fix: wait for HTTP server serve() termination

server/serverimpl.go

@@ -19,17 +19,17 @@
 	serverTypes "github.com/open-telemetry/opamp-go/server/types"
 )
 
-var (
-	errAlreadyStarted = errors.New("already started")
+var errAlreadyStarted = errors.New("already started")
+
+const (
+	defaultOpAMPPath      = "/v1/opamp"
+	headerContentType     = "Content-Type"
+	headerContentEncoding = "Content-Encoding"
+	headerAcceptEncoding  = "Accept-Encoding"
+	contentEncodingGzip   = "gzip"
+	contentTypeProtobuf   = "application/x-protobuf"
 )
 
-const defaultOpAMPPath = "/v1/opamp"
-const headerContentType = "Content-Type"
-const headerContentEncoding = "Content-Encoding"
-const headerAcceptEncoding = "Accept-Encoding"
-const contentEncodingGzip = "gzip"
-const contentTypeProtobuf = "application/x-protobuf"
-
 type server struct {
 	logger   types.Logger
 	settings Settings
@@ -39,7 +39,8 @@
 
 	// The listening HTTP Server after successful Start() call. Nil if Start()
 	// is not called or was not successful.
-	httpServer *http.Server
+	httpServer        *http.Server
+	httpServerServeWg *sync.WaitGroup
 
 	// The network address Server is listening on. Nil if not started.
 	addr net.Addr
@@ -108,6 +109,9 @@
 		ConnContext: contextWithConn,
 	}
 	s.httpServer = hs
+	httpServerServeWg := sync.WaitGroup{}
+	httpServerServeWg.Add(1)
+	s.httpServerServeWg = &httpServerServeWg
 
 	listenAddr := s.httpServer.Addr
 
@@ -118,15 +122,21 @@
 		}
 		err = s.startHttpServer(
 			listenAddr,
-			func(l net.Listener) error { return hs.ServeTLS(l, "", "") },
+			func(l net.Listener) error {
+				defer httpServerServeWg.Done()
+				return hs.ServeTLS(l, "", "")
+			},
 		)
 	} else {
 		if listenAddr == "" {
 			listenAddr = ":http"
 		}
 		err = s.startHttpServer(
 			listenAddr,
-			func(l net.Listener) error { return hs.Serve(l) },
+			func(l net.Listener) error {
+				defer httpServerServeWg.Done()
+				return hs.Serve(l)
+			},
 		)
 	}
 	return err
@@ -159,7 +169,11 @@
 		defer func() { s.httpServer = nil }()
 		// This stops accepting new connections. TODO: close existing
 		// connections and wait them to be terminated.
-		return s.httpServer.Shutdown(ctx)
+		err := s.httpServer.Shutdown(ctx)
+		if err != nil {
+			return err
+		}
+		s.httpServerServeWg.Wait()
 	}
 	return nil
 }
@@ -366,7 +380,6 @@
 		w.Header().Set(headerContentEncoding, contentEncodingGzip)
 	}
 	_, err = w.Write(bodyBytes)
-
 	if err != nil {
 		s.logger.Debugf(req.Context(), "Cannot send HTTP response: %v", err)
 	}

server/serverimpl_test.go

@@ -57,6 +57,23 @@ func TestServerStartStop(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+func TestServerStartStopIdempotency(t *testing.T) {
+	endpoint := testhelpers.GetAvailableLocalAddress()
+	for i := 0; i < 10; i++ {
+		t.Run(fmt.Sprintf("Attempt #%d: ", i), func(t *testing.T) {
+			srv := startServer(t, &StartSettings{
+				ListenEndpoint: endpoint,
+			})
+
+			err := srv.Start(StartSettings{})
+			assert.ErrorIs(t, err, errAlreadyStarted)
+
+			err = srv.Stop(context.Background())
+			assert.NoError(t, err)
+		})
+	}
+}
+
 func TestServerStartStopWithMiddleware(t *testing.T) {
 	var addedMiddleware atomic.Bool
 	assert.False(t, addedMiddleware.Load())
@@ -620,7 +637,6 @@ func TestServerAttachSendMessagePlainHTTP(t *testing.T) {
 }
 
 func TestServerHonoursClientRequestContentEncoding(t *testing.T) {
-
 	hc := http.Client{}
 	var rcvMsg atomic.Value
 	var onConnectedCalled, onCloseCalled int32
@@ -698,7 +714,6 @@ func TestServerHonoursClientRequestContentEncoding(t *testing.T) {
 }
 
 func TestServerHonoursAcceptEncoding(t *testing.T) {
-
 	hc := http.Client{}
 	var rcvMsg atomic.Value
 	var onConnectedCalled, onCloseCalled int32
@@ -985,7 +1000,6 @@ func BenchmarkSendToClient(b *testing.B) {
 	}
 	srv := New(&sharedinternal.NopLogger{})
 	err := srv.Start(*settings)
-
 	if err != nil {
 		b.Error(err)
 	}
@@ -1017,7 +1031,6 @@ func BenchmarkSendToClient(b *testing.B) {
 
 	for _, conn := range serverConnections {
 		err := conn.Send(context.Background(), &protobufs.ServerToAgent{})
-
 		if err != nil {
 			b.Error(err)
 		}
@@ -1026,5 +1039,4 @@ func BenchmarkSendToClient(b *testing.B) {
 	for _, conn := range clientConnections {
 		conn.Close()
 	}
-
 }