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Re: [PATCH 0/4] t5552: fix flakiness by introducing proper locking for GIT_TRACE

From
Jeff King <peff@peff.net>
Date
Aug 10, 2018, 14:09 UTC
Message-ID
<20180810140908.GA23507@sigill.intra.peff.net>
In-Reply-To
<xmqqo9ebb6z3.fsf@gitster-ct.c.googlers.com>
On Thu, Aug 09, 2018 at 01:49:52PM -0700, Junio C Hamano wrote:
Show 23 quoted lines
> Jeff King <peff@peff.net> writes:
> 
> > Are you sure that it's not well-defined? We open the path with O_APPEND,
> > which means every write() will be atomically positioned at the end of
> > file. So we would never lose or overwrite data.
> >
> > We do our own buffering in a strbuf, writing the result out in a single
> > write() call (modulo the OS returning a short write, but that should not
> > generally happen when writing short strings to a file). So we should get
> > individual trace lines as atomic units.
> >
> > The order of lines from the two processes is undefined, of course.
> 
> Correct.  But I am more worried about the "mixed/overwriting"
> breakage, if there is one; it means we may need to be prepared for
> systems that lack O_APPEND that works correctly.  I initially just
> assumed that it was what Dscho was seeing, but after re-reading his
> message, I am not sure anymore.
> 
> I think the "do not trace the other side" approach you suggest for
> these tests that only care about one side is more appropriate
> solution for this particular case.  We then do not have to worry
> about overwriting or output from both sides mixed randomly.

Here it is as a patch on top of jt/fetch-negotiator-skipping, which lets us pursue any fix for interleaved trace output on Windows without the pressure of an impending flaky test.

My gut says that looking into making O_APPEND work there is going to be the nicest solution, but my gut is not very well versed in Windows subtleties. ;)

-- >8 --
Subject: [PATCH] t5552: suppress upload-pack trace output

The t5552 test script uses GIT_TRACE_PACKET to monitor what git-fetch sends and receives. However, because we're accessing a local repository, the child upload-pack also sends trace output to the same file.

On Linux, this works out OK. We open the trace file with O_APPEND, so all writes are atomically positioned at the end of the file. No data can be overwritten or omitted. And since we prepare our small writes in a strbuf and write them with a single write(), we should see each line as an atomic unit. The order of lines between the two processes is undefined, but the test script greps only for "fetch>" or "fetch<" lines. So under Linux, the test results are deterministic.

The test fails intermittently on Windows, however, reportedly even overwriting bits of the output file (i.e., O_APPEND does not seem to give us an atomic position+write).

Since the test only cares about the trace output from fetch, we can just disable the output from upload-pack. That doesn't solve the greater question of O_APPEND/trace issues under Windows, but it easily fixes the flakiness from this test.

Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jeff King <peff@peff.net>
---
I'm assuming that this really isn't triggerable on Linux. I tried and
couldn't manage to get it to fail, and the reasoning above explains why.
But I wasn't 100% clear that Dscho hadn't seen it fail on non-Windows.
 t/t5552-skipping-fetch-negotiator.sh | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)
diff --git a/t/t5552-skipping-fetch-negotiator.sh b/t/t5552-skipping-fetch-negotiator.sh
index 0a8e0e42ed..0ad50dd839 100755
--- a/t/t5552-skipping-fetch-negotiator.sh
+++ b/t/t5552-skipping-fetch-negotiator.sh
@@ -28,6 +28,19 @@ have_not_sent () {
 	done
 }
 
+# trace_fetch <client_dir> <server_dir> [args]
+#
+# Trace the packet output of fetch, but make sure we disable the variable
+# in the child upload-pack, so we don't combine the results in the same file.
+trace_fetch () {
+	client=$1; shift
+	server=$1; shift
+	GIT_TRACE_PACKET="$(pwd)/trace" \
+	git -C "$client" fetch \
+	  --upload-pack 'unset GIT_TRACE_PACKET; git-upload-pack' \
+	  "$server" "$@"
+}
+
 test_expect_success 'commits with no parents are sent regardless of skip distance' '
 	git init server &&
 	test_commit -C server to_fetch &&
@@ -42,7 +55,7 @@ test_expect_success 'commits with no parents are sent regardless of skip distanc
 	# "c1" has no parent, it is still sent as "have" even though it would
 	# normally be skipped.
 	test_config -C client fetch.negotiationalgorithm skipping &&
-	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch "$(pwd)/server" &&
+	trace_fetch client "$(pwd)/server" &&
 	have_sent c7 c5 c2 c1 &&
 	have_not_sent c6 c4 c3
 '
@@ -65,7 +78,7 @@ test_expect_success 'when two skips collide, favor the larger one' '
 	# the next "have" sent will be "c1" (from "c6" skip 4) and not "c4"
 	# (from "c5side" skip 1).
 	test_config -C client fetch.negotiationalgorithm skipping &&
-	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch "$(pwd)/server" &&
+	trace_fetch client "$(pwd)/server" &&
 	have_sent c5side c11 c9 c6 c1 &&
 	have_not_sent c10 c8 c7 c5 c4 c3 c2
 '
@@ -91,7 +104,7 @@ test_expect_success 'use ref advertisement to filter out commits' '
 	# not need to send any ancestors of "c3", but we still need to send "c3"
 	# itself.
 	test_config -C client fetch.negotiationalgorithm skipping &&
-	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch origin to_fetch &&
+	trace_fetch client origin to_fetch &&
 	have_sent c5 c4^ c2side &&
 	have_not_sent c4 c4^^ c4^^^
 '
@@ -121,7 +134,7 @@ test_expect_success 'handle clock skew' '
 	# and sent, because (due to clock skew) its only parent has already been
 	# popped off the priority queue.
 	test_config -C client fetch.negotiationalgorithm skipping &&
-	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch "$(pwd)/server" &&
+	trace_fetch client "$(pwd)/server" &&
 	have_sent c2 c1 old4 old2 old1 &&
 	have_not_sent old3
 '
@@ -153,7 +166,7 @@ test_expect_success 'do not send "have" with ancestors of commits that server AC
 	test_commit -C server commit-on-b1 &&
 
 	test_config -C client fetch.negotiationalgorithm skipping &&
-	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch "$(pwd)/server" to_fetch &&
+	trace_fetch client "$(pwd)/server" to_fetch &&
 	grep "  fetch" trace &&
 
 	# fetch-pack sends 2 requests each containing 16 "have" lines before

-- 
2.18.0.1058.g0926f0b71f
Previous: Jeff KingNext: Junio C Hamano
Message 12 of 43 in “t5552: fix flakiness by introducing proper locking for GIT_TRACE”
  1. 0/4 t5552: fix flakiness by introducing proper locking for GIT_TRACEJohannes Schindelin via GitGitGadget, Aug 9, 2018
  2. 1/4 Introduce a function to lock/unlock file descriptors when appendingJohannes Schindelin via GitGitGadget, Aug 9, 2018
  3. Junio C HamanoAug 9, 2018
  4. Johannes SchindelinAug 10, 2018
  5. 2/4 mingw: implement lock_or_unlock_fd_for_appending()Johannes Schindelin via GitGitGadget, Aug 9, 2018
  6. 3/4 trace: lock the trace file to avoid racy trace_write() callsJohannes Schindelin via GitGitGadget, Aug 9, 2018
  7. 4/4 trace: verify that locking worksJohannes Schindelin via GitGitGadget, Aug 9, 2018
  8. Jeff KingAug 9, 2018
  9. Junio C HamanoAug 9, 2018
  10. Junio C HamanoAug 9, 2018
  11. Jeff KingAug 9, 2018
  12. Jeff KingAug 10, 2018
  13. Junio C HamanoAug 10, 2018
  14. Junio C HamanoAug 10, 2018
  15. Johannes SchindelinAug 10, 2018
  16. Jeff KingAug 10, 2018
  17. Junio C HamanoAug 10, 2018
  18. Johannes SchindelinAug 10, 2018
  19. Johannes SixtAug 10, 2018
  20. Jeff HostetlerAug 10, 2018
  21. Jeff HostetlerAug 10, 2018
  22. Johannes SixtAug 10, 2018
  23. Junio C HamanoAug 10, 2018
  24. Jeff KingAug 10, 2018
  25. mingw: enable atomic O_APPENDJohannes Sixt, Aug 13, 2018
  26. Junio C HamanoAug 13, 2018
  27. Johannes SixtAug 13, 2018
  28. Ævar Arnfjörð BjarmasonAug 13, 2018
  29. Junio C HamanoAug 13, 2018
  30. Jeff KingAug 13, 2018
  31. Ævar Arnfjörð BjarmasonAug 14, 2018
  32. Jeff KingAug 14, 2018
  33. Johannes SixtAug 14, 2018
  34. Jeff KingAug 14, 2018
  35. Jeff HostetlerAug 14, 2018
  36. Junio C HamanoAug 14, 2018
  37. 0/4 t5552: fix flakiness by introducing proper locking for GIT_TRACEJohannes Schindelin via GitGitGadget, Aug 10, 2018
  38. 1/4 Introduce a function to lock/unlock file descriptors when appendingJohannes Schindelin via GitGitGadget, Aug 10, 2018
  39. Junio C HamanoAug 10, 2018
  40. Johannes SchindelinAug 10, 2018
  41. 2/4 mingw: implement lock_or_unlock_fd_for_appending()Johannes Schindelin via GitGitGadget, Aug 10, 2018
  42. 3/4 trace: lock the trace file to avoid racy trace_write() callsJohannes Schindelin via GitGitGadget, Aug 10, 2018
  43. 4/4 trace: verify that locking worksJohannes Schindelin via GitGitGadget, Aug 10, 2018

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