stream: complete early if end of backing file is reached

It is possible to create an image that is larger than its backing file.
Reading beyond the end of the backing file produces zeroes if no writes
have been made to those sectors in the image file.

This patch finishes streaming early when the end of the backing file is
reached.  Without this patch the block job hangs and continually tries
to stream the first sectors beyond the end of the backing file.

To reproduce the hung block job bug:

  $ qemu-img create -f qcow2 backing.qcow2 128M
  $ qemu-img create -f qcow2 -o backing_file=backing.qcow2 image.qcow2 6G
  $ qemu -drive if=virtio,cache=none,file=image.qcow2
  (qemu) block_stream virtio0
  (qemu) info block-jobs

The qemu-iotests 030 streaming test still passes.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block/stream.c b/block/stream.c
index 37c46525d2e5bcc5a1cbc3a9ab7a9d5c059cfcef..c4f87dd5b6f450fd1d0295e95b38f4614483beba 100644 (file)
--- a/block/stream.c
+++ b/block/stream.c
@@ -122,6 +122,12 @@ wait:
              * known-unallocated area [sector_num, sector_num+n).  */
             ret = bdrv_co_is_allocated_above(bs->backing_hd, base,
                                              sector_num, n, &n);
+
+            /* Finish early if end of backing file has been reached */
+            if (ret == 0 && n == 0) {
+                n = end - sector_num;
+            }
+
             copy = (ret == 1);
         }
         trace_stream_one_iteration(s, sector_num, n, ret);