block-backend: add drained_poll

Allow block backends to poll their devices/users to check if they have
been quiesced when entering a drained section.

This will be used in the next patch to wait for the NBD server to be
completely quiesced.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Sergio Lopez <slp@redhat.com>
Message-Id: <20210602060552.17433-2-slp@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/block-backend.c

@@ -2386,8 +2386,13 @@ static void blk_root_drained_begin(BdrvChild *child)
 static bool blk_root_drained_poll(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
+    bool busy = false;
     assert(blk->quiesce_counter);
-    return !!blk->in_flight;
+
+    if (blk->dev_ops && blk->dev_ops->drained_poll) {
+        busy = blk->dev_ops->drained_poll(blk->dev_opaque);
+    }
+    return busy || !!blk->in_flight;
 }
 
 static void blk_root_drained_end(BdrvChild *child, int *drained_end_counter)

include/sysemu/block-backend.h

@@ -66,6 +66,10 @@ typedef struct BlockDevOps {
      * Runs when the backend's last drain request ends.
      */
     void (*drained_end)(void *opaque);
+    /*
+     * Is the device still busy?
+     */
+    bool (*drained_poll)(void *opaque);
 } BlockDevOps;
 
 /* This struct is embedded in (the private) BlockBackend struct and contains