[PATCH 02/10] t7422: fix flaky test caused by buffered stdout

[Patrick Steinhardt <ps@xxxxxx>]

One test in t7422 asserts that `git submodule status --recursive`
properly handles SIGPIPE. This test is flaky though and may sometimes
not see a SIGPIPE at all:

    expecting success of 7422.18 'git submodule status --recursive propagates SIGPIPE':
            { git submodule status --recursive 2>err; echo $?>status; } |
                    grep -q X/S &&
            test_must_be_empty err &&
            test_match_signal 13 "$(cat status)"
    ++ git submodule status --recursive
    ++ grep -q X/S
    ++ echo 0
    ++ test_must_be_empty err
    ++ test 1 -ne 1
    ++ test_path_is_file err
    ++ test 1 -ne 1
    ++ test -f err
    ++ test -s err
    +++ cat status
    ++ test_match_signal 13 0
    ++ test 0 = 141
    ++ test 0 = 269
    ++ return 1
    error: last command exited with $?=1
    not ok 18 - git submodule status --recursive propagates SIGPIPE

The issue is caused by us using grep(1) to terminate the pipe on the
first matching line in the recursing git-submodule(1) process. Standard
streams are typically buffered though, so this condition is racy and may
cause us to terminate the pipe after git-submodule(1) has already
exited, and in that case we wouldn't see the expected signal.

Fix the issue by converting standard streams to be unbuffered. I have
only been able to reproduce this issue a single time after running t7422
with `--stress` after an extended amount of time, so I cannot claim to
be fully certain that this fix is sufficient.

Signed-off-by: Patrick Steinhardt <ps@xxxxxx>
---
 t/t7422-submodule-output.sh | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/t/t7422-submodule-output.sh b/t/t7422-submodule-output.sh
index f21e9203678b94701281d5339ae8bfe53d5de0ed..ba843c02c9c2da198578aec5716813de32960b86 100755
--- a/t/t7422-submodule-output.sh
+++ b/t/t7422-submodule-output.sh
@@ -166,9 +166,13 @@ do
 	'
 done
 
-test_expect_success !MINGW 'git submodule status --recursive propagates SIGPIPE' '
-	{ git submodule status --recursive 2>err; echo $?>status; } |
-		grep -q X/S &&
+test_lazy_prereq STDBUF '
+	stdbuf --version
+'
+
+test_expect_success !MINGW,STDBUF 'git submodule status --recursive propagates SIGPIPE' '
+	{ stdbuf -oL git submodule status --recursive 2>err; echo $?>status; } |
+		stdbuf -i0 grep -q X/S &&
 	test_must_be_empty err &&
 	test_match_signal 13 "$(cat status)"
 '

-- 
2.48.0.rc1.241.g6c04ab211c.dirty