patch, 3 partsquarantine-push loose ends

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index aca9c33d8..b4469b3cc 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -772,7 +772,6 @@ static int run_update_hook(struct command *cmd)
 	proc.stdout_to_stderr = 1;
 	proc.err = use_sideband ? -1 : 0;
 	proc.argv = argv;
-	proc.env = tmp_objdir_env(tmp_objdir);
 
 	code = start_command(&proc);
 	if (code)

Documentation/git-receive-pack.txt, Documentation/githooks.txt

diff --git a/Documentation/git-receive-pack.txt b/Documentation/git-receive-pack.txt
index 0ccd5fbc7..7267ecfbe 100644
--- a/Documentation/git-receive-pack.txt
+++ b/Documentation/git-receive-pack.txt
@@ -114,6 +114,8 @@ will be performed, and the update, post-receive and post-update
 hooks will not be invoked either.  This can be useful to quickly
 bail out if the update is not to be supported.
 
+See the notes on the quarantine environment below.
+
 update Hook
 -----------
 Before each ref is updated, if $GIT_DIR/hooks/update file exists
@@ -214,6 +216,32 @@ if the repository is packed and is served via a dumb transport.
 	exec git update-server-info
 
 
+Quarantine Environment
+----------------------
+
+When `receive-pack` takes in objects, they are placed into a temporary
+"quarantine" directory within the `$GIT_DIR/objects` directory and
+migrated into the main object store only after the `pre-receive` hook
+has completed. If the push fails before then, the temporary directory is
+removed entirely.
+
+This has a few user-visible effects and caveats:
+
+  1. Pushes which fail due to problems with the incoming pack, missing
+     objects, or due to the `pre-receive` hook will not leave any
+     on-disk data. This is usually helpful to prevent repeated failed
+     pushes from filling up your disk, but can make debugging more
+     challenging.
+
+  2. Any objects created by the `pre-receive` hook will be created in
+     the quarantine directory (and migrated only if it succeeds).
+
+  3. The `pre-receive` hook MUST NOT update any refs to point to
+     quarantined objects. Other programs accessing the repository will
+     not be able to see the objects (and if the pre-receive hook fails,
+     those refs would become corrupted).
+
+
 SEE ALSO
 --------
 linkgit:git-send-pack[1], linkgit:gitnamespaces[7]
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 9565dc3fd..32343ae29 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -256,6 +256,9 @@ environment variables will not be set. If the client selects
 to use push options, but doesn't transmit any, the count variable
 will be set to zero, `GIT_PUSH_OPTION_COUNT=0`.
 
+See the section on "Quarantine Environment" in
+linkgit:git-receive-pack[1] for some caveats.
+
 [[update]]
 update
 ~~~~~~

Documentation/git-receive-pack.txt, refs.c, t/t5547-push-quarantine.sh

diff --git a/Documentation/git-receive-pack.txt b/Documentation/git-receive-pack.txt
index 7267ecfbe..86a4b32f0 100644
--- a/Documentation/git-receive-pack.txt
+++ b/Documentation/git-receive-pack.txt
@@ -239,7 +239,8 @@ This has a few user-visible effects and caveats:
   3. The `pre-receive` hook MUST NOT update any refs to point to
      quarantined objects. Other programs accessing the repository will
      not be able to see the objects (and if the pre-receive hook fails,
-     those refs would become corrupted).
+     those refs would become corrupted). For safety, any ref updates
+     from within `pre-receive` are automatically rejected.
 
 
 SEE ALSO
diff --git a/refs.c b/refs.c
index 0272e332c..62b405d0b 100644
--- a/refs.c
+++ b/refs.c
@@ -1516,6 +1516,12 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 {
 	struct ref_store *refs = get_ref_store(NULL);
 
+	if (getenv(GIT_QUARANTINE_ENVIRONMENT)) {
+		strbuf_addstr(err,
+			      _("ref updates forbidden inside quarantine environment"));
+		return -1;
+	}
+
 	return refs->be->transaction_commit(refs, transaction, err);
 }
 
diff --git a/t/t5547-push-quarantine.sh b/t/t5547-push-quarantine.sh
index af9fcd833..113c87007 100755
--- a/t/t5547-push-quarantine.sh
+++ b/t/t5547-push-quarantine.sh
@@ -58,4 +58,15 @@ test_expect_success 'push to repo path with path separator (colon)' '
 	git push "$(pwd)/xxx${pathsep}yyy.git" HEAD
 '
 
+test_expect_success 'updating a ref from quarantine is forbidden' '
+	git init --bare update.git &&
+	write_script update.git/hooks/pre-receive <<-\EOF &&
+	read old new refname
+	git update-ref refs/heads/unrelated $new
+	exit 1
+	EOF
+	test_must_fail git push update.git HEAD &&
+	git -C update.git fsck
+'
+
 test_done