[PATCH] builtin/gc: fix crash when running `git maintenance start`

[Patrick Steinhardt <ps@xxxxxx>]

It was reported on the mailing list that running `git maintenance start`
immediately segfaults starting with b6c3f8e12c (builtin/maintenance: fix
leak in `get_schedule_cmd()`, 2024-09-26). And indeed, this segfault is
trivial to reproduce up to a point where one is scratching their head
why we didn't catch this regression in our test suite.

The root cause of this error is `get_schedule_cmd()`, which does not
populate the `out` parameter in all cases anymore starting with the
mentioned commit. Callers do assume it to always be populated though and
will e.g. call `strvec_split()` on the returned value, which will of
course segfault when the variable is uninitialized.

So why didn't we catch this trivial regression? The reason is that our
tests always set up the "GIT_TEST_MAINT_SCHEDULER" environment variable
via "t/test-lib.sh", which allows us to override the scheduler command
with a custom one so that we don't accidentally modify the developer's
system. But the faulty code where we don't set the `out` parameter will
only get hit in case that environment variable is _not_ set, which is
never the case when executing our tests.

Fix the regression by again unconditionally allocating the value in the
`out` parameter, if provided. Add a test that unsets the environment
variable to catch future regressions in this area.

Reported-by: Shubham Kanodia <shubham.kanodia10@xxxxxxxxx>
Signed-off-by: Patrick Steinhardt <ps@xxxxxx>
---
 builtin/gc.c           |  7 +++++--
 t/t7900-maintenance.sh | 16 ++++++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/builtin/gc.c b/builtin/gc.c
index 6a7a2da006..d52735354c 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -1832,7 +1832,7 @@ static const char *get_extra_launchctl_strings(void) {
  *     | Input |                     Output                      |
  *     | *cmd  | return code |       *out        | *is_available |
  *     +-------+-------------+-------------------+---------------+
- *     | "foo" |    false    | NULL              |  (unchanged)  |
+ *     | "foo" |    false    | "foo" (allocated) |  (unchanged)  |
  *     +-------+-------------+-------------------+---------------+
  *
  *   GIT_TEST_MAINT_SCHEDULER set to “foo:./mock_foo.sh,bar:./mock_bar.sh”
@@ -1850,8 +1850,11 @@ static int get_schedule_cmd(const char *cmd, int *is_available, char **out)
 	struct string_list_item *item;
 	struct string_list list = STRING_LIST_INIT_NODUP;
 
-	if (!testing)
+	if (!testing) {
+		if (out)
+			*out = xstrdup(cmd);
 		return 0;
+	}
 
 	if (is_available)
 		*is_available = 0;
diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh
index a66d0e089d..4008e4e45e 100755
--- a/t/t7900-maintenance.sh
+++ b/t/t7900-maintenance.sh
@@ -646,6 +646,22 @@ test_expect_success !MINGW 'register and unregister with regex metacharacters' '
 		maintenance.repo "$(pwd)/$META"
 '
 
+test_expect_success 'start without GIT_TEST_MAINT_SCHEDULER' '
+	test_when_finished "rm -rf crontab.log script repo" &&
+	mkdir script &&
+	write_script script/crontab <<-EOF &&
+	echo "\$*" >>"$(pwd)"/crontab.log
+	EOF
+	git init repo &&
+	(
+		cd repo &&
+		sane_unset GIT_TEST_MAINT_SCHEDULER &&
+		PATH="$(pwd)/../script:$PATH" git maintenance start --scheduler=crontab
+	) &&
+	test_grep -- -l crontab.log &&
+	test_grep -- git_cron_edit_tmp crontab.log
+'
+
 test_expect_success 'start --scheduler=<scheduler>' '
 	test_expect_code 129 git maintenance start --scheduler=foo 2>err &&
 	test_grep "unrecognized --scheduler argument" err &&
-- 
2.47.0.rc1.33.g90fe3800b9.dirty