block: Introduce "drained begin/end" API

The semantics is that after bdrv_drained_begin(bs), bs will not get new external
requests until the matching bdrv_drained_end(bs).
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/io.c

@@ -2624,3 +2624,20 @@ void bdrv_flush_io_queue(BlockDriverState *bs)
     }
     bdrv_start_throttled_reqs(bs);
 }
+
+void bdrv_drained_begin(BlockDriverState *bs)
+{
+    if (!bs->quiesce_counter++) {
+        aio_disable_external(bdrv_get_aio_context(bs));
+    }
+    bdrv_drain(bs);
+}
+
+void bdrv_drained_end(BlockDriverState *bs)
+{
+    assert(bs->quiesce_counter > 0);
+    if (--bs->quiesce_counter > 0) {
+        return;
+    }
+    aio_enable_external(bdrv_get_aio_context(bs));
+}

include/block/block.h

@@ -610,4 +610,23 @@ void bdrv_io_plug(BlockDriverState *bs);
 void bdrv_io_unplug(BlockDriverState *bs);
 void bdrv_flush_io_queue(BlockDriverState *bs);
 
+/**
+ * bdrv_drained_begin:
+ *
+ * Begin a quiesced section for exclusive access to the BDS, by disabling
+ * external request sources including NBD server and device model. Note that
+ * this doesn't block timers or coroutines from submitting more requests, which
+ * means block_job_pause is still necessary.
+ *
+ * This function can be recursive.
+ */
+void bdrv_drained_begin(BlockDriverState *bs);
+
+/**
+ * bdrv_drained_end:
+ *
+ * End a quiescent section started by bdrv_drained_begin().
+ */
+void bdrv_drained_end(BlockDriverState *bs);
+
 #endif

include/block/block_int.h

@@ -448,6 +448,8 @@ struct BlockDriverState {
     /* threshold limit for writes, in bytes. "High water mark". */
     uint64_t write_threshold_offset;
     NotifierWithReturn write_threshold_notifier;
+
+    int quiesce_counter;
 };
 
 struct BlockBackendRootState {