ssh: synchronize unexpected response test

Wait for an ordered channel request after sending the unsolicited
response flood. This ensures that the client mux has discarded the
entire flood before SendRequest opens its response gate, avoiding
confusion between an in-flight unsolicited response and the expected
reply.

This fixes TestChannelUnexpectedResponsesDiscarded from flaking, where
otherwise running 1000000 iterations of the test with GOMAXPROCS=16
produced ~15 distinct failures of the form  "expected success reply".

Change-Id: Ic1125c1f44bd919a5cf32b889d331bbc94c06c25
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/817140
Reviewed-by: Neal Patel <neal@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: David Chase <drchase@google.com>

ssh/mux_test.go

@@ -1181,6 +1181,13 @@ func TestChannelUnexpectedResponsesDiscarded(t *testing.T) {
 				return
 			}
 		}
+		// Follow the flood with a request so the client can synchronize with
+		// its mux loop. Once the client receives this request, the mux loop has
+		// necessarily processed (and discarded) all the preceding responses.
+		if _, err := serverCh.SendRequest("sync", false, nil); err != nil {
+			done <- fmt.Errorf("send sync request: %w", err)
+			return
+		}
 		// Echo any legitimate request back.
 		for req := range serverCh.incomingRequests {
 			if req.WantReply {
@@ -1193,9 +1200,17 @@ func TestChannelUnexpectedResponsesDiscarded(t *testing.T) {
 		done <- nil
 	}()
 
+	req, ok := <-clientCh.incomingRequests
+	if !ok {
+		t.Fatal("channel closed before sync request")
+	}
+	if req.Type != "sync" {
+		t.Fatalf("unexpected sync request type %q", req.Type)
+	}
+
 	// If the flood had wedged the mux loop, this SendRequest would never
 	// receive a reply.
-	ok, err := clientCh.SendRequest("ping", true, []byte("hello"))
+	ok, err = clientCh.SendRequest("ping", true, []byte("hello"))
 	if err != nil {
 		t.Fatalf("SendRequest: %v", err)
 	}