ipn/ipn{server,test}: extract the LocalAPI test client and server into ipntest

In this PR, we extract the in-process LocalAPI client/server implementation from ipn/ipnserver/server_test.go
into a new ipntest package to be used in high‑level black‑box tests, such as those for the tailscale CLI.

Updates #15575

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/lapitest/example_test.go

@@ -0,0 +1,80 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lapitest
+
+import (
+	"context"
+	"testing"
+
+	"tailscale.com/ipn"
+)
+
+func TestClientServer(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+
+	// Create a server and two clients.
+	// Both clients represent the same user to make this work across platforms.
+	// On Windows we've been restricting the API usage to a single user at a time.
+	// While we're planning on changing this once a better permission model is in place,
+	// this test is currently limited to a single user (but more than one client is fine).
+	// Alternatively, we could override GOOS via envknobs to test as if we're
+	// on a different platform, but that would make the test depend on global state, etc.
+	s := NewServer(t, WithLogging(false))
+	c1 := s.ClientWithName("User-A")
+	c2 := s.ClientWithName("User-A")
+
+	// Start watching the IPN bus as the second client.
+	w2, _ := c2.WatchIPNBus(context.Background(), ipn.NotifyInitialPrefs)
+
+	// We're supposed to get a notification about the initial prefs,
+	// and WantRunning should be false.
+	n, err := w2.Next()
+	for ; err == nil; n, err = w2.Next() {
+		if n.Prefs == nil {
+			// Ignore non-prefs notifications.
+			continue
+		}
+		if n.Prefs.WantRunning() {
+			t.Errorf("WantRunning(initial): got %v, want false", n.Prefs.WantRunning())
+		}
+		break
+	}
+	if err != nil {
+		t.Fatalf("IPNBusWatcher.Next failed: %v", err)
+	}
+
+	// Now send an EditPrefs request from the first client to set WantRunning to true.
+	change := &ipn.MaskedPrefs{Prefs: ipn.Prefs{WantRunning: true}, WantRunningSet: true}
+	gotPrefs, err := c1.EditPrefs(ctx, change)
+	if err != nil {
+		t.Fatalf("EditPrefs failed: %v", err)
+	}
+	if !gotPrefs.WantRunning {
+		t.Fatalf("EditPrefs.WantRunning: got %v, want true", gotPrefs.WantRunning)
+	}
+
+	// We can check the backend directly to see if the prefs were set correctly.
+	if gotWantRunning := s.Backend().Prefs().WantRunning(); !gotWantRunning {
+		t.Fatalf("Backend.Prefs.WantRunning: got %v, want true", gotWantRunning)
+	}
+
+	// And can also wait for the second client with an IPN bus watcher to receive the notification
+	// about the prefs change.
+	n, err = w2.Next()
+	for ; err == nil; n, err = w2.Next() {
+		if n.Prefs == nil {
+			// Ignore non-prefs notifications.
+			continue
+		}
+		if !n.Prefs.WantRunning() {
+			t.Fatalf("WantRunning(changed): got %v, want true", n.Prefs.WantRunning())
+		}
+		break
+	}
+	if err != nil {
+		t.Fatalf("IPNBusWatcher.Next failed: %v", err)
+	}
+}