Add support for incoming emails (#22056)

closes #13585
fixes #9067
fixes #2386
ref #6226
ref #6219
fixes #745

This PR adds support to process incoming emails to perform actions.
Currently I added handling of replies and unsubscribing from
issues/pulls. In contrast to #13585 the IMAP IDLE command is used
instead of polling which results (in my opinion 😉) in cleaner code.

Procedure:
- When sending an issue/pull reply email, a token is generated which is
present in the Reply-To and References header.
- IMAP IDLE waits until a new email arrives
- The token tells which action should be performed

A possible signature and/or reply gets stripped from the content.

I added a new service to the drone pipeline to test the receiving of
incoming mails. If we keep this in, we may test our outgoing emails too
in future.

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/mailer/incoming/incoming_test.go

@@ -0,0 +1,138 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package incoming
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/jhillyerd/enmime"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIsAutomaticReply(t *testing.T) {
+	cases := []struct {
+		Headers  map[string]string
+		Expected bool
+	}{
+		{
+			Headers:  map[string]string{},
+			Expected: false,
+		},
+		{
+			Headers: map[string]string{
+				"Auto-Submitted": "no",
+			},
+			Expected: false,
+		},
+		{
+			Headers: map[string]string{
+				"Auto-Submitted": "yes",
+			},
+			Expected: true,
+		},
+		{
+			Headers: map[string]string{
+				"X-Autoreply": "no",
+			},
+			Expected: false,
+		},
+		{
+			Headers: map[string]string{
+				"X-Autoreply": "yes",
+			},
+			Expected: true,
+		},
+		{
+			Headers: map[string]string{
+				"X-Autorespond": "yes",
+			},
+			Expected: true,
+		},
+	}
+
+	for _, c := range cases {
+		b := enmime.Builder().
+			From("Dummy", "dummy@gitea.io").
+			To("Dummy", "dummy@gitea.io")
+		for k, v := range c.Headers {
+			b = b.Header(k, v)
+		}
+		root, err := b.Build()
+		assert.NoError(t, err)
+		env, err := enmime.EnvelopeFromPart(root)
+		assert.NoError(t, err)
+
+		assert.Equal(t, c.Expected, isAutomaticReply(env))
+	}
+}
+
+func TestGetContentFromMailReader(t *testing.T) {
+	mailString := "Content-Type: multipart/mixed; boundary=message-boundary\r\n" +
+		"\r\n" +
+		"--message-boundary\r\n" +
+		"Content-Type: multipart/alternative; boundary=text-boundary\r\n" +
+		"\r\n" +
+		"--text-boundary\r\n" +
+		"Content-Type: text/plain\r\n" +
+		"Content-Disposition: inline\r\n" +
+		"\r\n" +
+		"mail content\r\n" +
+		"--text-boundary--\r\n" +
+		"--message-boundary\r\n" +
+		"Content-Type: text/plain\r\n" +
+		"Content-Disposition: attachment; filename=attachment.txt\r\n" +
+		"\r\n" +
+		"attachment content\r\n" +
+		"--message-boundary--\r\n"
+
+	env, err := enmime.ReadEnvelope(strings.NewReader(mailString))
+	assert.NoError(t, err)
+	content := getContentFromMailReader(env)
+	assert.Equal(t, "mail content", content.Content)
+	assert.Len(t, content.Attachments, 1)
+	assert.Equal(t, "attachment.txt", content.Attachments[0].Name)
+	assert.Equal(t, []byte("attachment content"), content.Attachments[0].Content)
+
+	mailString = "Content-Type: multipart/mixed; boundary=message-boundary\r\n" +
+		"\r\n" +
+		"--message-boundary\r\n" +
+		"Content-Type: multipart/alternative; boundary=text-boundary\r\n" +
+		"\r\n" +
+		"--text-boundary\r\n" +
+		"Content-Type: text/html\r\n" +
+		"Content-Disposition: inline\r\n" +
+		"\r\n" +
+		"<p>mail content</p>\r\n" +
+		"--text-boundary--\r\n" +
+		"--message-boundary--\r\n"
+
+	env, err = enmime.ReadEnvelope(strings.NewReader(mailString))
+	assert.NoError(t, err)
+	content = getContentFromMailReader(env)
+	assert.Equal(t, "mail content", content.Content)
+	assert.Empty(t, content.Attachments)
+
+	mailString = "Content-Type: multipart/mixed; boundary=message-boundary\r\n" +
+		"\r\n" +
+		"--message-boundary\r\n" +
+		"Content-Type: multipart/alternative; boundary=text-boundary\r\n" +
+		"\r\n" +
+		"--text-boundary\r\n" +
+		"Content-Type: text/plain\r\n" +
+		"Content-Disposition: inline\r\n" +
+		"\r\n" +
+		"mail content without signature\r\n" +
+		"--\r\n" +
+		"signature\r\n" +
+		"--text-boundary--\r\n" +
+		"--message-boundary--\r\n"
+
+	env, err = enmime.ReadEnvelope(strings.NewReader(mailString))
+	assert.NoError(t, err)
+	content = getContentFromMailReader(env)
+	assert.NoError(t, err)
+	assert.Equal(t, "mail content without signature", content.Content)
+	assert.Empty(t, content.Attachments)
+}