blockjob: refactor backup_start as backup_job_create

Refactor backup_start as backup_job_create, which only creates the job,
but does not automatically start it. The old interface, 'backup_start',
is not kept in favor of limiting the number of nearly-identical interfaces
that would have to be edited to keep up with QAPI changes in the future.

Callers that wish to synchronously start the backup_block_job can
instead just call block_job_start immediately after calling
backup_job_create.

Transactions are updated to use the new interface, calling block_job_start
only during the .commit phase, which helps prevent race conditions where
jobs may finish before we even finish building the transaction. This may
happen, for instance, during empty block backup jobs.

Reported-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1478587839-9834-6-git-send-email-jsnow@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>

1 files changed, 12 insertions, 11 deletions

diff --git a/include/block/block_int.h b/include/block/block_int.h
index b02abbd..83a423c 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -748,7 +748,7 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
                   bool unmap, Error **errp);
 
 /*
- * backup_start:
+ * backup_job_create:
  * @job_id: The id of the newly-created job, or %NULL to use the
  * device name of @bs.
  * @bs: Block device to operate on.
@@ -764,18 +764,19 @@ void mirror_start(const char *job_id, BlockDriverState *bs,
  * @opaque: Opaque pointer value passed to @cb.
  * @txn: Transaction that this job is part of (may be NULL).
  *
- * Start a backup operation on @bs.  Clusters in @bs are written to @target
+ * Create a backup operation on @bs.  Clusters in @bs are written to @target
  * until the job is cancelled or manually completed.
  */
-void backup_start(const char *job_id, BlockDriverState *bs,
-                  BlockDriverState *target, int64_t speed,
-                  MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
-                  bool compress,
-                  BlockdevOnError on_source_error,
-                  BlockdevOnError on_target_error,
-                  int creation_flags,
-                  BlockCompletionFunc *cb, void *opaque,
-                  BlockJobTxn *txn, Error **errp);
+BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
+                            BlockDriverState *target, int64_t speed,
+                            MirrorSyncMode sync_mode,
+                            BdrvDirtyBitmap *sync_bitmap,
+                            bool compress,
+                            BlockdevOnError on_source_error,
+                            BlockdevOnError on_target_error,
+                            int creation_flags,
+                            BlockCompletionFunc *cb, void *opaque,
+                            BlockJobTxn *txn, Error **errp);
 
 void hmp_drive_add_node(Monitor *mon, const char *optstr);