builtin/merge.c: use fixed strings, not "strbuf", fix leak

Follow-up 465028e (merge: add missing strbuf_release(),
2021-10-07) and address the "msg" memory leak in this block. We could
free "&msg" before the "goto done" here, but even better is to avoid
allocating it in the first place.

By repeating the "Fast-forward" string here we can avoid using a
"struct strbuf" altogether.

Suggested-by: René Scharfe <[email protected]>
Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

builtin/merge.c

@@ -1560,7 +1560,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			!common->next &&
 			oideq(&common->item->object.oid, &head_commit->object.oid)) {
 		/* Again the most common case of merging one remote. */
-		struct strbuf msg = STRBUF_INIT;
+		const char *msg = have_message ?
+			"Fast-forward (no commit created; -m option ignored)" :
+			"Fast-forward";
 		struct commit *commit;
 
 		if (verbosity >= 0) {
@@ -1570,10 +1572,6 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			       find_unique_abbrev(&remoteheads->item->object.oid,
 						  DEFAULT_ABBREV));
 		}
-		strbuf_addstr(&msg, "Fast-forward");
-		if (have_message)
-			strbuf_addstr(&msg,
-				" (no commit created; -m option ignored)");
 		commit = remoteheads->item;
 		if (!commit) {
 			ret = 1;
@@ -1592,9 +1590,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 			goto done;
 		}
 
-		finish(head_commit, remoteheads, &commit->object.oid, msg.buf);
+		finish(head_commit, remoteheads, &commit->object.oid, msg);
 		remove_merge_branch_state(the_repository);
-		strbuf_release(&msg);
 		goto done;
 	} else if (!remoteheads->next && common->next)
 		;

t/t6439-merge-co-error-msgs.sh

@@ -5,6 +5,7 @@ test_description='unpack-trees error messages'
 GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
 export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh