control/controlclient: wait for c1 to receive a netmap.

This strictly sequences things such that c1 is fully registered in
the control server before c2 creates its poll. Failure to do this
can cause an inversion where c2's poll finishes establishing
before c1's poll starts, which results in c2 getting disconnected
rather than c1, and the test times out waiting for c1 to get kicked.

Fixes #98.

Signed-off-by: David Anderson <danderson@tailscale.com>

control/controlclient/direct_test.go

@@ -94,15 +94,24 @@ func TestClientsReusingKeys(t *testing.T) {
 	// test. We're expecting it to block happily, invoking the no-op
 	// update function periodically, then exit once c2 starts its own
 	// poll below.
+	gotNetmap := make(chan struct{}, 1)
 	pollErrCh := make(chan error)
 	go func() {
-		pollErrCh <- c1.PollNetMap(ctx, -1, func(netMap *NetworkMap) {})
+		pollErrCh <- c1.PollNetMap(ctx, -1, func(netMap *NetworkMap) {
+			select {
+			case gotNetmap <- struct{}{}:
+			default:
+			}
+		})
 	}()
 
 	select {
+	case <-gotNetmap:
+		t.Logf("c1: received initial netmap")
 	case err := <-pollErrCh:
 		t.Fatal(err)
-	default:
+	case <-time.After(5 * time.Second):
+		t.Fatal("c1 did not receive an initial netmap")
 	}
 
 	// Connect c2, reusing c1's credentials. In other words, c2 *is*