ipn/ipnserver: document potential race, start on test for it

ipn/ipnserver/server.go

@@ -116,7 +116,7 @@ func Run(rctx context.Context, logf logger.Logf, logid string, opts Options, e w
 
 	var s net.Conn
 	serverToClient := func(b []byte) {
-		if s != nil {
+		if s != nil { // TODO: racy access to s?
 			ipn.WriteMsg(s, b)
 		}
 	}

ipn/ipnserver/server_test.go

@@ -0,0 +1,77 @@
+// Copyright (c) 2020 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_test
+
+import (
+	"context"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"tailscale.com/ipn"
+	"tailscale.com/ipn/ipnserver"
+	"tailscale.com/safesocket"
+	"tailscale.com/wgengine"
+)
+
+func TestRunMultipleAccepts(t *testing.T) {
+	t.Skipf("TODO(bradfitz): finish this test, once other fires are out")
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	td, err := ioutil.TempDir("", "TestRunMultipleAccepts")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(td)
+	socketPath := filepath.Join(td, "tailscale.sock")
+
+	logf := func(format string, args ...interface{}) {
+		format = strings.TrimRight(format, "\n")
+		println(fmt.Sprintf(format, args...))
+		t.Logf(format, args...)
+	}
+
+	connect := func() {
+		for i := 1; i <= 2; i++ {
+			logf("connect %d ...", i)
+			c, err := safesocket.Connect(socketPath, 0)
+			if err != nil {
+				t.Fatalf("safesocket.Connect: %v\n", err)
+			}
+			clientToServer := func(b []byte) {
+				ipn.WriteMsg(c, b)
+			}
+			bc := ipn.NewBackendClient(logf, clientToServer)
+			prefs := ipn.NewPrefs()
+			bc.SetPrefs(prefs)
+			c.Close()
+		}
+	}
+
+	logTriggerTestf := func(format string, args ...interface{}) {
+		logf(format, args...)
+		if strings.HasPrefix(format, "Listening on ") {
+			connect()
+		}
+	}
+
+	eng, err := wgengine.NewFakeUserspaceEngine(logf, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer eng.Close()
+
+	opts := ipnserver.Options{
+		SocketPath: socketPath,
+	}
+	t.Logf("pre-Run")
+	err = ipnserver.Run(ctx, logTriggerTestf, "dummy_logid", opts, eng)
+	t.Logf("ipnserver.Run = %v", err)
+}