odb/transaction: make write_object_stream() pluggable

How an ODB transaction handles writing objects is expected to vary
between implementations. Introduce a new `write_object_stream()`
callback in `struct odb_transaction` to make this function pluggable.
Wire up `index_blob_packfile_transaction()` for use with `struct
odb_transaction_files` accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>

Author: jltobler

object-file.c

@@ -1574,9 +1574,10 @@ static void flush_packfile_transaction(struct odb_transaction_files *transaction
  * binary blobs, they generally do not want to get any conversion, and
  * callers should avoid this code path when filters are requested.
  */
-static int index_blob_packfile_transaction(struct odb_transaction *base,
-					   struct odb_write_stream *stream,
-					   size_t size, struct object_id *result_oid)
+static int odb_transaction_files_write_object_stream(struct odb_transaction *base,
+						     struct odb_write_stream *stream,
+						     size_t size,
+						     struct object_id *result_oid)
 {
 	struct odb_transaction_files *transaction = container_of(base,
 								 struct odb_transaction_files,
@@ -1660,10 +1661,10 @@ int index_fd(struct index_state *istate, struct object_id *oid,
 			struct object_database *odb = the_repository->objects;
 			struct odb_transaction *transaction = odb_transaction_begin(odb);
 
-			ret = index_blob_packfile_transaction(odb->transaction,
-							      &stream,
-							      xsize_t(st->st_size),
-							      oid);
+			ret = odb_transaction_write_object_stream(odb->transaction,
+								  &stream,
+								  xsize_t(st->st_size),
+								  oid);
 			odb_transaction_commit(transaction);
 		} else {
 			ret = hash_blob_stream(&stream,
@@ -2128,6 +2129,7 @@ struct odb_transaction *odb_transaction_files_begin(struct odb_source *source)
 	transaction = xcalloc(1, sizeof(*transaction));
 	transaction->base.source = source;
 	transaction->base.commit = odb_transaction_files_commit;
+	transaction->base.write_object_stream = odb_transaction_files_write_object_stream;
 
 	return &transaction->base;
 }

odb/transaction.h

@@ -12,14 +12,24 @@
  *
  * 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;
+	void (*commit)(struct odb_transaction *transaction);
+
+	/*
+	 * This callback is expected to write the given object stream into
+	 * the ODB transaction. Note that for now, only blobs support streaming.
+	 *
+	 * The resulting object ID shall be written into the out pointer. The
+	 * callback is expected to return 0 on success, a negative error code
+	 * otherwise.
+	 */
+	int (*write_object_stream)(struct odb_transaction *transaction,
+				   struct odb_write_stream *stream, size_t len,
+				   struct object_id *oid);
 };
 
 /*