1 files changed, 26 insertions, 0 deletions
diff --git a/lib/gitlab/email/service_desk/custom_email.rb b/lib/gitlab/email/service_desk/custom_email.rb
index 30ae435a6ec..1828f71984b 100644
--- a/lib/gitlab/email/service_desk/custom_email.rb
+++ b/lib/gitlab/email/service_desk/custom_email.rb
@@ -7,6 +7,9 @@ module Gitlab
       # support all features and methods of ingestable email addresses like
       # incoming_email and service_desk_email.
       module CustomEmail
+        REPLY_ADDRESS_KEY_REGEXP = /\+([0-9a-f]{32})@/
+        EMAIL_REGEXP = /\A[\w\-._]+@[\w\-.]+\.{1}[a-zA-Z]{2,}\z/
+
         class << self
           def reply_address(issue, reply_key)
             return if reply_key.nil?
@@ -18,6 +21,29 @@ module Gitlab
             # We don't have a placeholder.
             custom_email.sub('@', "+#{reply_key}@")
           end
+
+          def key_from_reply_address(email)
+            match_data = REPLY_ADDRESS_KEY_REGEXP.match(email)
+            return unless match_data
+
+            key = match_data[1]
+
+            settings = find_service_desk_setting_from_reply_address(email, key)
+            # We intentionally don't check whether custom email is enabled
+            # so we don't lose emails that are addressed to a disabled custom email address
+            return unless settings
+
+            key
+          end
+
+          private
+
+          def find_service_desk_setting_from_reply_address(email, key)
+            potential_custom_email = email.sub("+#{key}", '')
+            return unless EMAIL_REGEXP.match?(potential_custom_email)
+
+            ServiceDeskSetting.find_by_custom_email(potential_custom_email)
+          end
         end
       end
     end