block/backup: Fix hang for unaligned image size

When doing a block backup of an image with an unaligned size (with
respect to the BACKUP_CLUSTER_SIZE), qemu would check the allocation
status of sectors after the end of the image. bdrv_is_allocated()
returns a result that is valid for 0 sectors in this case, so the backup
job ran into an endless loop.

Stop looping when seeing a result valid for 0 sectors, we're at EOF then.

The test case looks somewhat unrelated at first sight because I
originally tried to reproduce a different suspected bug that turned out
to not exist. Still a good test case and it accidentally found this one.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

block/backup.c

@@ -307,7 +307,7 @@ static void coroutine_fn backup_run(void *opaque)
                                 BACKUP_SECTORS_PER_CLUSTER - i, &n);
                     i += n;
 
-                    if (alloced == 1) {
+                    if (alloced == 1 || n == 0) {
                         break;
                     }
                 }

tests/qemu-iotests/028

@@ -33,6 +33,7 @@ status=1	# failure is the default!
 
 _cleanup()
 {
+    rm -f "${TEST_IMG}.copy"
     _cleanup_test_img
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15
@@ -41,6 +42,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 . ./common.rc
 . ./common.filter
 . ./common.pattern
+. ./common.qemu
 
 # Any format supporting backing files except vmdk and qcow which do not support
 # smaller backing files.
@@ -99,6 +101,29 @@ _check_test_img
 # Rebase it on top of its base image
 $QEMU_IMG rebase -b "$TEST_IMG.base" "$TEST_IMG"
 
+echo
+echo block-backup
+echo
+
+qemu_comm_method="monitor"
+_launch_qemu -drive file="${TEST_IMG}",cache=${CACHEMODE},id=disk
+h=$QEMU_HANDLE
+QEMU_COMM_TIMEOUT=1
+
+_send_qemu_cmd $h "drive_backup disk ${TEST_IMG}.copy" "(qemu)"
+qemu_cmd_repeat=20 _send_qemu_cmd $h "info block-jobs" "No active jobs"
+_send_qemu_cmd $h 'quit' ""
+
+# Base image sectors
+TEST_IMG="${TEST_IMG}.copy" io readv $(( offset )) 512 1024 32
+
+# Image sectors
+TEST_IMG="${TEST_IMG}.copy" io readv $(( offset + 512 )) 512 1024 64
+
+# Zero sectors beyond end of base image
+TEST_IMG="${TEST_IMG}.copy" io_zero readv $(( offset + 32 * 1024 )) 512 1024 32
+
+
 _check_test_img
 
 # success, all done