odb/source: make free() function pluggable

Introduce a new callback function in `struct odb_source` to make the
function pluggable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

odb/source-files.c

@@ -18,10 +18,9 @@ static void odb_source_files_reparent(const char *name UNUSED,
 	files->base.path = path;
 }
 
-void odb_source_files_free(struct odb_source_files *files)
+static void odb_source_files_free(struct odb_source *source)
 {
-	if (!files)
-		return;
+	struct odb_source_files *files = odb_source_files_downcast(source);
 	chdir_notify_unregister(NULL, odb_source_files_reparent, files);
 	odb_source_loose_free(files->loose);
 	packfile_store_free(files->packed);
@@ -40,6 +39,8 @@ struct odb_source_files *odb_source_files_new(struct object_database *odb,
 	files->loose = odb_source_loose_new(&files->base);
 	files->packed = packfile_store_new(&files->base);
 
+	files->base.free = odb_source_files_free;
+
 	/*
 	 * Ideally, we would only ever store absolute paths in the source. This
 	 * is not (yet) possible though because we access and assume relative

odb/source-files.h

@@ -21,9 +21,6 @@ struct odb_source_files *odb_source_files_new(struct object_database *odb,
 					      const char *path,
 					      bool local);
 
-/* Free the object source and release all associated resources. */
-void odb_source_files_free(struct odb_source_files *files);
-
 /*
  * Cast the given object database source to the files backend. This will cause
  * a BUG in case the source doesn't use this backend.

odb/source.c

@@ -25,11 +25,9 @@ void odb_source_init(struct odb_source *source,
 
 void odb_source_free(struct odb_source *source)
 {
-	struct odb_source_files *files;
 	if (!source)
 		return;
-	files = odb_source_files_downcast(source);
-	odb_source_files_free(files);
+	source->free(source);
 }
 
 void odb_source_release(struct odb_source *source)

odb/source.h

@@ -51,6 +51,12 @@ struct odb_source {
 	 * the current working directory.
 	 */
 	char *path;
+
+	/*
+	 * This callback is expected to free the underlying object database source and
+	 * all associated resources. The function will never be called with a NULL pointer.
+	 */
+	void (*free)(struct odb_source *source);
 };
 
 /*