3 files changed, 83 insertions, 1 deletions
diff --git a/lib/gitlab/email/handler.rb b/lib/gitlab/email/handler.rb
index 1b8421d34f3..e71ea154355 100644
--- a/lib/gitlab/email/handler.rb
+++ b/lib/gitlab/email/handler.rb
@@ -11,6 +11,7 @@ module Gitlab
         [
           CreateNoteHandler,
           CreateIssueHandler,
+          CreateNoteOnIssuableHandler,
           UnsubscribeHandler,
           CreateMergeRequestHandler,
           ServiceDeskHandler
diff --git a/lib/gitlab/email/handler/create_note_on_issuable_handler.rb b/lib/gitlab/email/handler/create_note_on_issuable_handler.rb
new file mode 100644
index 00000000000..aed3647744a
--- /dev/null
+++ b/lib/gitlab/email/handler/create_note_on_issuable_handler.rb
@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+require 'gitlab/email/handler/base_handler'
+
+# Handles comment creation emails when sent/forwarded by an authorized
+# user. Attachments are allowed. Quoted material is _not_ stripped, just like
+# create issue emails
+# Supports these formats:
+#   incoming+gitlab-org-gitlab-ce-20-Author_Token12345678-issue-34@incoming.gitlab.com
+module Gitlab
+  module Email
+    module Handler
+      class CreateNoteOnIssuableHandler < BaseHandler
+        include ReplyProcessing
+
+        attr_reader :issuable_iid
+
+        HANDLER_REGEX = /\A#{HANDLER_ACTION_BASE_REGEX}-(?<incoming_email_token>.+)-issue-(?<issuable_iid>\d+)\z/.freeze
+
+        def initialize(mail, mail_key)
+          super(mail, mail_key)
+
+          if (matched = HANDLER_REGEX.match(mail_key.to_s))
+            @project_slug         = matched[:project_slug]
+            @project_id           = matched[:project_id]&.to_i
+            @incoming_email_token = matched[:incoming_email_token]
+            @issuable_iid         = matched[:issuable_iid]&.to_i
+          end
+        end
+
+        def can_handle?
+          incoming_email_token && project_id && issuable_iid
+        end
+
+        def execute
+          raise ProjectNotFound unless project
+
+          validate_permission!(:create_note)
+
+          raise NoteableNotFoundError unless noteable
+          raise EmptyEmailError if message_including_reply.blank?
+
+          verify_record!(
+            record: create_note,
+            invalid_exception: InvalidNoteError,
+            record_name: 'comment')
+        end
+
+        def metrics_event
+          :receive_email_create_note_issuable
+        end
+
+        def noteable
+          return unless issuable_iid
+
+          @noteable ||= project&.issues&.find_by_iid(issuable_iid)
+        end
+
+        private
+
+        # rubocop: disable CodeReuse/ActiveRecord
+        def author
+          @author ||= User.find_by(incoming_email_token: incoming_email_token)
+        end
+        # rubocop: enable CodeReuse/ActiveRecord
+
+        def create_note
+          Notes::CreateService.new(project, author, note_params).execute
+        end
+
+        def note_params
+          {
+            noteable_type: noteable.class.to_s,
+            noteable_id: noteable.id,
+            note: message_including_reply
+          }
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/email/handler/service_desk_handler.rb b/lib/gitlab/email/handler/service_desk_handler.rb
index 0bbe3980f67..f66e8a8794f 100644
--- a/lib/gitlab/email/handler/service_desk_handler.rb
+++ b/lib/gitlab/email/handler/service_desk_handler.rb
@@ -68,7 +68,7 @@ module Gitlab
         end
 
         def valid_project_key?(project, slug)
-          project.present? && slug == project.full_path_slug && Feature.enabled?(:service_desk_custom_address, project, default_enabled: true)
+          project.present? && slug == project.full_path_slug
         end
 
         def create_issue!