test $? -ne 1
@@ -119,7 +120,6 @@ require_svnserve () {
 }
 
 start_svnserve () {
-	SVNSERVE_PORT=${SVNSERVE_PORT-${this_test#t}}
 	svnserve --listen-port $SVNSERVE_PORT \
 		 --root "$rawsvnrepo" \
 		 --listen-once \
diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index a8729f8232..e465116ef9 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -82,7 +82,7 @@ case $(uname) in
 esac
 
 LIB_HTTPD_PATH=${LIB_HTTPD_PATH-"$DEFAULT_HTTPD_PATH"}
-LIB_HTTPD_PORT=${LIB_HTTPD_PORT-${this_test#t}}
+test_set_port LIB_HTTPD_PORT
 
 TEST_PATH="$TEST_DIRECTORY"/lib-httpd
 HTTPD_ROOT_PATH="$PWD"/httpd
diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index ba3887f178..0aca8d7588 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh
@@ -480,7 +480,6 @@ test_expect_success 'gc stops traversal when a missing but promised object is re
 	! grep "$TREE_HASH" out
 '
 
-LIB_HTTPD_PORT=12345  # default port, 410, cannot be used as non-root
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
diff --git a/t/t5512-ls-remote.sh b/t/t5512-ls-remote.sh
index 32e722db2e..cd9e60632d 100755
--- a/t/t5512-ls-remote.sh
+++ b/t/t5512-ls-remote.sh
@@ -260,7 +260,7 @@ test_lazy_prereq GIT_DAEMON '
 # This test spawns a daemon, so run it only if the user would be OK with
 # testing with git-daemon.
 test_expect_success PIPE,JGIT,GIT_DAEMON 'indicate no refs in standards-compliant empty remote' '
-	JGIT_DAEMON_PORT=${JGIT_DAEMON_PORT-${this_test#t}} &&
+	test_set_port JGIT_DAEMON_PORT &&
 	JGIT_DAEMON_PID= &&
 	git init --bare empty.git &&
 	>empty.git/git-daemon-export-ok &&
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 6b3bbf99e4..3746327bde 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1263,3 +1263,39 @@ test_oid () {
 	fi &&
 	eval "printf '%s' \"\${$var}\""
 }
+
+# Choose a port number based on the test script's number and store it in
+# the given variable name, unless that variable already contains a number.
+test_set_port () {
+	local var=$1 port
+
+	if test $# -ne 1 || test -z "$var"
+	then
+		BUG "test_set_port requires a variable name"
+	fi
+
+	eval port=\$$var
+	case "$port" in
+	"")
+		# No port is set in the given env var, use the test
+		# number as port number instead.
+		# Remove not only the leading 't', but all leading zeros
+		# as well, so the arithmetic below won't (mis)interpret
+		# a test number like '0123' as an octal value.
+		port=${this_test#${this_test%%[1-9]*}}
+		if test "${port:-0}" -lt 1024
+		then
+			# root-only port, use a larger one instead.
+			port=$(($port + 10000))
+		fi
+
+		eval $var=$port
+		;;
+	*[^0-9]*)
+		error >&7 "invalid port number: $port"
+		;;
+	*)
+		# The user has specified the port.
+		;;
+	esac
+}

t/README, t/test-lib-functions.sh, t/test-lib.sh

diff --git a/t/README b/t/README
index 28711cc508..11ce7675e3 100644
--- a/t/README
+++ b/t/README
@@ -186,6 +186,22 @@ appropriately before running "make".
 	this feature by setting the GIT_TEST_CHAIN_LINT environment
 	variable to "1" or "0", respectively.
 
+--stress::
+--stress=<N>::
+	Run the test script repeatedly in multiple parallel jobs until
+	one of them fails.  Useful for reproducing rare failures in
+	flaky tests.  The number of parallel jobs is, in order of
+	precedence: <N>, or the value of the GIT_TEST_STRESS_LOAD
+	environment variable, or twice the number of available
+	processors (as shown by the 'getconf' utility),	or 8.
+	Implies `--verbose -x --immediate` to get the most information
+	about the failure.  Note that the verbose output of each test
+	job is saved to 't/test-results/$TEST_NAME.stress-<nr>.out',
+	and only the output of the failed test job is shown on the
+	terminal.  The names of the trash directories get a
+	'.stress-<nr>' suffix, and the trash directory of the failed
+	test job is renamed to end with a '.stress-failed' suffix.
+
 You can also set the GIT_TEST_INSTALLED environment variable to
 the bindir of an existing git installation to test that installation.
 You still need to have built this git sandbox, from which various
@@ -425,7 +441,8 @@ This test harness library does the following things:
  - Creates an empty test directory with an empty .git/objects database
    and chdir(2) into it.  This directory is 't/trash
    directory.$test_name_without_dotsh', with t/ subject to change by
-   the --root option documented above.
+   the --root option documented above, and a '.stress-<N>' suffix
+   appended by the --stress option.
 
  - Defines standard test helper functions for your scripts to
    use.  These functions are designed to make all scripts behave
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 3746327bde..ee3435c6df 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1288,8 +1288,6 @@ test_set_port () {
 			# root-only port, use a larger one instead.
 			port=$(($port + 10000))
 		fi
-
-		eval $var=$port
 		;;
 	*[^0-9]*)
 		error >&7 "invalid port number: $port"
@@ -1298,4 +1296,9 @@ test_set_port () {
 		# The user has specified the port.
 		;;
 	esac
+
+	# Make sure that parallel '--stress' test jobs get different
+	# ports.
+	port=$(($port + ${GIT_TEST_STRESS_JOB_NR:-0}))
+	eval $var=$port
 }
diff --git a/t/test-lib.sh b/t/test-lib.sh
index d55d158580..e405191164 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -87,21 +87,110 @@ do
 		valgrind_only=${opt#--*=} ;;
 	--root=*)
 		root=${opt#--*=} ;;
+	--stress)
+		stress=t ;;
+	--stress=*)
+		stress=${opt#--*=} ;;
 	*)
 		# Other options will be handled later.
 	esac
 done
 
+TEST_STRESS_JOB_SFX="${GIT_TEST_STRESS_JOB_NR:+.stress-$GIT_TEST_STRESS_JOB_NR}"
 TEST_NAME="$(basename "$0" .sh)"
 TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
-TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
-TRASH_DIRECTORY="trash directory.$TEST_NAME"
+TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME$TEST_STRESS_JOB_SFX"
+TRASH_DIRECTORY="trash directory.$TEST_NAME$TEST_STRESS_JOB_SFX"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good
  *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
 esac
 
+# If --stress was passed, run this test repeatedly in several parallel loops.
+if test "$GIT_TEST_STRESS_STARTED" = "done"
+then
+	: # Don't stress test again.
+elif test -n "$stress"
+then
+	if test "$stress" != t
+	then
+		job_count=$stress
+	elif test -n "$GIT_TEST_STRESS_LOAD"
+	then
+		job_count="$GIT_TEST_STRESS_LOAD"
+	elif job_count=$(getconf _NPROCESSORS_ONLN 2>/dev/null) &&
+	     test -n "$job_count"
+	then
+		job_count=$((2 * $job_count))
+	else
+		job_count=8
+	fi
+
+	mkdir -p "$TEST_RESULTS_DIR"
+	stressfail="$TEST_RESULTS_BASE.stress-failed"
+	rm -f "$stressfail"
+
+	stress_exit=0
+	trap '
+		kill $job_pids 2>/dev/null
+		wait
+		stress_exit=1
+	' TERM INT HUP
+
+	job_pids=
+	job_nr=0
+	while test $job_nr -lt "$job_count"
+	do
+		(
+			GIT_TEST_STRESS_STARTED=done
+			GIT_TEST_STRESS_JOB_NR=$job_nr
+			export GIT_TEST_STRESS_STARTED GIT_TEST_STRESS_JOB_NR
+
+			trap '
+				kill $test_pid 2>/dev/null
+				wait
+				exit 1
+			' TERM INT
+
+			cnt=0
+			while ! test -e "$stressfail"
+			do
+				$TEST_SHELL_PATH "$0" "$@" >"$TEST_RESULTS_BASE.stress-$job_nr.out" 2>&1 &
+				test_pid=$!
+
+				if wait $test_pid
+				then
+					printf "OK   %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				else
+					echo $GIT_TEST_STRESS_JOB_NR >>"$stressfail"
+					printf "FAIL %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				fi
+				cnt=$(($cnt + 1))
+			done
+		) &
+		job_pids="$job_pids $!"
+		job_nr=$(($job_nr + 1))
+	done
+
+	wait
+
+	if test -f "$stressfail"
+	then
+		echo "Log(s) of failed test run(s):"
+		for failed_job_nr in $(sort -n "$stressfail")
+		do
+			echo "Contents of '$TEST_RESULTS_BASE.stress-$failed_job_nr.out':"
+			cat "$TEST_RESULTS_BASE.stress-$failed_job_nr.out"
+		done
+		rm -rf "$TRASH_DIRECTORY.stress-failed"
+		# Move the last one.
+		mv "$TRASH_DIRECTORY.stress-$failed_job_nr" "$TRASH_DIRECTORY.stress-failed"
+	fi
+
+	exit $stress_exit
+fi
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
 if test "$GIT_TEST_TEE_STARTED" = "done"
@@ -340,7 +429,8 @@ do
 	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind|\
 	--valgrind=*|\
 	--valgrind-only=*|\
-	--root=*)
+	--root=*|\
+	--stress|--stress=*)
 		shift ;; # These options were handled already.
 	*)
 		echo "error: unknown test option '$1'" >&2; exit 1 ;;
@@ -379,6 +469,13 @@ then
 	verbose=t
 fi
 
+if test -n "$stress"
+then
+	verbose=t
+	trace=t
+	immediate=t
+fi
+
 if test -n "$color"
 then
 	# Save the color control sequences now rather than run tput

diff --git a/t/test-lib.sh b/t/test-lib.sh
index e405191164..3e9916b39b 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -446,6 +446,13 @@ then
 	test -z "$verbose_log" && verbose=t
 fi
 
+if test -n "$stress"
+then
+	verbose=t
+	trace=t
+	immediate=t
+fi
+
 if test -n "$trace" && test -n "$test_untraceable"
 then
 	# '-x' tracing requested, but this test script can't be reliably
@@ -469,13 +476,6 @@ then
 	verbose=t
 fi
 
-if test -n "$stress"
-then
-	verbose=t
-	trace=t
-	immediate=t
-fi
-
 if test -n "$color"
 then
 	# Save the color control sequences now rather than run tput

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 0f1faa24b2..9a3f7930a3 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -476,7 +476,7 @@ die () {
 
 GIT_EXIT_OK=
 trap 'die' EXIT
-trap 'exit $?' INT
+trap 'exit $?' INT TERM HUP
 
 # The user-facing functions are loaded from a separate file so that
 # test_perf subshells can have them too

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 9a3f7930a3..ed7267962f 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -264,58 +264,59 @@ test "x$TERM" != "xdumb" && (
 	) &&
 	color=t
 
-while test "$#" -ne 0
+store_arg_to=
+prev_opt=
+for opt
 do
-	case "$1" in
+	if test -n "$store_arg_to"
+	then
+		eval $store_arg_to=\$opt
+		store_arg_to=
+		prev_opt=
+		continue
+	fi
+
+	case "$opt" in
 	-d|--d|--de|--deb|--debu|--debug)
-		debug=t; shift ;;
+		debug=t ;;
 	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
-		immediate=t; shift ;;
+		immediate=t ;;
 	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
-		GIT_TEST_LONG=t; export GIT_TEST_LONG; shift ;;
+		GIT_TEST_LONG=t; export GIT_TEST_LONG ;;
 	-r)
-		shift; test "$#" -ne 0 || {
-			echo 'error: -r requires an argument' >&2;
-			exit 1;
-		}
-		run_list=$1; shift ;;
+		store_arg_to=run_list
+		;;
 	--run=*)
-		run_list=${1#--*=}; shift ;;
+		run_list=${opt#--*=} ;;
 	-h|--h|--he|--hel|--help)
-		help=t; shift ;;
+		help=t ;;
 	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
-		verbose=t; shift ;;
+		verbose=t ;;
 	--verbose-only=*)
-		verbose_only=${1#--*=}
-		shift ;;
+		verbose_only=${opt#--*=}
+		;;
 	-q|--q|--qu|--qui|--quie|--quiet)
 		# Ignore --quiet under a TAP::Harness. Saying how many tests
 		# passed without the ok/not ok details is always an error.
-		test -z "$HARNESS_ACTIVE" && quiet=t; shift ;;
+		test -z "$HARNESS_ACTIVE" && quiet=t ;;
 	--with-dashes)
-		with_dashes=t; shift ;;
+		with_dashes=t ;;
 	--no-color)
-		color=; shift ;;
+		color= ;;
 	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
-		valgrind=memcheck
-		shift ;;
+		valgrind=memcheck ;;
 	--valgrind=*)
-		valgrind=${1#--*=}
-		shift ;;
+		valgrind=${opt#--*=} ;;
 	--valgrind-only=*)
-		valgrind_only=${1#--*=}
-		shift ;;
+		valgrind_only=${opt#--*=} ;;
 	--tee)
-		shift ;; # was handled already
+		;; # was handled already
 	--root=*)
-		root=${1#--*=}
-		shift ;;
+		root=${opt#--*=} ;;
 	--chain-lint)
-		GIT_TEST_CHAIN_LINT=1
-		shift ;;
+		GIT_TEST_CHAIN_LINT=1 ;;
 	--no-chain-lint)
-		GIT_TEST_CHAIN_LINT=0
-		shift ;;
+		GIT_TEST_CHAIN_LINT=0 ;;
 	-x)
 		# Some test scripts can't be reliably traced  with '-x',
 		# unless the test is run with a Bash version supporting
@@ -335,14 +336,20 @@ do
 		else
 			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
 		fi
-		shift ;;
+		;;
 	-V|--verbose-log)
-		verbose_log=t
-		shift ;;
+		verbose_log=t ;;
 	*)
-		echo "error: unknown test option '$1'" >&2; exit 1 ;;
+		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
 	esac
+
+	prev_opt=$opt
 done
+if test -n "$store_arg_to"
+then
+	echo "error: $prev_opt requires an argument" >&2
+	exit 1
+fi
 
 if test -n "$valgrind_only"
 then

diff --git a/t/test-lib.sh b/t/test-lib.sh
index ed7267962f..fcc04afdff 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -71,13 +71,125 @@ then
 	exit 1
 fi
 
+# Parse options while taking care to leave $@ intact, so we will still
+# have all the original command line options when executing the test
+# script again for '--tee' and '--verbose-log' below.
+store_arg_to=
+prev_opt=
+for opt
+do
+	if test -n "$store_arg_to"
+	then
+		eval $store_arg_to=\$opt
+		store_arg_to=
+		prev_opt=
+		continue
+	fi
+
+	case "$opt" in
+	-d|--d|--de|--deb|--debu|--debug)
+		debug=t ;;
+	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
+		immediate=t ;;
+	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
+		GIT_TEST_LONG=t; export GIT_TEST_LONG ;;
+	-r)
+		store_arg_to=run_list
+		;;
+	--run=*)
+		run_list=${opt#--*=} ;;
+	-h|--h|--he|--hel|--help)
+		help=t ;;
+	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
+		verbose=t ;;
+	--verbose-only=*)
+		verbose_only=${opt#--*=}
+		;;
+	-q|--q|--qu|--qui|--quie|--quiet)
+		# Ignore --quiet under a TAP::Harness. Saying how many tests
+		# passed without the ok/not ok details is always an error.
+		test -z "$HARNESS_ACTIVE" && quiet=t ;;
+	--with-dashes)
+		with_dashes=t ;;
+	--no-color)
+		color= ;;
+	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
+		valgrind=memcheck
+		tee=t
+		;;
+	--valgrind=*)
+		valgrind=${opt#--*=}
+		tee=t
+		;;
+	--valgrind-only=*)
+		valgrind_only=${opt#--*=}
+		tee=t
+		;;
+	--tee)
+		tee=t ;;
+	--root=*)
+		root=${opt#--*=} ;;
+	--chain-lint)
+		GIT_TEST_CHAIN_LINT=1 ;;
+	--no-chain-lint)
+		GIT_TEST_CHAIN_LINT=0 ;;
+	-x)
+		# Some test scripts can't be reliably traced  with '-x',
+		# unless the test is run with a Bash version supporting
+		# BASH_XTRACEFD (introduced in Bash v4.1).  Check whether
+		# this test is marked as such, and ignore '-x' if it
+		# isn't executed with a suitable Bash version.
+		if test -z "$test_untraceable" || {
+		     test -n "$BASH_VERSION" && {
+		       test ${BASH_VERSINFO[0]} -gt 4 || {
+			 test ${BASH_VERSINFO[0]} -eq 4 &&
+			 test ${BASH_VERSINFO[1]} -ge 1
+		       }
+		     }
+		   }
+		then
+			trace=t
+		else
+			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		fi
+		;;
+	-V|--verbose-log)
+		verbose_log=t
+		tee=t
+		;;
+	*)
+		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
+	esac
+
+	prev_opt=$opt
+done
+if test -n "$store_arg_to"
+then
+	echo "error: $prev_opt requires an argument" >&2
+	exit 1
+fi
+
+if test -n "$valgrind_only"
+then
+	test -z "$valgrind" && valgrind=memcheck
+	test -z "$verbose" && verbose_only="$valgrind_only"
+elif test -n "$valgrind"
+then
+	test -z "$verbose_log" && verbose=t
+fi
+
+if test -n "$trace" && test -z "$verbose_log"
+then
+	verbose=t
+fi
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
-case "$GIT_TEST_TEE_STARTED, $* " in
-done,*)
-	# do not redirect again
-	;;
-*' --tee '*|*' --va'*|*' -V '*|*' --verbose-log '*)
+if test "$GIT_TEST_TEE_STARTED" = "done"
+then
+	: # do not redirect again
+elif test -n "$tee"
+then
 	mkdir -p "$TEST_OUTPUT_DIRECTORY/test-results"
 	BASE="$TEST_OUTPUT_DIRECTORY/test-results/$(basename "$0" .sh)"
 
@@ -94,8 +206,7 @@ done,*)
 	 echo $? >"$BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
 	test "$(cat "$BASE.exit")" = 0
 	exit
-	;;
-esac
+fi
 
 # For repeatability, reset the environment to known value.
 # TERM is sanitized below, after saving color control sequences.
@@ -193,7 +304,7 @@ fi
 
 # Add libc MALLOC and MALLOC_PERTURB test
 # only if we are not executing the test with valgrind
-if expr " $GIT_TEST_OPTS " : ".* --valgrind " >/dev/null ||
+if test -n "$valgrind" ||
    test -n "$TEST_NO_MALLOC_CHECK"
 then
 	setup_malloc_check () {
@@ -264,107 +375,6 @@ test "x$TERM" != "xdumb" && (
 	) &&
 	color=t
 
-store_arg_to=
-prev_opt=
-for opt
-do
-	if test -n "$store_arg_to"
-	then
-		eval $store_arg_to=\$opt
-		store_arg_to=
-		prev_opt=
-		continue
-	fi
-
-	case "$opt" in
-	-d|--d|--de|--deb|--debu|--debug)
-		debug=t ;;
-	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
-		immediate=t ;;
-	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
-		GIT_TEST_LONG=t; export GIT_TEST_LONG ;;
-	-r)
-		store_arg_to=run_list
-		;;
-	--run=*)
-		run_list=${opt#--*=} ;;
-	-h|--h|--he|--hel|--help)
-		help=t ;;
-	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
-		verbose=t ;;
-	--verbose-only=*)
-		verbose_only=${opt#--*=}
-		;;
-	-q|--q|--qu|--qui|--quie|--quiet)
-		# Ignore --quiet under a TAP::Harness. Saying how many tests
-		# passed without the ok/not ok details is always an error.
-		test -z "$HARNESS_ACTIVE" && quiet=t ;;
-	--with-dashes)
-		with_dashes=t ;;
-	--no-color)
-		color= ;;
-	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
-		valgrind=memcheck ;;
-	--valgrind=*)
-		valgrind=${opt#--*=} ;;
-	--valgrind-only=*)
-		valgrind_only=${opt#--*=} ;;
-	--tee)
-		;; # was handled already
-	--root=*)
-		root=${opt#--*=} ;;
-	--chain-lint)
-		GIT_TEST_CHAIN_LINT=1 ;;
-	--no-chain-lint)
-		GIT_TEST_CHAIN_LINT=0 ;;
-	-x)
-		# Some test scripts can't be reliably traced  with '-x',
-		# unless the test is run with a Bash version supporting
-		# BASH_XTRACEFD (introduced in Bash v4.1).  Check whether
-		# this test is marked as such, and ignore '-x' if it
-		# isn't executed with a suitable Bash version.
-		if test -z "$test_untraceable" || {
-		     test -n "$BASH_VERSION" && {
-		       test ${BASH_VERSINFO[0]} -gt 4 || {
-			 test ${BASH_VERSINFO[0]} -eq 4 &&
-			 test ${BASH_VERSINFO[1]} -ge 1
-		       }
-		     }
-		   }
-		then
-			trace=t
-		else
-			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
-		fi
-		;;
-	-V|--verbose-log)
-		verbose_log=t ;;
-	*)
-		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
-	esac
-
-	prev_opt=$opt
-done
-if test -n "$store_arg_to"
-then
-	echo "error: $prev_opt requires an argument" >&2
-	exit 1
-fi
-
-if test -n "$valgrind_only"
-then
-	test -z "$valgrind" && valgrind=memcheck
-	test -z "$verbose" && verbose_only="$valgrind_only"
-elif test -n "$valgrind"
-then
-	test -z "$verbose_log" && verbose=t
-fi
-
-if test -n "$trace" && test -z "$verbose_log"
-then
-	verbose=t
-fi
-
 if test -n "$color"
 then
 	# Save the color control sequences now rather than run tput

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 2b88ba2de1..7d77a26d44 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -134,25 +134,7 @@ do
 	--no-chain-lint)
 		GIT_TEST_CHAIN_LINT=0 ;;
 	-x)
-		# Some test scripts can't be reliably traced  with '-x',
-		# unless the test is run with a Bash version supporting
-		# BASH_XTRACEFD (introduced in Bash v4.1).  Check whether
-		# this test is marked as such, and ignore '-x' if it
-		# isn't executed with a suitable Bash version.
-		if test -z "$test_untraceable" || {
-		     test -n "$BASH_VERSION" && {
-		       test ${BASH_VERSINFO[0]} -gt 4 || {
-			 test ${BASH_VERSINFO[0]} -eq 4 &&
-			 test ${BASH_VERSINFO[1]} -ge 1
-		       }
-		     }
-		   }
-		then
-			trace=t
-		else
-			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
-		fi
-		;;
+		trace=t ;;
 	-V|--verbose-log)
 		verbose_log=t
 		tee=t
@@ -178,6 +160,24 @@ then
 	test -z "$verbose_log" && verbose=t
 fi
 
+if test -n "$trace" && test -n "$test_untraceable"
+then
+	# '-x' tracing requested, but this test script can't be reliably
+	# traced, unless it is run with a Bash version supporting
+	# BASH_XTRACEFD (introduced in Bash v4.1).
+	if test -n "$BASH_VERSION" && {
+	     test ${BASH_VERSINFO[0]} -gt 4 || {
+	       test ${BASH_VERSINFO[0]} -eq 4 &&
+	       test ${BASH_VERSINFO[1]} -ge 1
+	     }
+	   }
+	then
+		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
+	else
+		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		trace=
+	fi
+fi
 if test -n "$trace" && test -z "$verbose_log"
 then
 	verbose=t

t/lib-git-daemon.sh, t/lib-git-p4.sh, t/lib-git-svn.sh, t/lib-httpd.sh, t/t0410-partial-clone.sh, t/t5512-ls-remote.sh, t/test-lib-functions.sh

diff --git a/t/lib-git-daemon.sh b/t/lib-git-daemon.sh
index f98de95c15..41eb1e3ae8 100644
--- a/t/lib-git-daemon.sh
+++ b/t/lib-git-daemon.sh
@@ -28,7 +28,7 @@ then
 	test_skip_or_die $GIT_TEST_GIT_DAEMON "file system does not support FIFOs"
 fi
 
-LIB_GIT_DAEMON_PORT=${LIB_GIT_DAEMON_PORT-${this_test#t}}
+test_set_port LIB_GIT_DAEMON_PORT
 
 GIT_DAEMON_PID=
 GIT_DAEMON_DOCUMENT_ROOT_PATH="$PWD"/repo
diff --git a/t/lib-git-p4.sh b/t/lib-git-p4.sh
index c27599474c..b3be3ba011 100644
--- a/t/lib-git-p4.sh
+++ b/t/lib-git-p4.sh
@@ -53,14 +53,7 @@ time_in_seconds () {
 	(cd / && "$PYTHON_PATH" -c 'import time; print(int(time.time()))')
 }
 
-# Try to pick a unique port: guess a large number, then hope
-# no more than one of each test is running.
-#
-# This does not handle the case where somebody else is running the
-# same tests and has chosen the same ports.
-testid=${this_test#t}
-git_p4_test_start=9800
-P4DPORT=$((10669 + ($testid - $git_p4_test_start)))
+test_set_port P4DPORT
 
 P4PORT=localhost:$P4DPORT
 P4CLIENT=client
diff --git a/t/lib-git-svn.sh b/t/lib-git-svn.sh
index a8130f9119..f3b478c307 100644
--- a/t/lib-git-svn.sh
+++ b/t/lib-git-svn.sh
@@ -13,6 +13,7 @@ fi
 GIT_DIR=$PWD/.git
 GIT_SVN_DIR=$GIT_DIR/svn/refs/remotes/git-svn
 SVN_TREE=$GIT_SVN_DIR/svn-tree
+test_set_port SVNSERVE_PORT
 
 svn >/dev/null 2>&1
 if test $? -ne 1
@@ -119,7 +120,6 @@ require_svnserve () {
 }
 
 start_svnserve () {
-	SVNSERVE_PORT=${SVNSERVE_PORT-${this_test#t}}
 	svnserve --listen-port $SVNSERVE_PORT \
 		 --root "$rawsvnrepo" \
 		 --listen-once \
diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index a8729f8232..e465116ef9 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -82,7 +82,7 @@ case $(uname) in
 esac
 
 LIB_HTTPD_PATH=${LIB_HTTPD_PATH-"$DEFAULT_HTTPD_PATH"}
-LIB_HTTPD_PORT=${LIB_HTTPD_PORT-${this_test#t}}
+test_set_port LIB_HTTPD_PORT
 
 TEST_PATH="$TEST_DIRECTORY"/lib-httpd
 HTTPD_ROOT_PATH="$PWD"/httpd
diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index ba3887f178..0aca8d7588 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh
@@ -480,7 +480,6 @@ test_expect_success 'gc stops traversal when a missing but promised object is re
 	! grep "$TREE_HASH" out
 '
 
-LIB_HTTPD_PORT=12345  # default port, 410, cannot be used as non-root
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
diff --git a/t/t5512-ls-remote.sh b/t/t5512-ls-remote.sh
index 32e722db2e..cd9e60632d 100755
--- a/t/t5512-ls-remote.sh
+++ b/t/t5512-ls-remote.sh
@@ -260,7 +260,7 @@ test_lazy_prereq GIT_DAEMON '
 # This test spawns a daemon, so run it only if the user would be OK with
 # testing with git-daemon.
 test_expect_success PIPE,JGIT,GIT_DAEMON 'indicate no refs in standards-compliant empty remote' '
-	JGIT_DAEMON_PORT=${JGIT_DAEMON_PORT-${this_test#t}} &&
+	test_set_port JGIT_DAEMON_PORT &&
 	JGIT_DAEMON_PID= &&
 	git init --bare empty.git &&
 	>empty.git/git-daemon-export-ok &&
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 6b3bbf99e4..4459bdda13 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1263,3 +1263,39 @@ test_oid () {
 	fi &&
 	eval "printf '%s' \"\${$var}\""
 }
+
+# Choose a port number based on the test script's number and store it in
+# the given variable name, unless that variable already contains a number.
+test_set_port () {
+	local var=$1 port
+
+	if test $# -ne 1 || test -z "$var"
+	then
+		BUG "test_set_port requires a variable name"
+	fi
+
+	eval port=\$$var
+	case "$port" in
+	"")
+		# No port is set in the given env var, use the test
+		# number as port number instead.
+		# Remove not only the leading 't', but all leading zeros
+		# as well, so the arithmetic below won't (mis)interpret
+		# a test number like '0123' as an octal value.
+		port=${this_test#${this_test%%[1-9]*}}
+		if test "${port:-0}" -lt 1024
+		then
+			# root-only port, use a larger one instead.
+			port=$(($port + 10000))
+		fi
+
+		eval $var=$port
+		;;
+	*[^0-9]*|0*)
+		error >&7 "invalid port number: $port"
+		;;
+	*)
+		# The user has specified the port.
+		;;
+	esac
+}

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 41457d1dcf..2b88ba2de1 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -186,6 +186,12 @@ fi
 TEST_NAME="$(basename "$0" .sh)"
 TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
 TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
+TRASH_DIRECTORY="trash directory.$TEST_NAME"
+test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
+case "$TRASH_DIRECTORY" in
+/*) ;; # absolute path is good
+ *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
+esac
 
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
@@ -1046,12 +1052,6 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$TEST_NAME"
-test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
-case "$TRASH_DIRECTORY" in
-/*) ;; # absolute path is good
- *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
-esac
 rm -fr "$TRASH_DIRECTORY" || {
 	GIT_EXIT_OK=t
 	echo >&5 "FATAL: Cannot prepare test area"

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 41457d1dcf..2b88ba2de1 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -186,6 +186,12 @@ fi
 TEST_NAME="$(basename "$0" .sh)"
 TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
 TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
+TRASH_DIRECTORY="trash directory.$TEST_NAME"
+test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
+case "$TRASH_DIRECTORY" in
+/*) ;; # absolute path is good
+ *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
+esac
 
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
@@ -1046,12 +1052,6 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$TEST_NAME"
-test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
-case "$TRASH_DIRECTORY" in
-/*) ;; # absolute path is good
- *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
-esac
 rm -fr "$TRASH_DIRECTORY" || {
 	GIT_EXIT_OK=t
 	echo >&5 "FATAL: Cannot prepare test area"

diff --git a/t/test-lib.sh b/t/test-lib.sh
index fcc04afdff..41457d1dcf 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -183,6 +183,10 @@ then
 	verbose=t
 fi
 
+TEST_NAME="$(basename "$0" .sh)"
+TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
+TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
 if test "$GIT_TEST_TEE_STARTED" = "done"
@@ -190,12 +194,11 @@ then
 	: # do not redirect again
 elif test -n "$tee"
 then
-	mkdir -p "$TEST_OUTPUT_DIRECTORY/test-results"
-	BASE="$TEST_OUTPUT_DIRECTORY/test-results/$(basename "$0" .sh)"
+	mkdir -p "$TEST_RESULTS_DIR"
 
 	# Make this filename available to the sub-process in case it is using
 	# --verbose-log.
-	GIT_TEST_TEE_OUTPUT_FILE=$BASE.out
+	GIT_TEST_TEE_OUTPUT_FILE=$TEST_RESULTS_BASE.out
 	export GIT_TEST_TEE_OUTPUT_FILE
 
 	# Truncate before calling "tee -a" to get rid of the results
@@ -203,8 +206,8 @@ then
 	>"$GIT_TEST_TEE_OUTPUT_FILE"
 
 	(GIT_TEST_TEE_STARTED=done ${TEST_SHELL_PATH} "$0" "$@" 2>&1;
-	 echo $? >"$BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
-	test "$(cat "$BASE.exit")" = 0
+	 echo $? >"$TEST_RESULTS_BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
+	test "$(cat "$TEST_RESULTS_BASE.exit")" = 0
 	exit
 fi
 
@@ -835,12 +838,9 @@ test_done () {
 
 	if test -z "$HARNESS_ACTIVE"
 	then
-		test_results_dir="$TEST_OUTPUT_DIRECTORY/test-results"
-		mkdir -p "$test_results_dir"
-		base=${0##*/}
-		test_results_path="$test_results_dir/${base%.sh}.counts"
+		mkdir -p "$TEST_RESULTS_DIR"
 
-		cat >"$test_results_path" <<-EOF
+		cat >"$TEST_RESULTS_BASE.counts" <<-EOF
 		total $test_count
 		success $test_success
 		fixed $test_fixed
@@ -1046,7 +1046,7 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$(basename "$0" .sh)"
+TRASH_DIRECTORY="trash directory.$TEST_NAME"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good

t/README, t/test-lib-functions.sh, t/test-lib.sh

diff --git a/t/README b/t/README
index 28711cc508..11ce7675e3 100644
--- a/t/README
+++ b/t/README
@@ -186,6 +186,22 @@ appropriately before running "make".
 	this feature by setting the GIT_TEST_CHAIN_LINT environment
 	variable to "1" or "0", respectively.
 
+--stress::
+--stress=<N>::
+	Run the test script repeatedly in multiple parallel jobs until
+	one of them fails.  Useful for reproducing rare failures in
+	flaky tests.  The number of parallel jobs is, in order of
+	precedence: <N>, or the value of the GIT_TEST_STRESS_LOAD
+	environment variable, or twice the number of available
+	processors (as shown by the 'getconf' utility),	or 8.
+	Implies `--verbose -x --immediate` to get the most information
+	about the failure.  Note that the verbose output of each test
+	job is saved to 't/test-results/$TEST_NAME.stress-<nr>.out',
+	and only the output of the failed test job is shown on the
+	terminal.  The names of the trash directories get a
+	'.stress-<nr>' suffix, and the trash directory of the failed
+	test job is renamed to end with a '.stress-failed' suffix.
+
 You can also set the GIT_TEST_INSTALLED environment variable to
 the bindir of an existing git installation to test that installation.
 You still need to have built this git sandbox, from which various
@@ -425,7 +441,8 @@ This test harness library does the following things:
  - Creates an empty test directory with an empty .git/objects database
    and chdir(2) into it.  This directory is 't/trash
    directory.$test_name_without_dotsh', with t/ subject to change by
-   the --root option documented above.
+   the --root option documented above, and a '.stress-<N>' suffix
+   appended by the --stress option.
 
  - Defines standard test helper functions for your scripts to
    use.  These functions are designed to make all scripts behave
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 4459bdda13..92cf8f812c 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1288,8 +1288,6 @@ test_set_port () {
 			# root-only port, use a larger one instead.
 			port=$(($port + 10000))
 		fi
-
-		eval $var=$port
 		;;
 	*[^0-9]*|0*)
 		error >&7 "invalid port number: $port"
@@ -1298,4 +1296,9 @@ test_set_port () {
 		# The user has specified the port.
 		;;
 	esac
+
+	# Make sure that parallel '--stress' test jobs get different
+	# ports.
+	port=$(($port + ${GIT_TEST_STRESS_JOB_NR:-0}))
+	eval $var=$port
 }
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 7d77a26d44..16f74bd524 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -139,6 +139,19 @@ do
 		verbose_log=t
 		tee=t
 		;;
+	--stress)
+		stress=t ;;
+	--stress=*)
+		stress=${opt#--*=}
+		case "$stress" in
+		*[^0-9]*|0*|"")
+			echo "error: --stress=<N> requires the number of jobs to run" >&2
+			exit 1
+			;;
+		*)	# Good.
+			;;
+		esac
+		;;
 	*)
 		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
 	esac
@@ -160,6 +173,13 @@ then
 	test -z "$verbose_log" && verbose=t
 fi
 
+if test -n "$stress"
+then
+	verbose=t
+	trace=t
+	immediate=t
+fi
+
 if test -n "$trace" && test -n "$test_untraceable"
 then
 	# '-x' tracing requested, but this test script can't be reliably
@@ -183,16 +203,101 @@ then
 	verbose=t
 fi
 
+TEST_STRESS_JOB_SFX="${GIT_TEST_STRESS_JOB_NR:+.stress-$GIT_TEST_STRESS_JOB_NR}"
 TEST_NAME="$(basename "$0" .sh)"
 TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
-TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
-TRASH_DIRECTORY="trash directory.$TEST_NAME"
+TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME$TEST_STRESS_JOB_SFX"
+TRASH_DIRECTORY="trash directory.$TEST_NAME$TEST_STRESS_JOB_SFX"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good
  *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
 esac
 
+# If --stress was passed, run this test repeatedly in several parallel loops.
+if test "$GIT_TEST_STRESS_STARTED" = "done"
+then
+	: # Don't stress test again.
+elif test -n "$stress"
+then
+	if test "$stress" != t
+	then
+		job_count=$stress
+	elif test -n "$GIT_TEST_STRESS_LOAD"
+	then
+		job_count="$GIT_TEST_STRESS_LOAD"
+	elif job_count=$(getconf _NPROCESSORS_ONLN 2>/dev/null) &&
+	     test -n "$job_count"
+	then
+		job_count=$((2 * $job_count))
+	else
+		job_count=8
+	fi
+
+	mkdir -p "$TEST_RESULTS_DIR"
+	stressfail="$TEST_RESULTS_BASE.stress-failed"
+	rm -f "$stressfail"
+
+	stress_exit=0
+	trap '
+		kill $job_pids 2>/dev/null
+		wait
+		stress_exit=1
+	' TERM INT HUP
+
+	job_pids=
+	job_nr=0
+	while test $job_nr -lt "$job_count"
+	do
+		(
+			GIT_TEST_STRESS_STARTED=done
+			GIT_TEST_STRESS_JOB_NR=$job_nr
+			export GIT_TEST_STRESS_STARTED GIT_TEST_STRESS_JOB_NR
+
+			trap '
+				kill $test_pid 2>/dev/null
+				wait
+				exit 1
+			' TERM INT
+
+			cnt=0
+			while ! test -e "$stressfail"
+			do
+				$TEST_SHELL_PATH "$0" "$@" >"$TEST_RESULTS_BASE.stress-$job_nr.out" 2>&1 &
+				test_pid=$!
+
+				if wait $test_pid
+				then
+					printf "OK   %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				else
+					echo $GIT_TEST_STRESS_JOB_NR >>"$stressfail"
+					printf "FAIL %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				fi
+				cnt=$(($cnt + 1))
+			done
+		) &
+		job_pids="$job_pids $!"
+		job_nr=$(($job_nr + 1))
+	done
+
+	wait
+
+	if test -f "$stressfail"
+	then
+		echo "Log(s) of failed test run(s):"
+		for failed_job_nr in $(sort -n "$stressfail")
+		do
+			echo "Contents of '$TEST_RESULTS_BASE.stress-$failed_job_nr.out':"
+			cat "$TEST_RESULTS_BASE.stress-$failed_job_nr.out"
+		done
+		rm -rf "$TRASH_DIRECTORY.stress-failed"
+		# Move the last one.
+		mv "$TRASH_DIRECTORY.stress-$failed_job_nr" "$TRASH_DIRECTORY.stress-failed"
+	fi
+
+	exit $stress_exit
+fi
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
 if test "$GIT_TEST_TEE_STARTED" = "done"

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 3f0b0c07df..a1abb1177a 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -180,29 +180,6 @@ then
 	immediate=t
 fi
 
-if test -n "$trace" && test -n "$test_untraceable"
-then
-	# '-x' tracing requested, but this test script can't be reliably
-	# traced, unless it is run with a Bash version supporting
-	# BASH_XTRACEFD (introduced in Bash v4.1).
-	if test -n "$BASH_VERSION" && eval '
-	     test ${BASH_VERSINFO[0]} -gt 4 || {
-	       test ${BASH_VERSINFO[0]} -eq 4 &&
-	       test ${BASH_VERSINFO[1]} -ge 1
-	     }
-	   '
-	then
-		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
-	else
-		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
-		trace=
-	fi
-fi
-if test -n "$trace" && test -z "$verbose_log"
-then
-	verbose=t
-fi
-
 TEST_STRESS_JOB_SFX="${GIT_TEST_STRESS_JOB_NR:+.stress-$GIT_TEST_STRESS_JOB_NR}"
 TEST_NAME="$(basename "$0" .sh)"
 TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
@@ -322,6 +299,34 @@ then
 	exit
 fi
 
+if test -n "$trace" && test -n "$test_untraceable"
+then
+	# '-x' tracing requested, but this test script can't be reliably
+	# traced, unless it is run with a Bash version supporting
+	# BASH_XTRACEFD (introduced in Bash v4.1).
+	#
+	# Perform this version check _after_ the test script was
+	# potentially re-executed with $TEST_SHELL_PATH for '--tee' or
+	# '--verbose-log', so the right shell is checked and the
+	# warning is issued only once.
+	if test -n "$BASH_VERSION" && eval '
+	     test ${BASH_VERSINFO[0]} -gt 4 || {
+	       test ${BASH_VERSINFO[0]} -eq 4 &&
+	       test ${BASH_VERSINFO[1]} -ge 1
+	     }
+	   '
+	then
+		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
+	else
+		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		trace=
+	fi
+fi
+if test -n "$trace" && test -z "$verbose_log"
+then
+	verbose=t
+fi
+
 # For repeatability, reset the environment to known value.
 # TERM is sanitized below, after saving color control sequences.
 LANG=C

diff --git a/t/test-lib.sh b/t/test-lib.sh
index c34831a4de..4c3744cce4 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -476,7 +476,7 @@ die () {
 
 GIT_EXIT_OK=
 trap 'die' EXIT
-trap 'exit $?' INT
+trap 'exit $?' INT TERM HUP
 
 # The user-facing functions are loaded from a separate file so that
 # test_perf subshells can have them too

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 4c3744cce4..1f02e2e25b 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -317,24 +317,7 @@ do
 		GIT_TEST_CHAIN_LINT=0
 		shift ;;
 	-x)
-		# Some test scripts can't be reliably traced  with '-x',
-		# unless the test is run with a Bash version supporting
-		# BASH_XTRACEFD (introduced in Bash v4.1).  Check whether
-		# this test is marked as such, and ignore '-x' if it
-		# isn't executed with a suitable Bash version.
-		if test -z "$test_untraceable" || {
-		     test -n "$BASH_VERSION" && eval '
-		       test ${BASH_VERSINFO[0]} -gt 4 || {
-			 test ${BASH_VERSINFO[0]} -eq 4 &&
-			 test ${BASH_VERSINFO[1]} -ge 1
-		       }
-		     '
-		   }
-		then
-			trace=t
-		else
-			echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
-		fi
+		trace=t
 		shift ;;
 	-V|--verbose-log)
 		verbose_log=t
@@ -353,6 +336,24 @@ then
 	test -z "$verbose_log" && verbose=t
 fi
 
+if test -n "$trace" && test -n "$test_untraceable"
+then
+	# '-x' tracing requested, but this test script can't be reliably
+	# traced, unless it is run with a Bash version supporting
+	# BASH_XTRACEFD (introduced in Bash v4.1).
+	if test -n "$BASH_VERSION" && eval '
+	     test ${BASH_VERSINFO[0]} -gt 4 || {
+	       test ${BASH_VERSINFO[0]} -eq 4 &&
+	       test ${BASH_VERSINFO[1]} -ge 1
+	     }
+	   '
+	then
+		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
+	else
+		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		trace=
+	fi
+fi
 if test -n "$trace" && test -z "$verbose_log"
 then
 	verbose=t

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 1f02e2e25b..3cf59a92f0 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -264,68 +264,74 @@ test "x$TERM" != "xdumb" && (
 	) &&
 	color=t
 
-while test "$#" -ne 0
+store_arg_to=
+prev_opt=
+for opt
 do
-	case "$1" in
+	if test -n "$store_arg_to"
+	then
+		eval $store_arg_to=\$opt
+		store_arg_to=
+		prev_opt=
+		continue
+	fi
+
+	case "$opt" in
 	-d|--d|--de|--deb|--debu|--debug)
-		debug=t; shift ;;
+		debug=t ;;
 	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
-		immediate=t; shift ;;
+		immediate=t ;;
 	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
-		GIT_TEST_LONG=t; export GIT_TEST_LONG; shift ;;
+		GIT_TEST_LONG=t; export GIT_TEST_LONG ;;
 	-r)
-		shift; test "$#" -ne 0 || {
-			echo 'error: -r requires an argument' >&2;
-			exit 1;
-		}
-		run_list=$1; shift ;;
+		store_arg_to=run_list
+		;;
 	--run=*)
-		run_list=${1#--*=}; shift ;;
+		run_list=${opt#--*=} ;;
 	-h|--h|--he|--hel|--help)
-		help=t; shift ;;
+		help=t ;;
 	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
-		verbose=t; shift ;;
+		verbose=t ;;
 	--verbose-only=*)
-		verbose_only=${1#--*=}
-		shift ;;
+		verbose_only=${opt#--*=}
+		;;
 	-q|--q|--qu|--qui|--quie|--quiet)
 		# Ignore --quiet under a TAP::Harness. Saying how many tests
 		# passed without the ok/not ok details is always an error.
-		test -z "$HARNESS_ACTIVE" && quiet=t; shift ;;
+		test -z "$HARNESS_ACTIVE" && quiet=t ;;
 	--with-dashes)
-		with_dashes=t; shift ;;
+		with_dashes=t ;;
 	--no-color)
-		color=; shift ;;
+		color= ;;
 	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
-		valgrind=memcheck
-		shift ;;
+		valgrind=memcheck ;;
 	--valgrind=*)
-		valgrind=${1#--*=}
-		shift ;;
+		valgrind=${opt#--*=} ;;
 	--valgrind-only=*)
-		valgrind_only=${1#--*=}
-		shift ;;
+		valgrind_only=${opt#--*=} ;;
 	--tee)
-		shift ;; # was handled already
+		;; # was handled already
 	--root=*)
-		root=${1#--*=}
-		shift ;;
+		root=${opt#--*=} ;;
 	--chain-lint)
-		GIT_TEST_CHAIN_LINT=1
-		shift ;;
+		GIT_TEST_CHAIN_LINT=1 ;;
 	--no-chain-lint)
-		GIT_TEST_CHAIN_LINT=0
-		shift ;;
+		GIT_TEST_CHAIN_LINT=0 ;;
 	-x)
-		trace=t
-		shift ;;
+		trace=t ;;
 	-V|--verbose-log)
-		verbose_log=t
-		shift ;;
+		verbose_log=t ;;
 	*)
-		echo "error: unknown test option '$1'" >&2; exit 1 ;;
+		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
 	esac
+
+	prev_opt=$opt
 done
+if test -n "$store_arg_to"
+then
+	echo "error: $prev_opt requires an argument" >&2
+	exit 1
+fi
 
 if test -n "$valgrind_only"
 then

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 3cf59a92f0..14ccb60838 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -71,13 +71,102 @@ then
 	exit 1
 fi
 
+# Parse options while taking care to leave $@ intact, so we will still
+# have all the original command line options when executing the test
+# script again for '--tee' and '--verbose-log' below.
+store_arg_to=
+prev_opt=
+for opt
+do
+	if test -n "$store_arg_to"
+	then
+		eval $store_arg_to=\$opt
+		store_arg_to=
+		prev_opt=
+		continue
+	fi
+
+	case "$opt" in
+	-d|--d|--de|--deb|--debu|--debug)
+		debug=t ;;
+	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
+		immediate=t ;;
+	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
+		GIT_TEST_LONG=t; export GIT_TEST_LONG ;;
+	-r)
+		store_arg_to=run_list
+		;;
+	--run=*)
+		run_list=${opt#--*=} ;;
+	-h|--h|--he|--hel|--help)
+		help=t ;;
+	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
+		verbose=t ;;
+	--verbose-only=*)
+		verbose_only=${opt#--*=}
+		;;
+	-q|--q|--qu|--qui|--quie|--quiet)
+		# Ignore --quiet under a TAP::Harness. Saying how many tests
+		# passed without the ok/not ok details is always an error.
+		test -z "$HARNESS_ACTIVE" && quiet=t ;;
+	--with-dashes)
+		with_dashes=t ;;
+	--no-color)
+		color= ;;
+	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
+		valgrind=memcheck
+		tee=t
+		;;
+	--valgrind=*)
+		valgrind=${opt#--*=}
+		tee=t
+		;;
+	--valgrind-only=*)
+		valgrind_only=${opt#--*=}
+		tee=t
+		;;
+	--tee)
+		tee=t ;;
+	--root=*)
+		root=${opt#--*=} ;;
+	--chain-lint)
+		GIT_TEST_CHAIN_LINT=1 ;;
+	--no-chain-lint)
+		GIT_TEST_CHAIN_LINT=0 ;;
+	-x)
+		trace=t ;;
+	-V|--verbose-log)
+		verbose_log=t
+		tee=t
+		;;
+	*)
+		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
+	esac
+
+	prev_opt=$opt
+done
+if test -n "$store_arg_to"
+then
+	echo "error: $prev_opt requires an argument" >&2
+	exit 1
+fi
+
+if test -n "$valgrind_only"
+then
+	test -z "$valgrind" && valgrind=memcheck
+	test -z "$verbose" && verbose_only="$valgrind_only"
+elif test -n "$valgrind"
+then
+	test -z "$verbose_log" && verbose=t
+fi
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
-case "$GIT_TEST_TEE_STARTED, $* " in
-done,*)
-	# do not redirect again
-	;;
-*' --tee '*|*' --va'*|*' -V '*|*' --verbose-log '*)
+if test "$GIT_TEST_TEE_STARTED" = "done"
+then
+	: # do not redirect again
+elif test -n "$tee"
+then
 	mkdir -p "$TEST_OUTPUT_DIRECTORY/test-results"
 	BASE="$TEST_OUTPUT_DIRECTORY/test-results/$(basename "$0" .sh)"
 
@@ -94,8 +183,35 @@ done,*)
 	 echo $? >"$BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
 	test "$(cat "$BASE.exit")" = 0
 	exit
-	;;
-esac
+fi
+
+if test -n "$trace" && test -n "$test_untraceable"
+then
+	# '-x' tracing requested, but this test script can't be reliably
+	# traced, unless it is run with a Bash version supporting
+	# BASH_XTRACEFD (introduced in Bash v4.1).
+	#
+	# Perform this version check _after_ the test script was
+	# potentially re-executed with $TEST_SHELL_PATH for '--tee' or
+	# '--verbose-log', so the right shell is checked and the
+	# warning is issued only once.
+	if test -n "$BASH_VERSION" && eval '
+	     test ${BASH_VERSINFO[0]} -gt 4 || {
+	       test ${BASH_VERSINFO[0]} -eq 4 &&
+	       test ${BASH_VERSINFO[1]} -ge 1
+	     }
+	   '
+	then
+		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
+	else
+		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		trace=
+	fi
+fi
+if test -n "$trace" && test -z "$verbose_log"
+then
+	verbose=t
+fi
 
 # For repeatability, reset the environment to known value.
 # TERM is sanitized below, after saving color control sequences.
@@ -193,7 +309,7 @@ fi
 
 # Add libc MALLOC and MALLOC_PERTURB test
 # only if we are not executing the test with valgrind
-if expr " $GIT_TEST_OPTS " : ".* --valgrind " >/dev/null ||
+if test -n "$valgrind" ||
    test -n "$TEST_NO_MALLOC_CHECK"
 then
 	setup_malloc_check () {
@@ -264,107 +380,6 @@ test "x$TERM" != "xdumb" && (
 	) &&
 	color=t
 
-store_arg_to=
-prev_opt=
-for opt
-do
-	if test -n "$store_arg_to"
-	then
-		eval $store_arg_to=\$opt
-		store_arg_to=
-		prev_opt=
-		continue
-	fi
-
-	case "$opt" in
-	-d|--d|--de|--deb|--debu|--debug)
-		debug=t ;;
-	-i|--i|--im|--imm|--imme|--immed|--immedi|--immedia|--immediat|--immediate)
-		immediate=t ;;
-	-l|--l|--lo|--lon|--long|--long-|--long-t|--long-te|--long-tes|--long-test|--long-tests)
-		GIT_TEST_LONG=t; export GIT_TEST_LONG ;;
-	-r)
-		store_arg_to=run_list
-		;;
-	--run=*)
-		run_list=${opt#--*=} ;;
-	-h|--h|--he|--hel|--help)
-		help=t ;;
-	-v|--v|--ve|--ver|--verb|--verbo|--verbos|--verbose)
-		verbose=t ;;
-	--verbose-only=*)
-		verbose_only=${opt#--*=}
-		;;
-	-q|--q|--qu|--qui|--quie|--quiet)
-		# Ignore --quiet under a TAP::Harness. Saying how many tests
-		# passed without the ok/not ok details is always an error.
-		test -z "$HARNESS_ACTIVE" && quiet=t ;;
-	--with-dashes)
-		with_dashes=t ;;
-	--no-color)
-		color= ;;
-	--va|--val|--valg|--valgr|--valgri|--valgrin|--valgrind)
-		valgrind=memcheck ;;
-	--valgrind=*)
-		valgrind=${opt#--*=} ;;
-	--valgrind-only=*)
-		valgrind_only=${opt#--*=} ;;
-	--tee)
-		;; # was handled already
-	--root=*)
-		root=${opt#--*=} ;;
-	--chain-lint)
-		GIT_TEST_CHAIN_LINT=1 ;;
-	--no-chain-lint)
-		GIT_TEST_CHAIN_LINT=0 ;;
-	-x)
-		trace=t ;;
-	-V|--verbose-log)
-		verbose_log=t ;;
-	*)
-		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
-	esac
-
-	prev_opt=$opt
-done
-if test -n "$store_arg_to"
-then
-	echo "error: $prev_opt requires an argument" >&2
-	exit 1
-fi
-
-if test -n "$valgrind_only"
-then
-	test -z "$valgrind" && valgrind=memcheck
-	test -z "$verbose" && verbose_only="$valgrind_only"
-elif test -n "$valgrind"
-then
-	test -z "$verbose_log" && verbose=t
-fi
-
-if test -n "$trace" && test -n "$test_untraceable"
-then
-	# '-x' tracing requested, but this test script can't be reliably
-	# traced, unless it is run with a Bash version supporting
-	# BASH_XTRACEFD (introduced in Bash v4.1).
-	if test -n "$BASH_VERSION" && eval '
-	     test ${BASH_VERSINFO[0]} -gt 4 || {
-	       test ${BASH_VERSINFO[0]} -eq 4 &&
-	       test ${BASH_VERSINFO[1]} -ge 1
-	     }
-	   '
-	then
-		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
-	else
-		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
-		trace=
-	fi
-fi
-if test -n "$trace" && test -z "$verbose_log"
-then
-	verbose=t
-fi
-
 if test -n "$color"
 then
 	# Save the color control sequences now rather than run tput

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 14ccb60838..5720292641 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -160,6 +160,10 @@ then
 	test -z "$verbose_log" && verbose=t
 fi
 
+TEST_NAME="$(basename "$0" .sh)"
+TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
+TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
 if test "$GIT_TEST_TEE_STARTED" = "done"
@@ -167,12 +171,11 @@ then
 	: # do not redirect again
 elif test -n "$tee"
 then
-	mkdir -p "$TEST_OUTPUT_DIRECTORY/test-results"
-	BASE="$TEST_OUTPUT_DIRECTORY/test-results/$(basename "$0" .sh)"
+	mkdir -p "$TEST_RESULTS_DIR"
 
 	# Make this filename available to the sub-process in case it is using
 	# --verbose-log.
-	GIT_TEST_TEE_OUTPUT_FILE=$BASE.out
+	GIT_TEST_TEE_OUTPUT_FILE=$TEST_RESULTS_BASE.out
 	export GIT_TEST_TEE_OUTPUT_FILE
 
 	# Truncate before calling "tee -a" to get rid of the results
@@ -180,8 +183,8 @@ then
 	>"$GIT_TEST_TEE_OUTPUT_FILE"
 
 	(GIT_TEST_TEE_STARTED=done ${TEST_SHELL_PATH} "$0" "$@" 2>&1;
-	 echo $? >"$BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
-	test "$(cat "$BASE.exit")" = 0
+	 echo $? >"$TEST_RESULTS_BASE.exit") | tee -a "$GIT_TEST_TEE_OUTPUT_FILE"
+	test "$(cat "$TEST_RESULTS_BASE.exit")" = 0
 	exit
 fi
 
@@ -840,12 +843,9 @@ test_done () {
 
 	if test -z "$HARNESS_ACTIVE"
 	then
-		test_results_dir="$TEST_OUTPUT_DIRECTORY/test-results"
-		mkdir -p "$test_results_dir"
-		base=${0##*/}
-		test_results_path="$test_results_dir/${base%.sh}.counts"
+		mkdir -p "$TEST_RESULTS_DIR"
 
-		cat >"$test_results_path" <<-EOF
+		cat >"$TEST_RESULTS_BASE.counts" <<-EOF
 		total $test_count
 		success $test_success
 		fixed $test_fixed
@@ -1051,7 +1051,7 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$(basename "$0" .sh)"
+TRASH_DIRECTORY="trash directory.$TEST_NAME"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 5720292641..0e9ac9118d 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -163,6 +163,12 @@ fi
 TEST_NAME="$(basename "$0" .sh)"
 TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
 TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
+TRASH_DIRECTORY="trash directory.$TEST_NAME"
+test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
+case "$TRASH_DIRECTORY" in
+/*) ;; # absolute path is good
+ *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
+esac
 
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
@@ -1051,12 +1057,6 @@ then
 fi
 
 # Test repository
-TRASH_DIRECTORY="trash directory.$TEST_NAME"
-test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
-case "$TRASH_DIRECTORY" in
-/*) ;; # absolute path is good
- *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
-esac
 rm -fr "$TRASH_DIRECTORY" || {
 	GIT_EXIT_OK=t
 	echo >&5 "FATAL: Cannot prepare test area"

t/README, t/test-lib-functions.sh, t/test-lib.sh

diff --git a/t/README b/t/README
index 28711cc508..11ce7675e3 100644
--- a/t/README
+++ b/t/README
@@ -186,6 +186,22 @@ appropriately before running "make".
 	this feature by setting the GIT_TEST_CHAIN_LINT environment
 	variable to "1" or "0", respectively.
 
+--stress::
+--stress=<N>::
+	Run the test script repeatedly in multiple parallel jobs until
+	one of them fails.  Useful for reproducing rare failures in
+	flaky tests.  The number of parallel jobs is, in order of
+	precedence: <N>, or the value of the GIT_TEST_STRESS_LOAD
+	environment variable, or twice the number of available
+	processors (as shown by the 'getconf' utility),	or 8.
+	Implies `--verbose -x --immediate` to get the most information
+	about the failure.  Note that the verbose output of each test
+	job is saved to 't/test-results/$TEST_NAME.stress-<nr>.out',
+	and only the output of the failed test job is shown on the
+	terminal.  The names of the trash directories get a
+	'.stress-<nr>' suffix, and the trash directory of the failed
+	test job is renamed to end with a '.stress-failed' suffix.
+
 You can also set the GIT_TEST_INSTALLED environment variable to
 the bindir of an existing git installation to test that installation.
 You still need to have built this git sandbox, from which various
@@ -425,7 +441,8 @@ This test harness library does the following things:
  - Creates an empty test directory with an empty .git/objects database
    and chdir(2) into it.  This directory is 't/trash
    directory.$test_name_without_dotsh', with t/ subject to change by
-   the --root option documented above.
+   the --root option documented above, and a '.stress-<N>' suffix
+   appended by the --stress option.
 
  - Defines standard test helper functions for your scripts to
    use.  These functions are designed to make all scripts behave
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 4459bdda13..92cf8f812c 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1288,8 +1288,6 @@ test_set_port () {
 			# root-only port, use a larger one instead.
 			port=$(($port + 10000))
 		fi
-
-		eval $var=$port
 		;;
 	*[^0-9]*|0*)
 		error >&7 "invalid port number: $port"
@@ -1298,4 +1296,9 @@ test_set_port () {
 		# The user has specified the port.
 		;;
 	esac
+
+	# Make sure that parallel '--stress' test jobs get different
+	# ports.
+	port=$(($port + ${GIT_TEST_STRESS_JOB_NR:-0}))
+	eval $var=$port
 }
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 0e9ac9118d..a1abb1177a 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -139,6 +139,19 @@ do
 		verbose_log=t
 		tee=t
 		;;
+	--stress)
+		stress=t ;;
+	--stress=*)
+		stress=${opt#--*=}
+		case "$stress" in
+		*[^0-9]*|0*|"")
+			echo "error: --stress=<N> requires the number of jobs to run" >&2
+			exit 1
+			;;
+		*)	# Good.
+			;;
+		esac
+		;;
 	*)
 		echo "error: unknown test option '$opt'" >&2; exit 1 ;;
 	esac
@@ -160,16 +173,108 @@ then
 	test -z "$verbose_log" && verbose=t
 fi
 
+if test -n "$stress"
+then
+	verbose=t
+	trace=t
+	immediate=t
+fi
+
+TEST_STRESS_JOB_SFX="${GIT_TEST_STRESS_JOB_NR:+.stress-$GIT_TEST_STRESS_JOB_NR}"
 TEST_NAME="$(basename "$0" .sh)"
 TEST_RESULTS_DIR="$TEST_OUTPUT_DIRECTORY/test-results"
-TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME"
-TRASH_DIRECTORY="trash directory.$TEST_NAME"
+TEST_RESULTS_BASE="$TEST_RESULTS_DIR/$TEST_NAME$TEST_STRESS_JOB_SFX"
+TRASH_DIRECTORY="trash directory.$TEST_NAME$TEST_STRESS_JOB_SFX"
 test -n "$root" && TRASH_DIRECTORY="$root/$TRASH_DIRECTORY"
 case "$TRASH_DIRECTORY" in
 /*) ;; # absolute path is good
  *) TRASH_DIRECTORY="$TEST_OUTPUT_DIRECTORY/$TRASH_DIRECTORY" ;;
 esac
 
+# If --stress was passed, run this test repeatedly in several parallel loops.
+if test "$GIT_TEST_STRESS_STARTED" = "done"
+then
+	: # Don't stress test again.
+elif test -n "$stress"
+then
+	if test "$stress" != t
+	then
+		job_count=$stress
+	elif test -n "$GIT_TEST_STRESS_LOAD"
+	then
+		job_count="$GIT_TEST_STRESS_LOAD"
+	elif job_count=$(getconf _NPROCESSORS_ONLN 2>/dev/null) &&
+	     test -n "$job_count"
+	then
+		job_count=$((2 * $job_count))
+	else
+		job_count=8
+	fi
+
+	mkdir -p "$TEST_RESULTS_DIR"
+	stressfail="$TEST_RESULTS_BASE.stress-failed"
+	rm -f "$stressfail"
+
+	stress_exit=0
+	trap '
+		kill $job_pids 2>/dev/null
+		wait
+		stress_exit=1
+	' TERM INT HUP
+
+	job_pids=
+	job_nr=0
+	while test $job_nr -lt "$job_count"
+	do
+		(
+			GIT_TEST_STRESS_STARTED=done
+			GIT_TEST_STRESS_JOB_NR=$job_nr
+			export GIT_TEST_STRESS_STARTED GIT_TEST_STRESS_JOB_NR
+
+			trap '
+				kill $test_pid 2>/dev/null
+				wait
+				exit 1
+			' TERM INT
+
+			cnt=0
+			while ! test -e "$stressfail"
+			do
+				$TEST_SHELL_PATH "$0" "$@" >"$TEST_RESULTS_BASE.stress-$job_nr.out" 2>&1 &
+				test_pid=$!
+
+				if wait $test_pid
+				then
+					printf "OK   %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				else
+					echo $GIT_TEST_STRESS_JOB_NR >>"$stressfail"
+					printf "FAIL %2d.%d\n" $GIT_TEST_STRESS_JOB_NR $cnt
+				fi
+				cnt=$(($cnt + 1))
+			done
+		) &
+		job_pids="$job_pids $!"
+		job_nr=$(($job_nr + 1))
+	done
+
+	wait
+
+	if test -f "$stressfail"
+	then
+		echo "Log(s) of failed test run(s):"
+		for failed_job_nr in $(sort -n "$stressfail")
+		do
+			echo "Contents of '$TEST_RESULTS_BASE.stress-$failed_job_nr.out':"
+			cat "$TEST_RESULTS_BASE.stress-$failed_job_nr.out"
+		done
+		rm -rf "$TRASH_DIRECTORY.stress-failed"
+		# Move the last one.
+		mv "$TRASH_DIRECTORY.stress-$failed_job_nr" "$TRASH_DIRECTORY.stress-failed"
+	fi
+
+	exit $stress_exit
+fi
+
 # if --tee was passed, write the output not only to the terminal, but
 # additionally to the file test-results/$BASENAME.out, too.
 if test "$GIT_TEST_TEE_STARTED" = "done"

t/lib-git-daemon.sh, t/lib-git-p4.sh, t/lib-git-svn.sh, t/lib-httpd.sh, t/t0410-partial-clone.sh, t/t5512-ls-remote.sh, t/test-lib-functions.sh

diff --git a/t/lib-git-daemon.sh b/t/lib-git-daemon.sh
index f98de95c15..41eb1e3ae8 100644
--- a/t/lib-git-daemon.sh
+++ b/t/lib-git-daemon.sh
@@ -28,7 +28,7 @@ then
 	test_skip_or_die $GIT_TEST_GIT_DAEMON "file system does not support FIFOs"
 fi
 
-LIB_GIT_DAEMON_PORT=${LIB_GIT_DAEMON_PORT-${this_test#t}}
+test_set_port LIB_GIT_DAEMON_PORT
 
 GIT_DAEMON_PID=
 GIT_DAEMON_DOCUMENT_ROOT_PATH="$PWD"/repo
diff --git a/t/lib-git-p4.sh b/t/lib-git-p4.sh
index c27599474c..b3be3ba011 100644
--- a/t/lib-git-p4.sh
+++ b/t/lib-git-p4.sh
@@ -53,14 +53,7 @@ time_in_seconds () {
 	(cd / && "$PYTHON_PATH" -c 'import time; print(int(time.time()))')
 }
 
-# Try to pick a unique port: guess a large number, then hope
-# no more than one of each test is running.
-#
-# This does not handle the case where somebody else is running the
-# same tests and has chosen the same ports.
-testid=${this_test#t}
-git_p4_test_start=9800
-P4DPORT=$((10669 + ($testid - $git_p4_test_start)))
+test_set_port P4DPORT
 
 P4PORT=localhost:$P4DPORT
 P4CLIENT=client
diff --git a/t/lib-git-svn.sh b/t/lib-git-svn.sh
index a8130f9119..f3b478c307 100644
--- a/t/lib-git-svn.sh
+++ b/t/lib-git-svn.sh
@@ -13,6 +13,7 @@ fi
 GIT_DIR=$PWD/.git
 GIT_SVN_DIR=$GIT_DIR/svn/refs/remotes/git-svn
 SVN_TREE=$GIT_SVN_DIR/svn-tree
+test_set_port SVNSERVE_PORT
 
 svn >/dev/null 2>&1
 if test $? -ne 1
@@ -119,7 +120,6 @@ require_svnserve () {
 }
 
 start_svnserve () {
-	SVNSERVE_PORT=${SVNSERVE_PORT-${this_test#t}}
 	svnserve --listen-port $SVNSERVE_PORT \
 		 --root "$rawsvnrepo" \
 		 --listen-once \
diff --git a/t/lib-httpd.sh b/t/lib-httpd.sh
index a8729f8232..e465116ef9 100644
--- a/t/lib-httpd.sh
+++ b/t/lib-httpd.sh
@@ -82,7 +82,7 @@ case $(uname) in
 esac
 
 LIB_HTTPD_PATH=${LIB_HTTPD_PATH-"$DEFAULT_HTTPD_PATH"}
-LIB_HTTPD_PORT=${LIB_HTTPD_PORT-${this_test#t}}
+test_set_port LIB_HTTPD_PORT
 
 TEST_PATH="$TEST_DIRECTORY"/lib-httpd
 HTTPD_ROOT_PATH="$PWD"/httpd
diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
index ba3887f178..0aca8d7588 100755
--- a/t/t0410-partial-clone.sh
+++ b/t/t0410-partial-clone.sh
@@ -480,7 +480,6 @@ test_expect_success 'gc stops traversal when a missing but promised object is re
 	! grep "$TREE_HASH" out
 '
 
-LIB_HTTPD_PORT=12345  # default port, 410, cannot be used as non-root
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
diff --git a/t/t5512-ls-remote.sh b/t/t5512-ls-remote.sh
index 32e722db2e..cd9e60632d 100755
--- a/t/t5512-ls-remote.sh
+++ b/t/t5512-ls-remote.sh
@@ -260,7 +260,7 @@ test_lazy_prereq GIT_DAEMON '
 # This test spawns a daemon, so run it only if the user would be OK with
 # testing with git-daemon.
 test_expect_success PIPE,JGIT,GIT_DAEMON 'indicate no refs in standards-compliant empty remote' '
-	JGIT_DAEMON_PORT=${JGIT_DAEMON_PORT-${this_test#t}} &&
+	test_set_port JGIT_DAEMON_PORT &&
 	JGIT_DAEMON_PID= &&
 	git init --bare empty.git &&
 	>empty.git/git-daemon-export-ok &&
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 6b3bbf99e4..4459bdda13 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1263,3 +1263,39 @@ test_oid () {
 	fi &&
 	eval "printf '%s' \"\${$var}\""
 }
+
+# Choose a port number based on the test script's number and store it in
+# the given variable name, unless that variable already contains a number.
+test_set_port () {
+	local var=$1 port
+
+	if test $# -ne 1 || test -z "$var"
+	then
+		BUG "test_set_port requires a variable name"
+	fi
+
+	eval port=\$$var
+	case "$port" in
+	"")
+		# No port is set in the given env var, use the test
+		# number as port number instead.
+		# Remove not only the leading 't', but all leading zeros
+		# as well, so the arithmetic below won't (mis)interpret
+		# a test number like '0123' as an octal value.
+		port=${this_test#${this_test%%[1-9]*}}
+		if test "${port:-0}" -lt 1024
+		then
+			# root-only port, use a larger one instead.
+			port=$(($port + 10000))
+		fi
+
+		eval $var=$port
+		;;
+	*[^0-9]*|0*)
+		error >&7 "invalid port number: $port"
+		;;
+	*)
+		# The user has specified the port.
+		;;
+	esac
+}