odb/source: make begin_transaction() 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

@@ -122,6 +122,16 @@ static int odb_source_files_write_object_stream(struct odb_source *source,
 	return odb_source_loose_write_stream(source, stream, len, oid);
 }
 
+static int odb_source_files_begin_transaction(struct odb_source *source,
+					      struct odb_transaction **out)
+{
+	struct odb_transaction *tx = odb_transaction_files_begin(source);
+	if (!tx)
+		return -1;
+	*out = tx;
+	return 0;
+}
+
 static int odb_source_files_read_alternates(struct odb_source *source,
 					    struct strvec *out)
 {
@@ -213,6 +223,7 @@ struct odb_source_files *odb_source_files_new(struct object_database *odb,
 	files->base.freshen_object = odb_source_files_freshen_object;
 	files->base.write_object = odb_source_files_write_object;
 	files->base.write_object_stream = odb_source_files_write_object_stream;
+	files->base.begin_transaction = odb_source_files_begin_transaction;
 	files->base.read_alternates = odb_source_files_read_alternates;
 	files->base.write_alternate = odb_source_files_write_alternate;
 

odb/source.h

@@ -53,6 +53,7 @@ enum object_info_flags {
 struct object_id;
 struct object_info;
 struct odb_read_stream;
+struct odb_transaction;
 struct odb_write_stream;
 struct strvec;
 
@@ -233,6 +234,19 @@ struct odb_source {
 				   struct odb_write_stream *stream, size_t len,
 				   struct object_id *oid);
 
+	/*
+	 * This callback is expected to create a new transaction that can be
+	 * used to write objects to. The objects shall only be persisted into
+	 * the object database when the transcation's commit function is
+	 * called. Otherwise, the objects shall be discarded.
+	 *
+	 * Returns 0 on success, in which case the `*out` pointer will have
+	 * been populated with the object database transaction. Returns a
+	 * negative error code otherwise.
+	 */
+	int (*begin_transaction)(struct odb_source *source,
+				 struct odb_transaction **out);
+
 	/*
 	 * This callback is expected to read the list of alternate object
 	 * database sources connected to it and write them into the `strvec`.
@@ -438,4 +452,17 @@ static inline int odb_source_write_alternate(struct odb_source *source,
 	return source->write_alternate(source, alternate);
 }
 
+/*
+ * Create a new transaction that can be used to write objects into a temporary
+ * staging area. The objects will only be persisted when the transaction is
+ * committed.
+ *
+ * Returns 0 on success, a negative error code otherwise.
+ */
+static inline int odb_source_begin_transaction(struct odb_source *source,
+					       struct odb_transaction **out)
+{
+	return source->begin_transaction(source, out);
+}
+
 #endif