config: skip r/o backends when writing

Configuration backends have a readonly-flag which is currently used to
distinguish configuration snapshots. But somewhat unexpectedly, we do
not use the flag to prevent writing to a readonly backend but happily
proceed to do so.

This commit modifies logic to also honor the readonly flag for
configuration setters. We will now traverse through all backends and
pick the first one which is not marked as read-only whenever we want to
write new configuration.

src/config.c

@@ -576,22 +576,50 @@ int git_config_foreach_match(
  * Setters
  **************/
 
-static int config_error_nofiles(const char *name)
+typedef enum {
+	BACKEND_USE_SET,
+	BACKEND_USE_DELETE
+} backend_use;
+
+static const char *uses[] = {
+    "set",
+    "delete"
+};
+
+static int get_backend_for_use(git_config_backend **out,
+	git_config *cfg, const char *name, backend_use use)
 {
+	size_t i;
+	file_internal *f;
+
+	*out = NULL;
+
+	if (git_vector_length(&cfg->files) == 0) {
+		giterr_set(GITERR_CONFIG,
+			"cannot %s value for '%s' when no config files exist",
+			uses[use], name);
+		return GIT_ENOTFOUND;
+	}
+
+	git_vector_foreach(&cfg->files, i, f) {
+		if (!f->file->readonly) {
+			*out = f->file;
+			return 0;
+		}
+	}
+
 	giterr_set(GITERR_CONFIG,
-		"cannot set value for '%s' when no config files exist", name);
+		"cannot %s value for '%s' when all config files are readonly",
+		uses[use], name);
 	return GIT_ENOTFOUND;
 }
 
 int git_config_delete_entry(git_config *cfg, const char *name)
 {
 	git_config_backend *file;
-	file_internal *internal;
 
-	internal = git_vector_get(&cfg->files, 0);
-	if (!internal || !internal->file)
-		return config_error_nofiles(name);
-	file = internal->file;
+	if (get_backend_for_use(&file, cfg, name, BACKEND_USE_DELETE) < 0)
+		return GIT_ENOTFOUND;
 
 	return file->del(file, name);
 }
@@ -617,17 +645,14 @@ int git_config_set_string(git_config *cfg, const char *name, const char *value)
 {
 	int error;
 	git_config_backend *file;
-	file_internal *internal;
 
 	if (!value) {
 		giterr_set(GITERR_CONFIG, "the value to set cannot be NULL");
 		return -1;
 	}
 
-	internal = git_vector_get(&cfg->files, 0);
-	if (!internal || !internal->file)
-		return config_error_nofiles(name);
-	file = internal->file;
+	if (get_backend_for_use(&file, cfg, name, BACKEND_USE_SET) < 0)
+		return GIT_ENOTFOUND;
 
 	error = file->set(file, name, value);
 
@@ -1032,12 +1057,9 @@ on_error:
 int git_config_set_multivar(git_config *cfg, const char *name, const char *regexp, const char *value)
 {
 	git_config_backend *file;
-	file_internal *internal;
 
-	internal = git_vector_get(&cfg->files, 0);
-	if (!internal || !internal->file)
-		return config_error_nofiles(name);
-	file = internal->file;
+	if (get_backend_for_use(&file, cfg, name, BACKEND_USE_DELETE) < 0)
+		return GIT_ENOTFOUND;
 
 	return file->set_multivar(file, name, regexp, value);
 }
@@ -1045,12 +1067,9 @@ int git_config_set_multivar(git_config *cfg, const char *name, const char *regex
 int git_config_delete_multivar(git_config *cfg, const char *name, const char *regexp)
 {
 	git_config_backend *file;
-	file_internal *internal;
 
-	internal = git_vector_get(&cfg->files, 0);
-	if (!internal || !internal->file)
-		return config_error_nofiles(name);
-	file = internal->file;
+	if (get_backend_for_use(&file, cfg, name, BACKEND_USE_DELETE) < 0)
+		return GIT_ENOTFOUND;
 
 	return file->del_multivar(file, name, regexp);
 }