block: simplify failure handling for bdrv_aio_multiwrite

Now that early failure of bdrv_aio_writev is not possible anymore,
mcb->num_requests can be set before the loop starts.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -2842,37 +2842,13 @@ int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
 
     trace_bdrv_aio_multiwrite(mcb, mcb->num_callbacks, num_reqs);
 
-    /*
-     * Run the aio requests. As soon as one request can't be submitted
-     * successfully, fail all requests that are not yet submitted (we must
-     * return failure for all requests anyway)
-     *
-     * num_requests cannot be set to the right value immediately: If
-     * bdrv_aio_writev fails for some request, num_requests would be too high
-     * and therefore multiwrite_cb() would never recognize the multiwrite
-     * request as completed. We also cannot use the loop variable i to set it
-     * when the first request fails because the callback may already have been
-     * called for previously submitted requests. Thus, num_requests must be
-     * incremented for each request that is submitted.
-     *
-     * The problem that callbacks may be called early also means that we need
-     * to take care that num_requests doesn't become 0 before all requests are
-     * submitted - multiwrite_cb() would consider the multiwrite request
-     * completed. A dummy request that is "completed" by a manual call to
-     * multiwrite_cb() takes care of this.
-     */
-    mcb->num_requests = 1;
-
-    // Run the aio requests
+    /* Run the aio requests. */
+    mcb->num_requests = num_reqs;
     for (i = 0; i < num_reqs; i++) {
-        mcb->num_requests++;
         bdrv_aio_writev(bs, reqs[i].sector, reqs[i].qiov,
             reqs[i].nb_sectors, multiwrite_cb, mcb);
     }
 
-    /* Complete the dummy request */
-    multiwrite_cb(mcb, 0);
-
     return 0;
 }