[PATCH v3 02/10] t7422: fix flaky test caused by buffered stdout

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 generating a couple thousand nested submodules and matching on the first nested submodule. This ensures that the recursive git-submodule(1) process completely fills its stdout buffer, which makes subsequent writes block until the downstream consumer of the pipe either fully drains it or closes it.

To verify that this works as expected one can apply the following patch to the preimage of this commit, which used to reliably trigger the race:

    diff --git a/t/t7422-submodule-output.sh b/t/t7422-submodule-output.sh
    index 3c5177cc30..df6001f8a0 100755
    --- a/t/t7422-submodule-output.sh
    +++ b/t/t7422-submodule-output.sh
    @@ -202,7 +202,7 @@ test_expect_success !MINGW 'git submodule status --recursive propagates SIGPIPE'
     		cd repo &&
     		GIT_ALLOW_PROTOCOL=file git submodule add "$(pwd)"/../submodule &&
     		{ git submodule status --recursive 2>err; echo $?>status; } |
    -			grep -q recursive-submodule-path-1 &&
    +			{ sleep 1 && grep -q recursive-submodule-path-1 && sleep 1; } &&
     		test_must_be_empty err &&
     		test_match_signal 13 "$(cat status)"
     	)

With the pipe-stuffing workaround the test runs successfully.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 t/t7422-submodule-output.sh | 43 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 39 insertions(+), 4 deletions(-)