t5410: avoid hangs in CI runs in the win+Meson test jobs

In the GitHub workflow used in Git's CI builds, the `vs test` jobs use a
subset of a specific revision of Git for Windows' SDK to run Git's test
suite. This revision is validated by another CI workflow to ensure that
said revision _can_ run Git's test suite successfully, skipping buggy
updates in Git for Windows' SDK.

The `win+Meson test` jobs do things differently, quite differently. They
use the Bash of the Git for Windows version that is installed on the
runners to run Git's test suite.

This difference has consequences.

When 68cb0b5 (builtin/receive-pack: add option to skip connectivity
check, 2025-05-20) introduced a test case that uses `tee <file> | git
receive-pack` as `--receive-pack` parameter (imitating an existing
pattern in the same test script), it hit just the sweet spot to trigger
a bug in the MSYS2 runtime shipped in Git for Windows v2.49.0. This
version is the one currently installed on GitHub's runners.

The problem is that the `git receive-pack` process finishes while the
`tee` process does not need to write anything anymore and therefore does
not receive an EOF. Instead, it should receive a SIGPIPE, but the bug in
the MSYS2 runtime prevents that from working as intended. As a
consequence, the `tee` process waits for more input from the `git.exe
send-pack` process but none is coming, and the test script patiently
waits until the 6h timeout hits.

Only every once in a while, the `git receive-pack` process manages to
send an EOF to the `tee` process and no hang occurs. Therefore, the
problem can be worked around by cancelling the clearly-hanging job after
twenty or so minutes and re-running it, repeating the process about half
a dozen times, until the hang was successfully avoided.

This bug in the MSYS2 runtime has been fixed in the meantime, which is
the reason why the same test case causes no problems in the `win test`
and the `vs test` jobs.

This will continue to be the case until the Git for Windows version on
the GitHub runners is upgraded to a version that distributes a newer
MSYS2 runtime version. However, as of time of writing, this _is_ the
latest Git for Windows version, and will be for another 1.5 weeks, until
Git v2.50.0 is scheduled to appear (and shortly thereafter Git for
Windows v2.50.0). Traditionally it takes a while before the runners pick
up the new version.

We could just wait it out, six hours at a time.

Here, I opt for an alternative: Detect the buggy MSYS2 runtime and
simply skip the test case. It's not like the `receive-pack` test cases
are specific to Windows, and even then, to my chagrin the CI runs in
git-for-windows/git spend around ten hours of compute time each and
every time to run the entire test suite on all the platforms, even the
tests that cover cross-platform code, and for Windows alone we do that
three times: with GCC, with MSVC, and with MSVC via Meson. Therefore, I
deem it more than acceptable to skip this test case in one of those
matrices.

For good luck, also the preceding test case is skipped in that scenario,
as it uses the same `--receive-pack=tee <file> | git receive-pack`
pattern, even though I never observed that test case to hang in
practice.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/t5410-receive-pack.sh

@@ -41,7 +41,19 @@ test_expect_success 'with core.alternateRefsPrefixes' '
 	test_cmp expect actual.haves
 '
 
-test_expect_success 'receive-pack missing objects fails connectivity check' '
+# The `tee.exe` shipped in Git for Windows v2.49.0 is known to hang frequently
+# when spawned from `git.exe` and piping its output to `git.exe`. This seems
+# related to MSYS2 runtime bug fixes regarding the signal handling; Let's just
+# skip the tests that need to exercise this when the faulty MSYS2 runtime is
+# detected; The test cases are exercised enough in other matrix jobs of the CI
+# runs.
+test_lazy_prereq TEE_DOES_NOT_HANG '
+	test_have_prereq !MINGW &&
+	case "$(uname -a)" in *3.5.7-463ebcdc.x86_64*) false;; esac
+'
+
+test_expect_success TEE_DOES_NOT_HANG \
+	'receive-pack missing objects fails connectivity check' '
 	test_when_finished rm -rf repo remote.git setup.git &&
 
 	git init repo &&
@@ -62,7 +74,8 @@ test_expect_success 'receive-pack missing objects fails connectivity check' '
 	test_must_fail git -C remote.git cat-file -e $(git -C repo rev-parse HEAD)
 '
 
-test_expect_success 'receive-pack missing objects bypasses connectivity check' '
+test_expect_success TEE_DOES_NOT_HANG \
+	'receive-pack missing objects bypasses connectivity check' '
 	test_when_finished rm -rf repo remote.git setup.git &&
 
 	git init repo &&