reduce_heads: fix memory leaks

We currently have seven callers of `reduce_heads(foo)`. Six of them do
not use the original list `foo` again, and actually, all six of those
end up leaking it.

Introduce and use `reduce_heads_replace(&foo)` as a leak-free version of
`foo = reduce_heads(foo)` to fix several of these. Fix the remaining
leaks using `free_commit_list()`.

While we're here, document `reduce_heads()` and mark it as `extern`.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -1728,7 +1728,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 				allow_fast_forward = 0;
 		}
 		if (allow_fast_forward)
-			parents = reduce_heads(parents);
+			reduce_heads_replace(&parents);
 	} else {
 		if (!reflog_msg)
 			reflog_msg = (whence == FROM_CHERRY_PICK)

builtin/fmt-merge-msg.c

@@ -571,7 +571,7 @@ static void find_merge_parents(struct merge_parents *result,
 	head_commit = lookup_commit(head);
 	if (head_commit)
 		commit_list_insert(head_commit, &parents);
-	parents = reduce_heads(parents);
+	reduce_heads_replace(&parents);
 
 	while (parents) {
 		struct commit *cmit = pop_commit(&parents);

builtin/merge-base.c

@@ -57,7 +57,7 @@ static int handle_independent(int count, const char **args)
 	for (i = count - 1; i >= 0; i--)
 		commit_list_insert(get_commit_reference(args[i]), &revs);
 
-	revs = reduce_heads(revs);
+	reduce_heads_replace(&revs);
 
 	if (!revs)
 		return 1;
@@ -78,7 +78,9 @@ static int handle_octopus(int count, const char **args, int show_all)
 	for (i = count - 1; i >= 0; i--)
 		commit_list_insert(get_commit_reference(args[i]), &revs);
 
-	result = reduce_heads(get_octopus_merge_bases(revs));
+	result = get_octopus_merge_bases(revs);
+	free_commit_list(revs);
+	reduce_heads_replace(&result);
 
 	if (!result)
 		return 1;

builtin/merge.c

@@ -999,6 +999,7 @@ static struct commit_list *reduce_parents(struct commit *head_commit,
 
 	/* Find what parents to record by checking independent ones. */
 	parents = reduce_heads(remoteheads);
+	free_commit_list(remoteheads);
 
 	remoteheads = NULL;
 	remotes = &remoteheads;

builtin/pull.c

@@ -745,12 +745,15 @@ static int get_octopus_merge_base(struct object_id *merge_base,
 	if (!is_null_oid(fork_point))
 		commit_list_insert(lookup_commit_reference(fork_point), &revs);
 
-	result = reduce_heads(get_octopus_merge_bases(revs));
+	result = get_octopus_merge_bases(revs);
 	free_commit_list(revs);
+	reduce_heads_replace(&result);
+
 	if (!result)
 		return 1;
 
 	oidcpy(merge_base, &result->item->object.oid);
+	free_commit_list(result);
 	return 0;
 }
 

commit.c

@@ -1090,6 +1090,13 @@ struct commit_list *reduce_heads(struct commit_list *heads)
 	return result;
 }
 
+void reduce_heads_replace(struct commit_list **heads)
+{
+	struct commit_list *result = reduce_heads(*heads);
+	free_commit_list(*heads);
+	*heads = result;
+}
+
 static const char gpg_sig_header[] = "gpgsig";
 static const int gpg_sig_header_len = sizeof(gpg_sig_header) - 1;
 

commit.h

@@ -313,7 +313,23 @@ extern int interactive_add(int argc, const char **argv, const char *prefix, int
 extern int run_add_interactive(const char *revision, const char *patch_mode,
 			       const struct pathspec *pathspec);
 
-struct commit_list *reduce_heads(struct commit_list *heads);
+/*
+ * Takes a list of commits and returns a new list where those
+ * have been removed that can be reached from other commits in
+ * the list. It is useful for, e.g., reducing the commits
+ * randomly thrown at the git-merge command and removing
+ * redundant commits that the user shouldn't have given to it.
+ *
+ * This function destroys the STALE bit of the commit objects'
+ * flags.
+ */
+extern struct commit_list *reduce_heads(struct commit_list *heads);
+
+/*
+ * Like `reduce_heads()`, except it replaces the list. Use this
+ * instead of `foo = reduce_heads(foo);` to avoid memory leaks.
+ */
+extern void reduce_heads_replace(struct commit_list **heads);
 
 struct commit_extra_header {
 	struct commit_extra_header *next;