block: Assert serialisation assumptions in pwritev

If a request calls wait_serialising_requests() and actually has to wait
in this function (i.e. a coroutine yield), other requests can run and
previously read data (like the head or tail buffer) could become
outdated. In this case, we would have to restart from the beginning to
read in the updated data.

However, we're lucky and don't actually need to do that: A request can
only wait in the first call of wait_serialising_requests() because we
mark it as serialising before that call, so any later requests would
wait. So as we don't wait in practice, we don't have to reload the data.

This is an important assumption that may not be broken or data
corruption will happen. Document it with some assertions.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>

block.c

@@ -2303,14 +2303,15 @@ static bool tracked_request_overlaps(BdrvTrackedRequest *req,
     return true;
 }
 
-static void coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
+static bool coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
 {
     BlockDriverState *bs = self->bs;
     BdrvTrackedRequest *req;
     bool retry;
+    bool waited = false;
 
     if (!bs->serialising_in_flight) {
-        return;
+        return false;
     }
 
     do {
@@ -2336,11 +2337,14 @@ static void coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
                     qemu_co_queue_wait(&req->wait_queue);
                     self->waiting_for = NULL;
                     retry = true;
+                    waited = true;
                     break;
                 }
             }
         }
     } while (retry);
+
+    return waited;
 }
 
 /*
@@ -3191,6 +3195,7 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
     QEMUIOVector *qiov, int flags)
 {
     BlockDriver *drv = bs->drv;
+    bool waited;
     int ret;
 
     int64_t sector_num = offset >> BDRV_SECTOR_BITS;
@@ -3199,7 +3204,8 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
     assert((offset & (BDRV_SECTOR_SIZE - 1)) == 0);
     assert((bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
 
-    wait_serialising_requests(req);
+    waited = wait_serialising_requests(req);
+    assert(!waited || !req->serialising);
 
     ret = notifier_with_return_list_notify(&bs->before_write_notifiers, req);
 
@@ -3299,9 +3305,11 @@ static int coroutine_fn bdrv_co_do_pwritev(BlockDriverState *bs,
         QEMUIOVector tail_qiov;
         struct iovec tail_iov;
         size_t tail_bytes;
+        bool waited;
 
         mark_request_serialising(&req, align);
-        wait_serialising_requests(&req);
+        waited = wait_serialising_requests(&req);
+        assert(!waited || !use_local_qiov);
 
         tail_buf = qemu_blockalign(bs, align);
         tail_iov = (struct iovec) {