merge: fix leak with merge.defaultToUpstream

By default the setting 'merge.defaultToUpstream' for git-merge(1) is set
to 'true', which means when `git merge` is invoked with no arguments it
merges the upstream branch configured for the current branch.

With this configuration set to 'true', setup_with_upstream() is called.
That function allocates an array of arguments and hands it back to
cmd_merge() via its `argv` parameter. This array is never freed, so
cmd_merge() leaks it on every invocation.

Track the allocated array in a separate variable and free it at the end.

The leak has been present since 93e535a5b7 (merge: merge with the
default upstream branch without argument, 2011-03-24). Although the leak
sanitizer was enabled for tests in fc1ddf42af (t: remove
TEST_PASSES_SANITIZE_LEAK annotations, 2024-11-21), it went unnoticed
because no test calls `git merge` without arguments, exercising the
default-to-upstream path. Add such a test in t7600, which fails under
the leak sanitizer without this fix.

Signed-off-by: Toon Claes <toon@iotcl.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

diff --git a/builtin/merge.c b/builtin/merge.c
index 2cbce56..ecd7a89 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -1372,7 +1372,7 @@ int cmd_merge(int argc,
 	struct commit_list *common = NULL;
 	const char *best_strategy = NULL, *wt_strategy = NULL;
 	struct commit_list *remoteheads = NULL, *p;
-	void *branch_to_free;
+	void *branch_to_free, *argv_to_free = NULL;
 	int orig_argc = argc;
 	int merge_log_config = -1;
 
@@ -1516,8 +1516,10 @@ int cmd_merge(int argc,
 		option_commit = 1;
 
 	if (!argc) {
-		if (default_to_upstream)
+		if (default_to_upstream) {
 			argc = setup_with_upstream(&argv);
+			argv_to_free = argv;
+		}
 		else
 			die(_("No commit specified and merge.defaultToUpstream not set."));
 	} else if (argc == 1 && !strcmp(argv[0], "-")) {
@@ -1885,6 +1887,7 @@ int cmd_merge(int argc,
 	}
 	strbuf_release(&buf);
 	free(branch_to_free);
+	free(argv_to_free);
 	free(pull_twohead);
 	free(pull_octopus);
 	discard_index(the_repository->index);

t/t7600-merge.sh

diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 9838094..b3a2164 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -1165,4 +1165,21 @@
 	grep remotes/origin/not-local stderr
 '
 
+test_expect_success 'merge with no argument defaults to upstream' '
+	test_when_finished "rm -rf upstream downstream" &&
+	git init upstream &&
+	(
+		cd upstream &&
+		test_commit one &&
+		test_commit two
+	) &&
+	git clone upstream downstream &&
+	(
+		cd downstream &&
+		git reset --hard HEAD^ &&
+		git merge &&
+		test_cmp_rev origin/main HEAD
+	)
+'
+
 test_done