performance-metrics: Add sync drain completions helper

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

performance-metrics/src/util.rs

@@ -8,6 +8,7 @@ use std::io::{ErrorKind, Seek, SeekFrom, Write};
 use std::thread;
 use std::time::Duration;
 
+use block::async_io::AsyncIo;
 use block::qcow::{QcowFile, RawFile};
 use block::qcow_async::QcowDiskAsync;
 use block::qcow_sync::QcowDiskSync;
@@ -65,6 +66,13 @@ pub fn qcow_async_tempfile(num_clusters: usize) -> (TempFile, QcowDiskAsync) {
     (tmp, disk)
 }
 
+/// Drain `count` completions from a synchronous async_io backend.
+pub fn drain_completions(async_io: &mut dyn AsyncIo, count: usize) {
+    for _ in 0..count {
+        async_io.next_completed_request();
+    }
+}
+
 /// Spin and wait until the given eventfd becomes readable.
 pub fn wait_for_eventfd(notifier: &EventFd) {
     loop {