block: add new BlockDriver handler: bdrv_cancel_in_flight

It will be used to stop retrying NBD requests on mirror cancel.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210205163720.887197-2-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/io.c

@@ -3460,3 +3460,14 @@ out:
 
     return ret;
 }
+
+void bdrv_cancel_in_flight(BlockDriverState *bs)
+{
+    if (!bs || !bs->drv) {
+        return;
+    }
+
+    if (bs->drv->bdrv_cancel_in_flight) {
+        bs->drv->bdrv_cancel_in_flight(bs);
+    }
+}

include/block/block.h

@@ -849,4 +849,7 @@ int coroutine_fn bdrv_co_copy_range(BdrvChild *src, int64_t src_offset,
                                     BdrvChild *dst, int64_t dst_offset,
                                     int64_t bytes, BdrvRequestFlags read_flags,
                                     BdrvRequestFlags write_flags);
+
+void bdrv_cancel_in_flight(BlockDriverState *bs);
+
 #endif

include/block/block_int.h

@@ -352,6 +352,15 @@ struct BlockDriver {
         bool want_zero, int64_t offset, int64_t bytes, int64_t *pnum,
         int64_t *map, BlockDriverState **file);
 
+    /*
+     * This informs the driver that we are no longer interested in the result
+     * of in-flight requests, so don't waste the time if possible.
+     *
+     * One example usage is to avoid waiting for an nbd target node reconnect
+     * timeout during job-cancel.
+     */
+    void (*bdrv_cancel_in_flight)(BlockDriverState *bs);
+
     /*
      * Invalidate any cached meta-data.
      */