[PATCH v2 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 making the writer fill the pipe buffer before we execute git-submodule(1). Ideally, it would be git-submodule(1) itself that does produce all that data, but it would require us to create a large amount of submodules, which is inefficient. Instead, we use Perl to print gibberish until the buffer is filled.

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 f21e920367..9338c75626 100755
    --- a/t/t7422-submodule-output.sh
    +++ b/t/t7422-submodule-output.sh
    @@ -168,7 +168,7 @@ done

     test_expect_success !MINGW 'git submodule status --recursive propagates SIGPIPE' '
            { git submodule status --recursive 2>err; echo $?>status; } |
    -		grep -q X/S &&
    +		{ sleep 1 && grep -q X/S; } &&
            test_must_be_empty err &&
            test_match_signal 13 "$(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 | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)