imap: use low level tcp connection

In preparation for tcp keepalive options, we need access to the
net.TCPConn object associated with an IMAP connection. The only way to
do this is to create the connection ourselves.

No functional change.

Signed-off-by: Robin Jarry <robin@jarry.cc>

worker/imap/worker.go

@@ -3,6 +3,7 @@ package imap
 import (
 	"crypto/tls"
 	"fmt"
+	"net"
 	"net/url"
 	"strings"
 
@@ -214,29 +215,44 @@ func (w *IMAPWorker) handleImapUpdate(update client.Update) {
 
 func (w *IMAPWorker) connect() (*client.Client, error) {
 	var (
-		c   *client.Client
-		err error
+		conn *net.TCPConn
+		c    *client.Client
 	)
+
+	addr, err := net.ResolveTCPAddr("tcp", w.config.addr)
+	if err != nil {
+		return nil, err
+	}
+
+	conn, err = net.DialTCP("tcp", nil, addr)
+	if err != nil {
+		return nil, err
+	}
+
+	serverName, _, _ := net.SplitHostPort(w.config.addr)
+	tlsConfig := &tls.Config{ServerName: serverName}
+
 	switch w.config.scheme {
 	case "imap":
-		c, err = client.Dial(w.config.addr)
+		c, err = client.New(conn)
 		if err != nil {
 			return nil, err
 		}
-
 		if !w.config.insecure {
-			if err := c.StartTLS(&tls.Config{}); err != nil {
+			if err = c.StartTLS(tlsConfig); err != nil {
 				return nil, err
 			}
 		}
 	case "imaps":
-		c, err = client.DialTLS(w.config.addr, &tls.Config{})
+		tlsConn := tls.Client(conn, tlsConfig)
+		c, err = client.New(tlsConn)
 		if err != nil {
 			return nil, err
 		}
 	default:
 		return nil, fmt.Errorf("Unknown IMAP scheme %s", w.config.scheme)
 	}
+
 	c.ErrorLog = w.worker.Logger
 
 	if w.config.user != nil {