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, 35 insertions, 10 deletions

diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 95edf6f23..4d3dd17f9 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -185,17 +185,27 @@ struct ref_update *ref_transaction_add_update(
 
 /*
  * Transaction states.
- * OPEN:   The transaction is in a valid state and can accept new updates.
- *         An OPEN transaction can be committed.
- * CLOSED: A closed transaction is no longer active and no other operations
- *         than free can be used on it in this state.
- *         A transaction can either become closed by successfully committing
- *         an active transaction or if there is a failure while building
- *         the transaction thus rendering it failed/inactive.
+ *
+ * OPEN:   The transaction is initialized and new updates can still be
+ *         added to it. An OPEN transaction can be prepared,
+ *         committed, freed, or aborted (freeing and aborting an open
+ *         transaction are equivalent).
+ *
+ * PREPARED: ref_transaction_prepare(), which locks all of the
+ *         references involved in the update and checks that the
+ *         update has no errors, has been called successfully for the
+ *         transaction. A PREPARED transaction can be committed or
+ *         aborted.
+ *
+ * CLOSED: The transaction is no longer active. A transaction becomes
+ *         CLOSED if there is a failure while building the transaction
+ *         or if a transaction is committed or aborted. A CLOSED
+ *         transaction can only be freed.
  */
 enum ref_transaction_state {
-	REF_TRANSACTION_OPEN   = 0,
-	REF_TRANSACTION_CLOSED = 1
+	REF_TRANSACTION_OPEN     = 0,
+	REF_TRANSACTION_PREPARED = 1,
+	REF_TRANSACTION_CLOSED   = 2
 };
 
 /*
@@ -497,6 +507,18 @@ typedef struct ref_store *ref_store_init_fn(const char *gitdir,
 
 typedef int ref_init_db_fn(struct ref_store *refs, struct strbuf *err);
 
+typedef int ref_transaction_prepare_fn(struct ref_store *refs,
+				       struct ref_transaction *transaction,
+				       struct strbuf *err);
+
+typedef int ref_transaction_finish_fn(struct ref_store *refs,
+				      struct ref_transaction *transaction,
+				      struct strbuf *err);
+
+typedef int ref_transaction_abort_fn(struct ref_store *refs,
+				     struct ref_transaction *transaction,
+				     struct strbuf *err);
+
 typedef int ref_transaction_commit_fn(struct ref_store *refs,
 				      struct ref_transaction *transaction,
 				      struct strbuf *err);
@@ -600,7 +622,10 @@ struct ref_storage_be {
 	const char *name;
 	ref_store_init_fn *init;
 	ref_init_db_fn *init_db;
-	ref_transaction_commit_fn *transaction_commit;
+
+	ref_transaction_prepare_fn *transaction_prepare;
+	ref_transaction_finish_fn *transaction_finish;
+	ref_transaction_abort_fn *transaction_abort;
 	ref_transaction_commit_fn *initial_transaction_commit;
 
 	pack_refs_fn *pack_refs;