ipn/{ipnserver,localapi}: fix InUseOtherUser handling with WatchIPNBus

Updates tailscale/corp#8222

Change-Id: I2d6fa6514c7b8d0f89fded35a2d44e7df27e6fb1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server_test.go

@@ -0,0 +1,47 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ipnserver
+
+import (
+	"context"
+	"sync"
+	"testing"
+)
+
+func TestWaiterSet(t *testing.T) {
+	var s waiterSet
+
+	wantLen := func(want int, when string) {
+		t.Helper()
+		if got := len(s); got != want {
+			t.Errorf("%s: len = %v; want %v", when, got, want)
+		}
+	}
+	wantLen(0, "initial")
+	var mu sync.Mutex
+	ctx, cancel := context.WithCancel(context.Background())
+
+	ready, cleanup := s.add(&mu, ctx)
+	wantLen(1, "after add")
+
+	select {
+	case <-ready:
+		t.Fatal("should not be ready")
+	default:
+	}
+	s.wakeAll()
+	<-ready
+
+	wantLen(1, "after fire")
+	cleanup()
+	wantLen(0, "after cleanup")
+
+	// And again but on an already-expired ctx.
+	cancel()
+	ready, cleanup = s.add(&mu, ctx)
+	<-ready // shouldn't block
+	cleanup()
+	wantLen(0, "at end")
+}