remote: remove branch->merge_name and fix branch_release()

The branch structure has both branch->merge_name and branch->merge for
tracking the merge information. The former is allocated by add_merge()
and stores the names read from the configuration file. The latter is
allocated by set_merge() which is called by branch_get() when an
external caller requests a branch.

This leads to the confusing situation where branch->merge_nr tracks both
the size of branch->merge (once its allocated) and branch->merge_name.
The branch_release() function incorrectly assumes that branch->merge is
always set when branch->merge_nr is non-zero, and can potentially crash
if read_config() is called without branch_get() being called on every
branch.

In addition, branch_release() fails to free some of the memory
associated with the structure including:

 * Failure to free the refspec_item containers in branch->merge[i]
 * Failure to free the strings in branch->merge_name[i]
 * Failure to free the branch->merge_name parent array.

The set_merge() function sets branch->merge_nr to 0 when there is no
valid remote_name, to avoid external callers seeing a non-zero merge_nr
but a NULL merge array. This results in failure to release most of the
merge data as well.

These issues could be fixed directly, and indeed I initially proposed
such a change at [1] in the past. While this works, there was some
confusion during review because of the inconsistencies.

Instead, its time to clean up the situation properly. Remove
branch->merge_name entirely. Instead, allocate branch->merge earlier
within add_merge() instead of within set_merge(). Instead of having
set_merge() copy from merge_name[i] to merge[i]->src, just have
add_merge() directly initialize merge[i]->src.

Modify the add_merge() to call xstrdup() itself, instead of having
the caller of add_merge() do so. This makes it more obvious which code
owns the memory.

Update all callers which use branch->merge_name[i] to use
branch->merge[i]->src instead.

Add a merge_clear() function which properly releases all of the
merge-related memory, and which sets branch->merge_nr to zero. Use this
both in branch_release() and in set_merge(), fixing the leak when
set_merge() finds no valid remote_name.

Add a set_merge variable to the branch structure, which indicates
whether set_merge() has been called. This replaces the previous use of a
NULL check against the branch->merge array.

With these changes, the merge array is always allocated when merge_nr is
non-zero.

This use of refspec_item to store the names should be safe. External
callers should be using branch_get() to obtain a pointer to the branch,
which will call set_merge(), and the callers internal to remote.c
already handle the partially initialized refpsec_item structure safely.

This end result is cleaner, and avoids duplicating the merge names
twice.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Link: [1] https://lore.kernel.org/git/20250617-jk-submodule-helper-use-url-v2-1-04cbb003177d@gmail.com/
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -230,7 +230,7 @@ static int inherit_tracking(struct tracking *tracking, const char *orig_ref)
 		return -1;
 	}
 
-	if (branch->merge_nr < 1 || !branch->merge_name || !branch->merge_name[0]) {
+	if (branch->merge_nr < 1 || !branch->merge || !branch->merge[0] || !branch->merge[0]->src) {
 		warning(_("asked to inherit tracking from '%s', but no merge configuration is set"),
 			bare_ref);
 		return -1;
@@ -238,7 +238,7 @@ static int inherit_tracking(struct tracking *tracking, const char *orig_ref)
 
 	tracking->remote = branch->remote_name;
 	for (i = 0; i < branch->merge_nr; i++)
-		string_list_append(tracking->srcs, branch->merge_name[i]);
+		string_list_append(tracking->srcs, branch->merge[i]->src);
 	return 0;
 }
 

builtin/pull.c

@@ -487,7 +487,7 @@ static void NORETURN die_no_merge_candidates(const char *repo, const char **refs
 	} else
 		fprintf_ln(stderr, _("Your configuration specifies to merge with the ref '%s'\n"
 			"from the remote, but no such ref was fetched."),
-			*curr_branch->merge_name);
+			curr_branch->merge[0]->src);
 	exit(1);
 }
 

remote.c

@@ -174,9 +174,15 @@ static void remote_clear(struct remote *remote)
 
 static void add_merge(struct branch *branch, const char *name)
 {
-	ALLOC_GROW(branch->merge_name, branch->merge_nr + 1,
+	struct refspec_item *merge;
+
+	ALLOC_GROW(branch->merge, branch->merge_nr + 1,
 		   branch->merge_alloc);
-	branch->merge_name[branch->merge_nr++] = name;
+
+	merge = xcalloc(1, sizeof(*merge));
+	merge->src = xstrdup(name);
+
+	branch->merge[branch->merge_nr++] = merge;
 }
 
 struct branches_hash_key {
@@ -247,15 +253,23 @@ static struct branch *make_branch(struct remote_state *remote_state,
 	return ret;
 }
 
+static void merge_clear(struct branch *branch)
+{
+	for (int i = 0; i < branch->merge_nr; i++) {
+		refspec_item_clear(branch->merge[i]);
+		free(branch->merge[i]);
+	}
+	FREE_AND_NULL(branch->merge);
+	branch->merge_nr = 0;
+}
+
 static void branch_release(struct branch *branch)
 {
 	free((char *)branch->name);
 	free((char *)branch->refname);
 	free(branch->remote_name);
 	free(branch->pushremote_name);
-	for (int i = 0; i < branch->merge_nr; i++)
-		refspec_item_clear(branch->merge[i]);
-	free(branch->merge);
+	merge_clear(branch);
 }
 
 static struct rewrite *make_rewrite(struct rewrites *r,
@@ -429,7 +443,7 @@ static int handle_config(const char *key, const char *value,
 		} else if (!strcmp(subkey, "merge")) {
 			if (!value)
 				return config_error_nonbool(key);
-			add_merge(branch, xstrdup(value));
+			add_merge(branch, value);
 		}
 		return 0;
 	}
@@ -692,7 +706,7 @@ char *remote_ref_for_branch(struct branch *branch, int for_push)
 	if (branch) {
 		if (!for_push) {
 			if (branch->merge_nr) {
-				return xstrdup(branch->merge_name[0]);
+				return xstrdup(branch->merge[0]->src);
 			}
 		} else {
 			char *dst;
@@ -1731,32 +1745,30 @@ static void set_merge(struct remote_state *remote_state, struct branch *ret)
 
 	if (!ret)
 		return; /* no branch */
-	if (ret->merge)
+	if (ret->set_merge)
 		return; /* already run */
 	if (!ret->remote_name || !ret->merge_nr) {
 		/*
 		 * no merge config; let's make sure we don't confuse callers
 		 * with a non-zero merge_nr but a NULL merge
 		 */
-		ret->merge_nr = 0;
+		merge_clear(ret);
 		return;
 	}
+	ret->set_merge = 1;
 
 	remote = remotes_remote_get(remote_state, ret->remote_name);
 
-	CALLOC_ARRAY(ret->merge, ret->merge_nr);
 	for (i = 0; i < ret->merge_nr; i++) {
-		ret->merge[i] = xcalloc(1, sizeof(**ret->merge));
-		ret->merge[i]->src = xstrdup(ret->merge_name[i]);
 		if (!remote_find_tracking(remote, ret->merge[i]) ||
 		    strcmp(ret->remote_name, "."))
 			continue;
-		if (repo_dwim_ref(the_repository, ret->merge_name[i],
-				  strlen(ret->merge_name[i]), &oid, &ref,
+		if (repo_dwim_ref(the_repository, ret->merge[i]->src,
+				  strlen(ret->merge[i]->src), &oid, &ref,
 				  0) == 1)
 			ret->merge[i]->dst = ref;
 		else
-			ret->merge[i]->dst = xstrdup(ret->merge_name[i]);
+			ret->merge[i]->dst = xstrdup(ret->merge[i]->src);
 	}
 }
 
@@ -1776,7 +1788,7 @@ struct branch *branch_get(const char *name)
 
 int branch_has_merge_config(struct branch *branch)
 {
-	return branch && !!branch->merge;
+	return branch && branch->set_merge;
 }
 
 int branch_merge_matches(struct branch *branch,

remote.h

@@ -315,8 +315,8 @@ struct branch {
 
 	char *pushremote_name;
 
-	/* An array of the "merge" lines in the configuration. */
-	const char **merge_name;
+	/* True if set_merge() has been called to finalize the merge array */
+	int set_merge;
 
 	/**
 	 * An array of the struct refspecs used for the merge lines. That is,