odb: prepare `struct odb_transaction` to become generic

An ODB transaction handles how objects are stored temporarily and
eventually committed. Due to object storage being implemented
differently for a given ODB source, the ODB transactions must be
implemented in a manner specific to the source the objects are being
written to. To provide generic transactions, `struct odb_transaction` is
updated to store a commit callback that can be configured to support a
specific ODB source. For now `struct odb_transaction_files` is the
only transaction type and what is always returned when starting a
transaction.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/odb.h b/odb.h
index bab07755f4..83d3a37805 100644
--- a/odb.h
+++ b/odb.h
@@ -77,7 +77,24 @@ struct odb_source {
 struct packed_git;
 struct packfile_store;
 struct cached_object_entry;
+
+/*
+ * A transaction may be started for an object database prior to writing new
+ * objects via odb_transaction_begin(). These objects are not committed until
+ * odb_transaction_commit() is invoked. Only a single transaction may be pending
+ * at a time.
+ *
+ * Each ODB source is expected to implement its own transaction handling.
+ */
 struct odb_transaction;
+typedef void (*odb_transaction_commit_fn)(struct odb_transaction *transaction);
+struct odb_transaction {
+	/* The ODB source the transaction is opened against. */
+	struct odb_source *source;
+
+	/* The ODB source specific callback invoked to commit a transaction. */
+	odb_transaction_commit_fn commit;
+};
 
 /*
  * The object database encapsulates access to objects in a repository. It