block: Use blk_make_empty() after commits

bdrv_commit() already has a BlockBackend pointing to the BDS that we
want to empty, it just has the wrong permissions.

qemu-img commit has no BlockBackend pointing to the old backing file
yet, but introducing one is simple.

After this commit, bdrv_make_empty() is the only remaining caller of
BlockDriver.bdrv_make_empty().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200429141126.85159-5-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[kwolf: Fixed up reference output for 098]
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 9 insertions, 7 deletions

diff --git a/block/commit.c b/block/commit.c
index 87f6096d90..ba60fb7955 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -414,7 +414,9 @@ int bdrv_commit(BlockDriverState *bs)
     }
 
     ctx = bdrv_get_aio_context(bs);
-    src = blk_new(ctx, BLK_PERM_CONSISTENT_READ, BLK_PERM_ALL);
+    /* WRITE_UNCHANGED is required for bdrv_make_empty() */
+    src = blk_new(ctx, BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED,
+                  BLK_PERM_ALL);
     backing = blk_new(ctx, BLK_PERM_WRITE | BLK_PERM_RESIZE, BLK_PERM_ALL);
 
     ret = blk_insert_bs(src, bs, &local_err);
@@ -492,14 +494,14 @@ int bdrv_commit(BlockDriverState *bs)
         }
     }
 
-    if (drv->bdrv_make_empty) {
-        ret = drv->bdrv_make_empty(bs);
-        if (ret < 0) {
-            goto ro_cleanup;
-        }
-        blk_flush(src);
+    ret = blk_make_empty(src, NULL);
+    /* Ignore -ENOTSUP */
+    if (ret < 0 && ret != -ENOTSUP) {
+        goto ro_cleanup;
     }
 
+    blk_flush(src);
+
     /*
      * Make sure all data we wrote to the backing device is actually
      * stable on disk.