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/server.go

@@ -0,0 +1,132 @@
+// 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 serverutil
+
+import (
+	"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"
+)
+
+// Server which can handle disconnected users and lost internet connection.
+type Server interface {
+	HandlePanic()
+	DisconnectUser(string)
+	ListenRetryAndServe(int, time.Duration)
+	Close()
+	Port() int
+	IsRunning() bool
+}
+
+func monitorDisconnectedUsers(s Server, l listener.Listener) {
+	ch := make(chan string)
+	l.Add(events.CloseConnectionEvent, ch)
+	for address := range ch {
+		s.DisconnectUser(address)
+	}
+}
+
+func redirectInternetEventsToOneChannel(l listener.Listener) (isInternetOn chan bool) {
+	on := make(chan string)
+	l.Add(events.InternetOnEvent, on)
+	off := make(chan string)
+	l.Add(events.InternetOffEvent, off)
+
+	// Redirect two channels into one. When select was used the algorithm
+	// first read all on channels and then read all off channels.
+	isInternetOn = make(chan bool, 20)
+	go func() {
+		for {
+			logrus.WithField("try", <-on).Trace("Internet ON")
+			isInternetOn <- true
+		}
+	}()
+
+	go func() {
+		for {
+			logrus.WithField("try", <-off).Trace("Internet OFF")
+			isInternetOn <- false
+		}
+	}()
+	return
+}
+
+const (
+	recheckPortAfter    = 50 * time.Millisecond
+	stopPortChecksAfter = 15 * time.Second
+	retryListnerAfter   = 5 * time.Second
+)
+
+func monitorInternetConnection(s Server, l listener.Listener) {
+	isInternetOn := redirectInternetEventsToOneChannel(l)
+	for {
+		var expectedIsPortFree bool
+		if <-isInternetOn {
+			if s.IsRunning() {
+				continue
+			}
+			go func() {
+				defer s.HandlePanic()
+				// We had issues on Mac that from time to time something
+				// blocked our port for a bit after we closed IMAP server
+				// due to connection issues.
+				// Restart always helped, so we do retry to not bother user.
+				s.ListenRetryAndServe(10, retryListnerAfter)
+			}()
+			expectedIsPortFree = false
+		} else {
+			if !s.IsRunning() {
+				continue
+			}
+			s.Close()
+			expectedIsPortFree = true
+		}
+		start := time.Now()
+		for {
+			isPortFree := ports.IsPortFree(s.Port())
+			logrus.
+				WithField("port", s.Port()).
+				WithField("isFree", isPortFree).
+				WithField("wantToBeFree", expectedIsPortFree).
+				Trace("Check port")
+			if isPortFree == expectedIsPortFree {
+				break
+			}
+			// Safety stop if something went wrong.
+			if time.Since(start) > stopPortChecksAfter {
+				logrus.WithField("expectedIsPortFree", expectedIsPortFree).Warn("Server start/stop check timeouted")
+				break
+			}
+			time.Sleep(recheckPortAfter)
+		}
+	}
+}
+
+// ListenAndServe starts the server and keeps it on based on internet
+// availability. It also monitors and disconnect users if requested.
+func ListenAndServe(s Server, l listener.Listener) {
+	go monitorDisconnectedUsers(s, l)
+	go monitorInternetConnection(s, l)
+
+	// When starting the Bridge, we don't want to retry to notify user
+	// quickly about the issue. Very probably retry will not help anyway.
+	s.ListenRetryAndServe(0, 0)
+}