config_file: handle error when trying to lock strmap

Accessing the current values map is handled through the
`refcounder_strmap_take` function, which first acquires a mutex
before accessing its values. While this assures everybody is
trying to access the values with the mutex only we do not check
if the locking actually succeeds.

Fix the issue by checking if acquiring the lock succeeds and
returning `NULL` if we encounter an error. Adjust callers.

src/config_file.c

@@ -232,7 +232,10 @@ static refcounted_strmap *refcounted_strmap_take(diskfile_header *h)
 {
 	refcounted_strmap *map;
 
-	git_mutex_lock(&h->values_mutex);
+	if (git_mutex_lock(&h->values_mutex) < 0) {
+	    giterr_set(GITERR_OS, "Failed to lock config backend");
+	    return NULL;
+	}
 
 	map = h->values;
 	git_atomic_inc(&map->refcount);
@@ -318,7 +321,10 @@ static int config__refresh(git_config_backend *cfg)
 	if ((error = config_read(values->values, b, reader, b->level, 0)) < 0)
 		goto out;
 
-	git_mutex_lock(&b->header.values_mutex);
+	if ((error = git_mutex_lock(&b->header.values_mutex)) < 0) {
+		giterr_set(GITERR_OS, "Failed to lock config backend");
+		goto out;
+	}
 
 	tmp = b->header.values;
 	b->header.values = values;
@@ -460,7 +466,8 @@ static int config_set(git_config_backend *cfg, const char *name, const char *val
 	if ((rval = git_config__normalize_name(name, &key)) < 0)
 		return rval;
 
-	map = refcounted_strmap_take(&b->header);
+	if ((map = refcounted_strmap_take(&b->header)) == NULL)
+		return -1;
 	values = map->values;
 
 	/*
@@ -527,7 +534,8 @@ static int config_get(git_config_backend *cfg, const char *key, git_config_entry
 	if (!h->parent.readonly && ((error = config_refresh(cfg)) < 0))
 		return error;
 
-	map = refcounted_strmap_take(h);
+	if ((map = refcounted_strmap_take(h)) == NULL)
+		return -1;
 	values = map->values;
 
 	pos = git_strmap_lookup_index(values, key);
@@ -565,7 +573,8 @@ static int config_set_multivar(
 	if ((result = git_config__normalize_name(name, &key)) < 0)
 		return result;
 
-	map = refcounted_strmap_take(&b->header);
+	if ((map = refcounted_strmap_take(&b->header)) == NULL)
+		return -1;
 	values = b->header.values->values;
 
 	pos = git_strmap_lookup_index(values, key);
@@ -610,7 +619,8 @@ static int config_delete(git_config_backend *cfg, const char *name)
 	if ((result = git_config__normalize_name(name, &key)) < 0)
 		return result;
 
-	map = refcounted_strmap_take(&b->header);
+	if ((map = refcounted_strmap_take(&b->header)) == NULL)
+		return -1;
 	values = b->header.values->values;
 
 	pos = git_strmap_lookup_index(values, key);
@@ -649,7 +659,8 @@ static int config_delete_multivar(git_config_backend *cfg, const char *name, con
 	if ((result = git_config__normalize_name(name, &key)) < 0)
 		return result;
 
-	map = refcounted_strmap_take(&b->header);
+	if ((map = refcounted_strmap_take(&b->header)) == NULL)
+		return -1;
 	values = b->header.values->values;
 
 	pos = git_strmap_lookup_index(values, key);
@@ -832,7 +843,8 @@ static int config_readonly_open(git_config_backend *cfg, git_config_level_t leve
 	/* We're just copying data, don't care about the level */
 	GIT_UNUSED(level);
 
-	src_map = refcounted_strmap_take(src_header);
+	if ((src_map = refcounted_strmap_take(src_header)) == NULL)
+		return -1;
 	b->header.values = src_map;
 
 	return 0;