block: add blk_in_drain() API

The BlockBackend quiesce_counter is greater than zero during drained
sections. Add an API to check whether the BlockBackend is in a drained
section.

The next patch will use this API.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-10-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/block-backend.c

@@ -1287,6 +1287,13 @@ blk_check_byte_request(BlockBackend *blk, int64_t offset, int64_t bytes)
     return 0;
 }
 
+/* Are we currently in a drained section? */
+bool blk_in_drain(BlockBackend *blk)
+{
+    GLOBAL_STATE_CODE(); /* change to IO_OR_GS_CODE(), if necessary */
+    return qatomic_read(&blk->quiesce_counter);
+}
+
 /* To be called between exactly one pair of blk_inc/dec_in_flight() */
 static void coroutine_fn blk_wait_while_drained(BlockBackend *blk)
 {

include/sysemu/block-backend-global-state.h

@@ -81,6 +81,7 @@ void blk_activate(BlockBackend *blk, Error **errp);
 int blk_make_zero(BlockBackend *blk, BdrvRequestFlags flags);
 void blk_aio_cancel(BlockAIOCB *acb);
 int blk_commit_all(void);
+bool blk_in_drain(BlockBackend *blk);
 void blk_drain(BlockBackend *blk);
 void blk_drain_all(void);
 void blk_set_on_error(BlockBackend *blk, BlockdevOnError on_read_error,