hook API: don't segfault on strbuf_addf() to NULL "out"

Fix a logic error in a082345372e (hook API: fix v2.36.0 regression:
hooks should be connected to a TTY, 2022-06-07). When it started using
the "ungroup" API added in fd3aaf53f71 (run-command: add an "ungroup"
option to run_process_parallel(), 2022-06-07) it should have made the
same sort of change that fd3aaf53f71 itself made in
"t/helper/test-run-command.c".

The correct way to emit this "Couldn't start" output with "ungroup"
would be:

	fprintf(stderr, _("Couldn't start hook '%s'\n"), hook_path);

But we should instead remove the emitting of this output. As the added
test shows we already emit output when we can't run the child. The
"cannot run" output here is emitted by run-command.c's
child_err_spew().

So the addition of the "Couldn't start hook" output here in
96e7225b310 (hook: add 'run' subcommand, 2021-12-22) was always
redundant. For the pre-commit hook we'll now emit exactly the same
output as we did before f443246b9f2 (commit: convert
{pre-commit,prepare-commit-msg} hook to hook.h, 2021-12-22) (and
likewise for others).

We could at this point add this to the pick_next_hook() callbacks in
hook.c:

	assert(!out);
	assert(!*pp_task_cb);

And this to notify_start_failure() and notify_hook_finished() (in the
latter case the parameter is called "pp_task_cp"):

	assert(!out);
	assert(!pp_task_cb);

But let's leave any such instrumentation for some eventual cleanup of
the "ungroup" API.

Reported-by: Ilya K <me@0upti.me>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Emily Shaffer <emilyshaffer@google.com>
Reviewed-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/t/t1800-hook.sh b/t/t1800-hook.sh
index 210f429887..64096adac7 100755
--- a/t/t1800-hook.sh
+++ b/t/t1800-hook.sh
@@ -151,4 +151,30 @@ test_expect_success TTY 'git commit: stdout and stderr are connected to a TTY' '
 	test_hook_tty commit -m"B.new"
 '
 
+test_expect_success 'git hook run a hook with a bad shebang' '
+	test_when_finished "rm -rf bad-hooks" &&
+	mkdir bad-hooks &&
+	write_script bad-hooks/test-hook "/bad/path/no/spaces" </dev/null &&
+
+	# TODO: We should emit the same (or at least a more similar)
+	# error on Windows and !Windows. See the OS-specific code in
+	# start_command()
+	if test_have_prereq !WINDOWS
+	then
+		cat >expect <<-\EOF
+		fatal: cannot run bad-hooks/test-hook: ...
+		EOF
+	else
+		cat >expect <<-\EOF
+		error: cannot spawn bad-hooks/test-hook: ...
+		EOF
+	fi &&
+	test_expect_code 1 git \
+		-c core.hooksPath=bad-hooks \
+		hook run test-hook >out 2>err &&
+	test_must_be_empty out &&
+	sed -e "s/test-hook: .*/test-hook: .../" <err >actual &&
+	test_cmp expect actual
+'
+
 test_done