Create merge request from email

* new merge request can be created by sending an email to the specific
email address (similar to creating issues by email)
* for the first iteration, source branch must be specified in the mail
subject, other merge request parameters can not be set yet
* user should enable "Receive notifications about your own activity" in
user settings to receive a notification about created merge request

Part of #32878

3 files changed, 73 insertions, 2 deletions

diff --git a/lib/gitlab/email/handler.rb b/lib/gitlab/email/handler.rb
index b07c68d1498..e08b5be8984 100644
--- a/lib/gitlab/email/handler.rb
+++ b/lib/gitlab/email/handler.rb
@@ -1,3 +1,4 @@
+require 'gitlab/email/handler/create_merge_request_handler'
 require 'gitlab/email/handler/create_note_handler'
 require 'gitlab/email/handler/create_issue_handler'
 require 'gitlab/email/handler/unsubscribe_handler'
@@ -8,6 +9,7 @@ module Gitlab
       HANDLERS = [
         UnsubscribeHandler,
         CreateNoteHandler,
+        CreateMergeRequestHandler,
         CreateIssueHandler
       ].freeze
 
diff --git a/lib/gitlab/email/handler/create_merge_request_handler.rb b/lib/gitlab/email/handler/create_merge_request_handler.rb
new file mode 100644
index 00000000000..c63666b98c1
--- /dev/null
+++ b/lib/gitlab/email/handler/create_merge_request_handler.rb
@@ -0,0 +1,67 @@
+require 'gitlab/email/handler/base_handler'
+require 'gitlab/email/handler/reply_processing'
+
+module Gitlab
+  module Email
+    module Handler
+      class CreateMergeRequestHandler < BaseHandler
+        include ReplyProcessing
+        attr_reader :project_path, :incoming_email_token
+
+        def initialize(mail, mail_key)
+          super(mail, mail_key)
+          if m = /\A([^\+]*)\+merge-request\+(.*)/.match(mail_key.to_s)
+            @project_path, @incoming_email_token = m.captures
+          end
+        end
+
+        def can_handle?
+          @project_path && @incoming_email_token
+        end
+
+        def execute
+          raise ProjectNotFound unless project
+
+          validate_permission!(:create_merge_request)
+
+          verify_record!(
+            record: create_merge_request,
+            invalid_exception: InvalidMergeRequestError,
+            record_name: 'merge_request')
+        end
+
+        def author
+          @author ||= User.find_by(incoming_email_token: incoming_email_token)
+        end
+
+        def project
+          @project ||= Project.find_by_full_path(project_path)
+        end
+
+        def metrics_params
+          super.merge(project: project&.full_path)
+        end
+
+        private
+
+        def create_merge_request
+          merge_request = MergeRequests::BuildService.new(project, author, merge_request_params).execute
+
+          if merge_request.errors.any?
+            merge_request
+          else
+            MergeRequests::CreateService.new(project, author).create(merge_request)
+          end
+        end
+
+        def merge_request_params
+          {
+            source_project_id: project.id,
+            source_branch: mail.subject,
+            target_project_id: project.id
+          }
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/email/receiver.rb b/lib/gitlab/email/receiver.rb
index c8f4591d060..d8c594ad0e7 100644
--- a/lib/gitlab/email/receiver.rb
+++ b/lib/gitlab/email/receiver.rb
@@ -13,8 +13,10 @@ module Gitlab
     UserBlockedError = Class.new(ProcessingError)
     UserNotAuthorizedError = Class.new(ProcessingError)
     NoteableNotFoundError = Class.new(ProcessingError)
-    InvalidNoteError = Class.new(ProcessingError)
-    InvalidIssueError = Class.new(ProcessingError)
+    InvalidRecordError = Class.new(ProcessingError)
+    InvalidNoteError = Class.new(InvalidRecordError)
+    InvalidIssueError = Class.new(InvalidRecordError)
+    InvalidMergeRequestError = Class.new(InvalidRecordError)
     UnknownIncomingEmail = Class.new(ProcessingError)
 
     class Receiver