ref_transaction_prepare(): new optional step for reference updates

In the future, compound reference stores will sometimes need to modify
references in two different reference stores at the same time, meaning
that a single logical reference transaction might have to be
implemented as two internal sub-transactions. They won't want to call
`ref_transaction_commit()` for the two sub-transactions one after the
other, because that wouldn't be atomic (the first commit could succeed
and the second one fail). Instead, they will want to prepare both
sub-transactions (i.e., obtain any necessary locks and do any
pre-checks), and only if both prepare steps succeed, then commit both
sub-transactions.

Start preparing for that day by adding a new, optional
`ref_transaction_prepare()` step to the reference transaction
sequence, which obtains the locks and does any prechecks, reporting
any errors that occur. Also add a `ref_transaction_abort()` function
that can be used to abort a sub-transaction even if it has already
been prepared.

That is on the side of the public-facing API. On the side of the
`ref_store` VTABLE, get rid of `transaction_commit` and instead add
methods `transaction_prepare`, `transaction_finish`, and
`transaction_abort`. A `ref_transaction_commit()` now basically calls
methods `transaction_prepare` then `transaction_finish`.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 50 insertions, 13 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index a4261d4683..19842d2e56 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -2855,22 +2855,19 @@ static void files_transaction_cleanup(struct ref_transaction *transaction)
 	transaction->state = REF_TRANSACTION_CLOSED;
 }
 
-static int files_transaction_commit(struct ref_store *ref_store,
-				    struct ref_transaction *transaction,
-				    struct strbuf *err)
+static int files_transaction_prepare(struct ref_store *ref_store,
+				     struct ref_transaction *transaction,
+				     struct strbuf *err)
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE,
-			       "ref_transaction_commit");
+			       "ref_transaction_prepare");
 	size_t i;
 	int ret = 0;
-	struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
-	struct string_list_item *ref_to_delete;
 	struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
 	char *head_ref = NULL;
 	int head_type;
 	struct object_id head_oid;
-	struct strbuf sb = STRBUF_INIT;
 
 	assert(err);
 
@@ -2934,6 +2931,8 @@ static int files_transaction_commit(struct ref_store *ref_store,
 	 * that new values are valid, and write new values to the
 	 * lockfiles, ready to be activated. Only keep one lockfile
 	 * open at a time to avoid running out of file descriptors.
+	 * Note that lock_ref_for_update() might append more updates
+	 * to the transaction.
 	 */
 	for (i = 0; i < transaction->nr; i++) {
 		struct ref_update *update = transaction->updates[i];
@@ -2941,7 +2940,38 @@ static int files_transaction_commit(struct ref_store *ref_store,
 		ret = lock_ref_for_update(refs, update, transaction,
 					  head_ref, &affected_refnames, err);
 		if (ret)
-			goto cleanup;
+			break;
+	}
+
+cleanup:
+	free(head_ref);
+	string_list_clear(&affected_refnames, 0);
+
+	if (ret)
+		files_transaction_cleanup(transaction);
+	else
+		transaction->state = REF_TRANSACTION_PREPARED;
+
+	return ret;
+}
+
+static int files_transaction_finish(struct ref_store *ref_store,
+				    struct ref_transaction *transaction,
+				    struct strbuf *err)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, 0, "ref_transaction_finish");
+	size_t i;
+	int ret = 0;
+	struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
+	struct string_list_item *ref_to_delete;
+	struct strbuf sb = STRBUF_INIT;
+
+	assert(err);
+
+	if (!transaction->nr) {
+		transaction->state = REF_TRANSACTION_CLOSED;
+		return 0;
 	}
 
 	/* Perform updates first so live commits remain referenced */
@@ -3022,7 +3052,6 @@ static int files_transaction_commit(struct ref_store *ref_store,
 
 cleanup:
 	files_transaction_cleanup(transaction);
-	strbuf_release(&sb);
 
 	for (i = 0; i < transaction->nr; i++) {
 		struct ref_update *update = transaction->updates[i];
@@ -3039,13 +3068,19 @@ cleanup:
 		}
 	}
 
+	strbuf_release(&sb);
 	string_list_clear(&refs_to_delete, 0);
-	free(head_ref);
-	string_list_clear(&affected_refnames, 0);
-
 	return ret;
 }
 
+static int files_transaction_abort(struct ref_store *ref_store,
+				   struct ref_transaction *transaction,
+				   struct strbuf *err)
+{
+	files_transaction_cleanup(transaction);
+	return 0;
+}
+
 static int ref_present(const char *refname,
 		       const struct object_id *oid, int flags, void *cb_data)
 {
@@ -3316,7 +3351,9 @@ struct ref_storage_be refs_be_files = {
 	"files",
 	files_ref_store_create,
 	files_init_db,
-	files_transaction_commit,
+	files_transaction_prepare,
+	files_transaction_finish,
+	files_transaction_abort,
 	files_initial_transaction_commit,
 
 	files_pack_refs,