ref-filter: fix leak when formatting %(push:remoteref)

When we expand the %(upstream) or %(push) placeholders, we rely on
remote.c's remote_ref_for_branch() to fill in the ":refname" argument.
But that function has confusing memory ownership semantics: it may or
may not return an allocated string, depending on whether we are in
"upstream" mode or "push" mode. The caller in ref-filter.c always
duplicates the result, meaning that we leak the original in the case of
%(push:refname).

To solve this, let's make the return value from remote_ref_for_branch()
consistent, by always returning an allocated pointer. Note that the
switch to returning a non-const pointer has a ripple effect inside the
function, too. We were storing the "dst" result as a const pointer, too,
even though it is always allocated! It is the return value from
apply_refspecs(), which is always a non-const allocated string.

And then on the caller side in ref-filter.c (and this is the only caller
at all), we just need to avoid the extra duplication when the return
value is non-NULL.

This clears up one case that LSan finds in t6300, but there are more.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

ref-filter.c

@@ -2237,7 +2237,7 @@ static void fill_remote_ref_details(struct used_atom *atom, const char *refname,
 		const char *merge;
 
 		merge = remote_ref_for_branch(branch, atom->u.remote_ref.push);
-		*s = xstrdup(merge ? merge : "");
+		*s = merge ? merge : xstrdup("");
 	} else
 		BUG("unhandled RR_* enum");
 }

remote.c

@@ -632,19 +632,19 @@ const char *pushremote_for_branch(struct branch *branch, int *explicit)
 static struct remote *remotes_remote_get(struct remote_state *remote_state,
 					 const char *name);
 
-const char *remote_ref_for_branch(struct branch *branch, int for_push)
+char *remote_ref_for_branch(struct branch *branch, int for_push)
 {
 	read_config(the_repository, 0);
 	die_on_missing_branch(the_repository, branch);
 
 	if (branch) {
 		if (!for_push) {
 			if (branch->merge_nr) {
-				return branch->merge_name[0];
+				return xstrdup(branch->merge_name[0]);
 			}
 		} else {
-			const char *dst,
-				*remote_name = remotes_pushremote_for_branch(
+			char *dst;
+			const char *remote_name = remotes_pushremote_for_branch(
 					the_repository->remote_state, branch,
 					NULL);
 			struct remote *remote = remotes_remote_get(

remote.h

@@ -329,7 +329,7 @@ struct branch {
 struct branch *branch_get(const char *name);
 const char *remote_for_branch(struct branch *branch, int *explicit);
 const char *pushremote_for_branch(struct branch *branch, int *explicit);
-const char *remote_ref_for_branch(struct branch *branch, int for_push);
+char *remote_ref_for_branch(struct branch *branch, int for_push);
 
 /* returns true if the given branch has merge configuration given. */
 int branch_has_merge_config(struct branch *branch);