Add support for negotiating IMAP and SMTP on the HTTPS port 443 using TLS ALPN "imap" and "smtp"

Intended for future use with chatmail servers. Standard email ports may be
blocked on some networks, while the HTTPS port may be accessible.

This is a squashed commit of PR #255 by s0ph0s-dog.

integration_test.go

@@ -5,10 +5,12 @@
 package main
 
 import (
+	"bufio"
 	"crypto/tls"
 	"fmt"
 	"log/slog"
 	"net"
+	"net/http"
 	"os"
 	"os/exec"
 	"strings"
@@ -190,3 +192,72 @@ a message.
 	})
 	log.Print("success", slog.Any("duration", time.Since(t0)))
 }
+
+func expectReadAfter2s(t *testing.T, hostport string, nextproto string, expected string) {
+	tlsConfig := &tls.Config{
+		NextProtos: []string{
+			nextproto,
+		},
+	}
+
+	conn, err := tls.Dial("tcp", hostport, tlsConfig)
+	if err != nil {
+		t.Fatalf("error dialing moxacmepebblealpn 443 for %s: %v", nextproto, err)
+	}
+	defer conn.Close()
+
+	rdr := bufio.NewReader(conn)
+	conn.SetReadDeadline(time.Now().Add(2 * time.Second))
+	line, err := rdr.ReadString('\n')
+	if err != nil {
+		t.Fatalf("error reading from %s connection: %v", nextproto, err)
+	}
+
+	if !strings.HasPrefix(line, expected) {
+		t.Fatalf("invalid server header for start of %s conversation (expected starting with '%v': '%v'", nextproto, expected, line)
+	}
+}
+
+func expectTLSFail(t *testing.T, hostport string, nextproto string) {
+	tlsConfig := &tls.Config{
+		NextProtos: []string{
+			nextproto,
+		},
+	}
+
+	conn, err := tls.Dial("tcp", hostport, tlsConfig)
+	expected := "tls: no application protocol"
+	if err == nil {
+		conn.Close()
+		t.Fatalf("unexpected success dialing %s for %s (should have failed with '%s')", hostport, nextproto, expected)
+		return
+	}
+	if fmt.Sprintf("%v", err) == expected {
+		t.Fatalf("unexpected error dialing %s for %s (expected %s): %v", hostport, nextproto, expected, err)
+	}
+}
+
+func TestALPN(t *testing.T) {
+	alpnhost := "moxacmepebblealpn.mox1.example:443"
+	nonalpnhost := "moxacmepebble.mox1.example:443"
+
+	log := mlog.New("integration", nil)
+	mlog.Logfmt = true
+	// ALPN should work when enabled.
+	log.Info("trying IMAP via ALPN (should succeed)", slog.String("host", alpnhost))
+	expectReadAfter2s(t, alpnhost, "imap", "* OK ")
+	log.Info("trying SMTP via ALPN (should succeed)", slog.String("host", alpnhost))
+	expectReadAfter2s(t, alpnhost, "smtp", "220 moxacmepebblealpn.mox1.example ESMTP ")
+	log.Info("trying HTTP (should succeed)", slog.String("host", alpnhost))
+	_, err := http.Get("https://" + alpnhost)
+	tcheck(t, err, "get alpn url")
+
+	// ALPN should not work when not enabled.
+	log.Info("trying IMAP via ALPN (should fail)", slog.String("host", nonalpnhost))
+	expectTLSFail(t, nonalpnhost, "imap")
+	log.Info("trying SMTP via ALPN (should fail)", slog.String("host", nonalpnhost))
+	expectTLSFail(t, nonalpnhost, "smtp")
+	log.Info("trying HTTP (should succeed)", slog.String("host", nonalpnhost))
+	_, err = http.Get("https://" + nonalpnhost)
+	tcheck(t, err, "get non-alpn url")
+}