odb/source: make close() 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.c

@@ -1062,10 +1062,8 @@ struct object_database *odb_new(struct repository *repo,
 void odb_close(struct object_database *o)
 {
 	struct odb_source *source;
-	for (source = o->sources; source; source = source->next) {
-		struct odb_source_files *files = odb_source_files_downcast(source);
-		packfile_store_close(files->packed);
-	}
+	for (source = o->sources; source; source = source->next)
+		odb_source_close(source);
 	close_commit_graph(o);
 }
 

odb/source-files.c

@@ -28,6 +28,12 @@ static void odb_source_files_free(struct odb_source *source)
 	free(files);
 }
 
+static void odb_source_files_close(struct odb_source *source)
+{
+	struct odb_source_files *files = odb_source_files_downcast(source);
+	packfile_store_close(files->packed);
+}
+
 static void odb_source_files_reprepare(struct odb_source *source)
 {
 	struct odb_source_files *files = odb_source_files_downcast(source);
@@ -47,6 +53,7 @@ struct odb_source_files *odb_source_files_new(struct object_database *odb,
 	files->packed = packfile_store_new(&files->base);
 
 	files->base.free = odb_source_files_free;
+	files->base.close = odb_source_files_close;
 	files->base.reprepare = odb_source_files_reprepare;
 
 	/*

odb/source.h

@@ -58,6 +58,14 @@ struct odb_source {
 	 */
 	void (*free)(struct odb_source *source);
 
+	/*
+	 * This callback is expected to close any open resources, like for
+	 * example file descriptors or connections. The source is expected to
+	 * still be usable after it has been closed. Closed resources may need
+	 * to be reopened in that case.
+	 */
+	void (*close)(struct odb_source *source);
+
 	/*
 	 * This callback is expected to clear underlying caches of the object
 	 * database source. The function is called when the repository has for
@@ -103,6 +111,16 @@ void odb_source_free(struct odb_source *source);
  */
 void odb_source_release(struct odb_source *source);
 
+/*
+ * Close the object database source without releasing he underlying data. The
+ * source can still be used going forward, but it first needs to be reopened.
+ * This can be useful to reduce resource usage.
+ */
+static inline void odb_source_close(struct odb_source *source)
+{
+	source->close(source);
+}
+
 /*
  * Reprepare the object database source and clear any caches. Depending on the
  * backend used this may have the effect that concurrently-written objects