Fix a test flake (#1626)

Avoid a race in a test of read only downstairs that can connect before
or after IO is sent.  We avoid this race by waiting for all downstairs we
expect to be present to show up before we send the IO.

Fixes #1623

upstairs/src/dummy_downstairs_tests.rs

@@ -2988,14 +2988,34 @@ async fn test_ro_activate_from_list(activate: [bool; 3]) {
         guest,
     };
 
+    // Wait for all downstairs we expect to be active to actually be active.
+    // If we don't do this, it's possible for a downstairs to join after
+    // we send the IO, which will cause the read to be skipped, and we
+    // will end up acking the following flush.
+    loop {
+        tokio::time::sleep(Duration::from_secs(1)).await;
+        let ds = harness.guest.downstairs_state().await.unwrap();
+        if activate[0] && ds[ClientId::new(0)] != DsState::Active {
+            continue;
+        }
+        if activate[1] && ds[ClientId::new(1)] != DsState::Active {
+            continue;
+        }
+        if activate[2] && ds[ClientId::new(2)] != DsState::Active {
+            continue;
+        }
+        break;
+    }
+
     // We must `spawn` here because `read` will wait for the response to
     // come back before returning
     let h = harness.spawn(|guest| async move {
         let mut buffer = Buffer::new(1, 512);
         guest.read(BlockIndex(0), &mut buffer).await.unwrap();
     });
 
-    // Ack the read on the downstairs that are active.
+    // Ack the read on the downstairs that are active, but, only if a
+    // downstairs has actually joined.
     if activate[0] {
         harness.ds1().ack_read().await;
     }