[PATCH v3 1/6] bulk-checkin: remove ODB transaction nesting

[Justin Tobler <jltobler@gmail.com>]

ODB transactions support being nested. Only the outermost
{begin,end}_odb_transaction() start and finish a transaction. This
allows internal object write codepaths to be optimized with ODB
transactions without worrying about whether a transaction is already
active. When {begin,end}_odb_transaction() is invoked during an active
transaction, these operations are essentially treated as no-ops. This
can make the interface a bit awkward to use, as calling
end_odb_transaction() does not guarantee that a transaction is actually
ended. Thus, in situations where a transaction needs to be explicitly
flushed, flush_odb_transaction() must be used.

To remove the need for an explicit transaction flush operation via
flush_odb_transaction() and better clarify transaction semantics, drop
the transaction nesting mechanism in favor of begin_odb_transaction()
returning a NULL transaction value to signal it was a no-op, and
end_odb_transaction() behaving as a no-op when a NULL transaction value
is passed. This is safe for existing callers as the transaction value
wired to end_odb_transaction() already comes from
begin_odb_transaction() and thus continues the same no-op behavior when
a transaction is already pending. With this model, passing a pending
transaction to end_odb_transaction() ensures it is committed at that
point in time.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
---
 bulk-checkin.c | 22 ++++++++++------------
 bulk-checkin.h |  8 +++-----
 object-file.c  |  2 +-
 3 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/bulk-checkin.c b/bulk-checkin.c
index 124c493067..eb6ef704c3 100644
--- a/bulk-checkin.c
+++ b/bulk-checkin.c
@@ -33,7 +33,6 @@ struct bulk_checkin_packfile {
 struct odb_transaction {
 	struct object_database *odb;
 
-	int nesting;
 	struct tmp_objdir *objdir;
 	struct bulk_checkin_packfile packfile;
 };
@@ -368,12 +367,11 @@ void fsync_loose_object_bulk_checkin(struct odb_transaction *transaction,
 
 struct odb_transaction *begin_odb_transaction(struct object_database *odb)
 {
-	if (!odb->transaction) {
-		CALLOC_ARRAY(odb->transaction, 1);
-		odb->transaction->odb = odb;
-	}
+	if (odb->transaction)
+		return NULL;
 
-	odb->transaction->nesting += 1;
+	CALLOC_ARRAY(odb->transaction, 1);
+	odb->transaction->odb = odb;
 
 	return odb->transaction;
 }
@@ -389,14 +387,14 @@ void flush_odb_transaction(struct odb_transaction *transaction)
 
 void end_odb_transaction(struct odb_transaction *transaction)
 {
-	if (!transaction || transaction->nesting == 0)
-		BUG("Unbalanced ODB transaction nesting");
-
-	transaction->nesting -= 1;
-
-	if (transaction->nesting)
+	if (!transaction)
 		return;
 
+	/*
+	 * Ensure the transaction ending matches the pending transaction.
+	 */
+	ASSERT(transaction == transaction->odb->transaction);
+
 	flush_odb_transaction(transaction);
 	transaction->odb->transaction = NULL;
 	free(transaction);
diff --git a/bulk-checkin.h b/bulk-checkin.h
index ac8887f476..51d0ac6134 100644
--- a/bulk-checkin.h
+++ b/bulk-checkin.h
@@ -38,9 +38,8 @@ int index_blob_bulk_checkin(struct odb_transaction *transaction,
 /*
  * Tell the object database to optimize for adding
  * multiple objects. end_odb_transaction must be called
- * to make new objects visible. Transactions can be nested,
- * and objects are only visible after the outermost transaction
- * is complete or the transaction is flushed.
+ * to make new objects visible. If a transaction is already
+ * pending, NULL is returned.
  */
 struct odb_transaction *begin_odb_transaction(struct object_database *odb);
 
@@ -53,8 +52,7 @@ void flush_odb_transaction(struct odb_transaction *transaction);
 
 /*
  * Tell the object database to make any objects from the
- * current transaction visible if this is the final nested
- * transaction.
+ * current transaction visible.
  */
 void end_odb_transaction(struct odb_transaction *transaction);
 
diff --git a/object-file.c b/object-file.c
index bc15af4245..5e76573549 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1267,7 +1267,7 @@ int index_fd(struct index_state *istate, struct object_id *oid,
 		struct odb_transaction *transaction;
 
 		transaction = begin_odb_transaction(the_repository->objects);
-		ret = index_blob_bulk_checkin(transaction,
+		ret = index_blob_bulk_checkin(the_repository->objects->transaction,
 					      oid, fd, xsize_t(st->st_size),
 					      path, flags);
 		end_odb_transaction(transaction);
-- 
2.51.0.193.g4975ec3473b