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_handler.go

@@ -0,0 +1,171 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package incoming
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	access_model "code.gitea.io/gitea/models/perm/access"
+	repo_model "code.gitea.io/gitea/models/repo"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/upload"
+	"code.gitea.io/gitea/modules/util"
+	attachment_service "code.gitea.io/gitea/services/attachment"
+	issue_service "code.gitea.io/gitea/services/issue"
+	incoming_payload "code.gitea.io/gitea/services/mailer/incoming/payload"
+	"code.gitea.io/gitea/services/mailer/token"
+	pull_service "code.gitea.io/gitea/services/pull"
+)
+
+type MailHandler interface {
+	Handle(ctx context.Context, content *MailContent, doer *user_model.User, payload []byte) error
+}
+
+var handlers = map[token.HandlerType]MailHandler{
+	token.ReplyHandlerType:       &ReplyHandler{},
+	token.UnsubscribeHandlerType: &UnsubscribeHandler{},
+}
+
+// ReplyHandler handles incoming emails to create a reply from them
+type ReplyHandler struct{}
+
+func (h *ReplyHandler) Handle(ctx context.Context, content *MailContent, doer *user_model.User, payload []byte) error {
+	if doer == nil {
+		return util.NewInvalidArgumentErrorf("doer can't be nil")
+	}
+
+	ref, err := incoming_payload.GetReferenceFromPayload(ctx, payload)
+	if err != nil {
+		return err
+	}
+
+	var issue *issues_model.Issue
+
+	switch r := ref.(type) {
+	case *issues_model.Issue:
+		issue = r
+	case *issues_model.Comment:
+		comment := r
+
+		if err := comment.LoadIssue(ctx); err != nil {
+			return err
+		}
+
+		issue = comment.Issue
+	default:
+		return util.NewInvalidArgumentErrorf("unsupported reply reference: %v", ref)
+	}
+
+	if err := issue.LoadRepo(ctx); err != nil {
+		return err
+	}
+
+	perm, err := access_model.GetUserRepoPermission(ctx, issue.Repo, doer)
+	if err != nil {
+		return err
+	}
+
+	if !perm.CanWriteIssuesOrPulls(issue.IsPull) || issue.IsLocked && !doer.IsAdmin {
+		log.Debug("can't write issue or pull")
+		return nil
+	}
+
+	switch r := ref.(type) {
+	case *issues_model.Issue:
+		attachmentIDs := make([]string, 0, len(content.Attachments))
+		if setting.Attachment.Enabled {
+			for _, attachment := range content.Attachments {
+				a, err := attachment_service.UploadAttachment(bytes.NewReader(attachment.Content), setting.Attachment.AllowedTypes, &repo_model.Attachment{
+					Name:       attachment.Name,
+					UploaderID: doer.ID,
+					RepoID:     issue.Repo.ID,
+				})
+				if err != nil {
+					if upload.IsErrFileTypeForbidden(err) {
+						log.Info("Skipping disallowed attachment type: %s", attachment.Name)
+						continue
+					}
+					return err
+				}
+				attachmentIDs = append(attachmentIDs, a.UUID)
+			}
+		}
+
+		if content.Content == "" && len(attachmentIDs) == 0 {
+			return nil
+		}
+
+		_, err = issue_service.CreateIssueComment(ctx, doer, issue.Repo, issue, content.Content, attachmentIDs)
+		if err != nil {
+			return fmt.Errorf("CreateIssueComment failed: %w", err)
+		}
+	case *issues_model.Comment:
+		comment := r
+
+		if content.Content == "" {
+			return nil
+		}
+
+		if comment.Type == issues_model.CommentTypeCode {
+			_, err := pull_service.CreateCodeComment(
+				ctx,
+				doer,
+				nil,
+				issue,
+				comment.Line,
+				content.Content,
+				comment.TreePath,
+				false,
+				comment.ReviewID,
+				"",
+			)
+			if err != nil {
+				return fmt.Errorf("CreateCodeComment failed: %w", err)
+			}
+		}
+	}
+	return nil
+}
+
+// UnsubscribeHandler handles unwatching issues/pulls
+type UnsubscribeHandler struct{}
+
+func (h *UnsubscribeHandler) Handle(ctx context.Context, _ *MailContent, doer *user_model.User, payload []byte) error {
+	if doer == nil {
+		return util.NewInvalidArgumentErrorf("doer can't be nil")
+	}
+
+	ref, err := incoming_payload.GetReferenceFromPayload(ctx, payload)
+	if err != nil {
+		return err
+	}
+
+	switch r := ref.(type) {
+	case *issues_model.Issue:
+		issue := r
+
+		if err := issue.LoadRepo(ctx); err != nil {
+			return err
+		}
+
+		perm, err := access_model.GetUserRepoPermission(ctx, issue.Repo, doer)
+		if err != nil {
+			return err
+		}
+
+		if !perm.CanReadIssuesOrPulls(issue.IsPull) {
+			log.Debug("can't read issue or pull")
+			return nil
+		}
+
+		return issues_model.CreateOrUpdateIssueWatch(doer.ID, issue.ID, false)
+	}
+
+	return fmt.Errorf("unsupported unsubscribe reference: %v", ref)
+}