GODT-1159 SMTP server not restarting after restored internet

- [x] write tests to check that IMAP and SMTP servers are closed when there
  is no internet
- [x] always create new go-smtp instance during listenAndServe(int)

internal/serverutil/mocks/server.go

@@ -0,0 +1,150 @@
+// Copyright (c) 2021 Proton Technologies AG
+//
+// This file is part of ProtonMail Bridge.
+//
+// ProtonMail Bridge is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// ProtonMail Bridge is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with ProtonMail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package mocks
+
+import (
+	"fmt"
+	"net/http"
+	"sync/atomic"
+	"time"
+
+	"github.com/ProtonMail/proton-bridge/internal/events"
+	"github.com/ProtonMail/proton-bridge/pkg/listener"
+	"github.com/ProtonMail/proton-bridge/pkg/ports"
+	"github.com/sirupsen/logrus"
+	"github.com/stretchr/testify/require"
+)
+
+type DummyPanicHandler struct{}
+
+func (ph *DummyPanicHandler) HandlePanic() {}
+
+type TestServer struct {
+	PanicHandler  *DummyPanicHandler
+	WantPort      int
+	EventListener listener.Listener
+
+	isRunning atomic.Value
+	srv       *http.Server
+}
+
+func NewTestServer(port int) *TestServer {
+	s := &TestServer{
+		PanicHandler:  &DummyPanicHandler{},
+		EventListener: listener.New(),
+		WantPort:      ports.FindFreePortFrom(port),
+	}
+	s.isRunning.Store(false)
+	return s
+}
+
+func (s *TestServer) IsPortFree() bool {
+	return true
+}
+
+func (s *TestServer) IsPortOccupied() bool {
+	return true
+}
+
+func (s *TestServer) Emit(event string, try, iEvt int) int {
+	// Emit has separate go routine so it is needed to wait here to
+	// prevent event race condition.
+	time.Sleep(100 * time.Millisecond)
+	iEvt++
+	s.EventListener.Emit(event, fmt.Sprintf("%d:%d", try, iEvt))
+	return iEvt
+}
+
+func (s *TestServer) HandlePanic()          {}
+func (s *TestServer) DisconnectUser(string) {}
+func (s *TestServer) Port() int             { return s.WantPort }
+func (s *TestServer) IsRunning() bool       { return s.isRunning.Load().(bool) }
+
+func (s *TestServer) ListenRetryAndServe(retries int, retryAfter time.Duration) {
+	if s.isRunning.Load().(bool) {
+		return
+	}
+	s.isRunning.Store(true)
+
+	// There can be delay when starting server
+	time.Sleep(200 * time.Millisecond)
+
+	s.srv = &http.Server{
+		Addr: fmt.Sprintf("127.0.0.1:%d", s.WantPort),
+	}
+
+	err := s.srv.ListenAndServe()
+	if err != nil {
+		s.isRunning.Store(false)
+		if retries > 0 {
+			time.Sleep(retryAfter)
+			s.ListenRetryAndServe(retries-1, retryAfter)
+		}
+	}
+
+	if s.IsRunning() {
+		logrus.Error("Not serving but isRunning is true")
+		s.isRunning.Store(false)
+	}
+}
+
+func (s *TestServer) Close() {
+	if !s.isRunning.Load().(bool) {
+		return
+	}
+	s.isRunning.Store(false)
+
+	// There can be delay when stopping server
+	time.Sleep(200 * time.Millisecond)
+	if err := s.srv.Close(); err != nil {
+		logrus.WithError(err).Error("Closing dummy server")
+	}
+}
+
+func (s *TestServer) RunServerTests(r *require.Assertions) {
+	// NOTE About choosing tick durations:
+	// In order to avoid ticks to synchronise and cause occasional race
+	// condition we choose the tick duration around 100ms but not exactly
+	// to have large common multiple.
+	r.Eventually(s.IsPortOccupied, 5*time.Second, 97*time.Millisecond)
+
+	// There was an issue where second time we were not able to restore server.
+	for try := 0; try < 3; try++ {
+		i := s.Emit(events.InternetOffEvent, try, 0)
+		r.Eventually(s.IsPortFree, 10*time.Second, 99*time.Millisecond, "signal off try %d : %d", try, i)
+
+		i = s.Emit(events.InternetOnEvent, try, i)
+		i = s.Emit(events.InternetOffEvent, try, i)
+		i = s.Emit(events.InternetOffEvent, try, i)
+		i = s.Emit(events.InternetOffEvent, try, i)
+		i = s.Emit(events.InternetOffEvent, try, i)
+		i = s.Emit(events.InternetOnEvent, try, i)
+		i = s.Emit(events.InternetOnEvent, try, i)
+		i = s.Emit(events.InternetOffEvent, try, i)
+		// Wait a bit longer if needed to process all events
+		r.Eventually(s.IsPortFree, 20*time.Second, 101*time.Millisecond, "again signal off number %d : %d", try, i)
+
+		i = s.Emit(events.InternetOnEvent, try, i)
+		r.Eventually(s.IsPortOccupied, 10*time.Second, 103*time.Millisecond, "signal on number %d : %d", try, i)
+
+		i = s.Emit(events.InternetOffEvent, try, i)
+		i = s.Emit(events.InternetOnEvent, try, i)
+		i = s.Emit(events.InternetOnEvent, try, i)
+		r.Eventually(s.IsPortOccupied, 10*time.Second, 107*time.Millisecond, "again signal on number %d : %d", try, i)
+	}
+}