feat(GODT-2814): Standalone Server Manager

Convert ServerManger into a standalone service so that it can become a
self contained module.

internal/services/imapsmtpserver/smtp.go

@@ -0,0 +1,69 @@
+// Copyright (c) 2023 Proton AG
+//
+// This file is part of Proton Mail Bridge.
+//
+// Proton Mail 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.
+//
+// Proton Mail 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 Proton Mail Bridge.  If not, see <https://www.gnu.org/licenses/>.
+
+package imapsmtpserver
+
+import (
+	"crypto/tls"
+
+	"github.com/ProtonMail/proton-bridge/v3/internal/constants"
+	"github.com/ProtonMail/proton-bridge/v3/internal/identifier"
+	"github.com/ProtonMail/proton-bridge/v3/internal/logging"
+	smtpservice "github.com/ProtonMail/proton-bridge/v3/internal/services/smtp"
+	"github.com/emersion/go-sasl"
+	"github.com/emersion/go-smtp"
+	"github.com/sirupsen/logrus"
+)
+
+type SMTPSettingsProvider interface {
+	TLSConfig() *tls.Config
+	Log() bool
+	Port() int
+	SetPort(int) error
+	UseSSL() bool
+	Identifier() identifier.UserAgentUpdater
+}
+
+func newSMTPServer(accounts *smtpservice.Accounts, settings SMTPSettingsProvider) *smtp.Server {
+	logrus.WithField("logSMTP", settings.Log()).Info("Creating SMTP server")
+
+	smtpServer := smtp.NewServer(smtpservice.NewBackend(accounts, settings.Identifier()))
+
+	smtpServer.TLSConfig = settings.TLSConfig()
+	smtpServer.Domain = constants.Host
+	smtpServer.AllowInsecureAuth = true
+	smtpServer.MaxLineLength = 1 << 16
+	smtpServer.ErrorLog = logging.NewSMTPLogger()
+
+	// go-smtp suppors SASL PLAIN but not LOGIN. We need to add LOGIN support ourselves.
+	smtpServer.EnableAuth(sasl.Login, func(conn *smtp.Conn) sasl.Server {
+		return sasl.NewLoginServer(func(username, password string) error {
+			return conn.Session().AuthPlain(username, password)
+		})
+	})
+
+	if settings.Log() {
+		log := logrus.WithField("protocol", "SMTP")
+		log.Warning("================================================")
+		log.Warning("THIS LOG WILL CONTAIN **DECRYPTED** MESSAGE DATA")
+		log.Warning("================================================")
+
+		smtpServer.Debug = logging.NewSMTPDebugLogger()
+	}
+
+	return smtpServer
+}