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, 14 insertions, 12 deletions

diff --git a/block/backup.c b/block/backup.c
index ae1b99aa84..ea38733849 100644
--- a/block/backup.c
+++ b/block/backup.c
@@ -543,7 +543,7 @@ static const BlockJobDriver backup_job_driver = {
     .drain                  = backup_drain,
 };
 
-void backup_start(const char *job_id, BlockDriverState *bs,
+BlockJob *backup_job_create(const char *job_id, BlockDriverState *bs,
                   BlockDriverState *target, int64_t speed,
                   MirrorSyncMode sync_mode, BdrvDirtyBitmap *sync_bitmap,
                   bool compress,
@@ -563,52 +563,52 @@ void backup_start(const char *job_id, BlockDriverState *bs,
 
     if (bs == target) {
         error_setg(errp, "Source and target cannot be the same");
-        return;
+        return NULL;
     }
 
     if (!bdrv_is_inserted(bs)) {
         error_setg(errp, "Device is not inserted: %s",
                    bdrv_get_device_name(bs));
-        return;
+        return NULL;
     }
 
     if (!bdrv_is_inserted(target)) {
         error_setg(errp, "Device is not inserted: %s",
                    bdrv_get_device_name(target));
-        return;
+        return NULL;
     }
 
     if (compress && target->drv->bdrv_co_pwritev_compressed == NULL) {
         error_setg(errp, "Compression is not supported for this drive %s",
                    bdrv_get_device_name(target));
-        return;
+        return NULL;
     }
 
     if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP_SOURCE, errp)) {
-        return;
+        return NULL;
     }
 
     if (bdrv_op_is_blocked(target, BLOCK_OP_TYPE_BACKUP_TARGET, errp)) {
-        return;
+        return NULL;
     }
 
     if (sync_mode == MIRROR_SYNC_MODE_INCREMENTAL) {
         if (!sync_bitmap) {
             error_setg(errp, "must provide a valid bitmap name for "
                              "\"incremental\" sync mode");
-            return;
+            return NULL;
         }
 
         /* Create a new bitmap, and freeze/disable this one. */
         if (bdrv_dirty_bitmap_create_successor(bs, sync_bitmap, errp) < 0) {
-            return;
+            return NULL;
         }
     } else if (sync_bitmap) {
         error_setg(errp,
                    "a sync_bitmap was provided to backup_run, "
                    "but received an incompatible sync_mode (%s)",
                    MirrorSyncMode_lookup[sync_mode]);
-        return;
+        return NULL;
     }
 
     len = bdrv_getlength(bs);
@@ -655,8 +655,8 @@ void backup_start(const char *job_id, BlockDriverState *bs,
     block_job_add_bdrv(&job->common, target);
     job->common.len = len;
     block_job_txn_add_job(txn, &job->common);
-    block_job_start(&job->common);
-    return;
+
+    return &job->common;
 
  error:
     if (sync_bitmap) {
@@ -666,4 +666,6 @@ void backup_start(const char *job_id, BlockDriverState *bs,
         backup_clean(&job->common);
         block_job_unref(&job->common);
     }
+
+    return NULL;
 }