mirror: Fix qiov size for short requests

When mirroring an image of a size that is not a multiple of the
mirror job granularity, the last request would have the right nb_sectors
argument, but a qiov that is rounded up to the next multiple of the
granularity. Don't do this.

This fixes a segfault that is caused by raw-posix being confused by this
and allocating a buffer with request length, but operating on it with
qiov length.

[s/Driver/Drive/ in qemu-iotests 041 as suggested by Eric
--Stefan]

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/mirror.c

@@ -265,9 +265,11 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
     next_sector = sector_num;
     while (nb_chunks-- > 0) {
         MirrorBuffer *buf = QSIMPLEQ_FIRST(&s->buf_free);
+        size_t remaining = (nb_sectors * BDRV_SECTOR_SIZE) - op->qiov.size;
+
         QSIMPLEQ_REMOVE_HEAD(&s->buf_free, next);
         s->buf_free_count--;
-        qemu_iovec_add(&op->qiov, buf, s->granularity);
+        qemu_iovec_add(&op->qiov, buf, MIN(s->granularity, remaining));
 
         /* Advance the HBitmapIter in parallel, so that we do not examine
          * the same sector twice.

tests/qemu-iotests/041

@@ -217,6 +217,11 @@ class TestSingleDriveZeroLength(TestSingleDrive):
     test_small_buffer2 = None
     test_large_cluster = None
 
+class TestSingleDriveUnalignedLength(TestSingleDrive):
+    image_len = 1025 * 1024
+    test_small_buffer2 = None
+    test_large_cluster = None
+
 class TestMirrorNoBacking(ImageMirroringTestCase):
     image_len = 2 * 1024 * 1024 # MB
 

tests/qemu-iotests/041.out

@@ -1,5 +1,5 @@
-..............................................
+......................................................
 ----------------------------------------------------------------------
-Ran 46 tests
+Ran 54 tests
 
 OK