canvas-lms/app/models/incoming_mail/message_handler.rb

# frozen_string_literal: true

#
# Copyright (C) 2014 - present Instructure, Inc.
#
# This file is part of Canvas.
#
# Canvas is free software: you can redistribute it and/or modify it under
# the terms of the GNU Affero General Public License as published by the Free
# Software Foundation, version 3 of the License.
#
# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
# details.
#
# You should have received a copy of the GNU Affero General Public License along
# with this program. If not, see <http://www.gnu.org/licenses/>.
#

module IncomingMail
  class MessageHandler
    def handle(outgoing_from_address, body, html_body, incoming_message, tag)
      secure_id, original_message_id, timestamp = parse_tag(tag)
      return unless original_message_id

      original_message = get_original_message(original_message_id, timestamp)
      # This prevents us from rebouncing users that have auto-replies setup -- only bounce something
      # that was sent out because of a notification.
      return unless original_message&.notification_id
      return unless valid_secure_id?(original_message_id, secure_id)

      from_channel = nil
      original_message.shard.activate do
        context = original_message.context
        user = original_message.user
        raise IncomingMail::Errors::UnknownAddress unless valid_user_and_context?(context, user)
        raise IncomingMail::Errors::UserSuspended if user.suspended?

        from_channel = sent_from_channel(user, incoming_message)
        raise IncomingMail::Errors::UnknownSender unless from_channel
        raise IncomingMail::Errors::MessageTooLong if body.length > ActiveRecord::Base.maximum_text_length
        raise IncomingMail::Errors::BlankMessage if body.blank?

        # Check if html_body is too long, if yes, set html_body to nil so that the plain text is used instead
        if html_body.length > ActiveRecord::Base.maximum_text_length
          html_body = nil
        end

        Rails.cache.fetch(["incoming_mail_reply_from", context, incoming_message.message_id].cache_key, expires_in: 7.days) do
          context.reply_from({
                               purpose: "general",
                               user:,
                               subject: IncomingMailProcessor::IncomingMessageProcessor.utf8ify(incoming_message.subject, incoming_message.header[:subject].try(:charset)),
                               html: html_body,
                               text: body
                             })
          true
        end
      rescue IncomingMail::Errors::ReplyFrom => e
        bounce_message(original_message, incoming_message, e, outgoing_from_address, from_channel)
      rescue => e
        Canvas::Errors.capture_exception("IncomingMailProcessor", e)
      end
    end

    private

    def bounce_message(original_message, incoming_message, error, outgoing_from_address, from_channel)
      incoming_from = from_channel.try(:path) || incoming_message.from.try(:first)
      incoming_subject = incoming_message.subject
      return unless incoming_from

      ndr_subject, ndr_body = bounce_message_strings(incoming_subject, error)
      outgoing_message = Message.new({
                                       to: incoming_from,
                                       from: outgoing_from_address,
                                       subject: ndr_subject,
                                       body: ndr_body,
                                       delay_for: 0,
                                       context: nil,
                                       path_type: "email",
                                       from_name: "Instructure",
                                     })

      outgoing_message_delivered = false

      original_message.shard.activate do
        comch = from_channel || CommunicationChannel.active.email.by_path(incoming_from).first
        outgoing_message.communication_channel = comch
        outgoing_message.user = comch.try(:user)
        if outgoing_message.communication_channel
          outgoing_message.save
          outgoing_message.deliver
          outgoing_message_delivered = true
        end
      end

      unless outgoing_message_delivered
        # Can't use our usual mechanisms, so just try to send it once now
        begin
          Mailer.deliver(Mailer.create_message(outgoing_message))
        rescue
          # TODO: put some kind of error logging here?
        end
      end
    end

    def bounce_message_strings(subject, error)
      ndr_subject = I18n.t("Undelivered message")
      ndr_body = case error
                 when IncomingMail::Errors::ReplyToDeletedDiscussion
                   InstStatsd::Statsd.increment("incoming_mail_processor.message_processing_error.reply_to_deleted_discussion")
                   I18n.t(<<~TEXT, subject:).gsub(/^ +/, "")
                     The message titled "%{subject}" could not be delivered because the discussion topic has been deleted. If you are trying to contact someone through Canvas you can try logging in to your account and sending them a message using the Inbox tool.

                     Thank you,
                     Canvas Support
                   TEXT
                 when IncomingMail::Errors::ReplyToLockedTopic
                   InstStatsd::Statsd.increment("incoming_mail_processor.message_processing_error.reply_to_locked_topic")
                   I18n.t("lib.incoming_message_processor.locked_topic.body", <<~TEXT, subject:).gsub(/^ +/, "")
                     The message titled "%{subject}" could not be delivered because the discussion topic is locked. If you are trying to contact someone through Canvas you can try logging in to your account and sending them a message using the Inbox tool.

                     Thank you,
                     Canvas Support
                   TEXT
                 when IncomingMail::Errors::UnknownSender
                   InstStatsd::Statsd.increment("incoming_mail_processor.message_processing_error.unknown_sender")
                   I18n.t(<<~TEXT, subject:, link: I18n.t(:"community.guides_home")).gsub(/^ +/, "")
                     The message you sent with the subject line "%{subject}" was not delivered. To reply to Canvas messages from this email, it must first be a confirmed communication channel in your Canvas profile. Please visit your profile and resend the confirmation email for this email address. You may also contact this person via the Canvas Inbox. For help, please see the Inbox chapter for your user role in the Canvas Guides. [See %{link}].

                     Thank you,
                     Canvas Support
                   TEXT
                 when IncomingMail::Errors::UserSuspended
                   InstStatsd::Statsd.increment("incoming_mail_processor.message_processing_error.user_suspended")
                   I18n.t(<<~TEXT, subject:).gsub(/^ +/, "")
                     The message you sent with the subject line "%{subject}" was not delivered because your account has been suspended.

                     Thank you,
                     Canvas Support
                   TEXT
                 else # including IncomingMessageProcessor::UnknownAddressError
                   InstStatsd::Statsd.increment("incoming_mail_processor.message_processing_error.catch_all")
                   error_info = { tags: { type: :message_processing_error_catch_all }, extra: { ref: get_ref_uuid } }
                   Canvas::Errors.capture(error, error_info, :error)
                   I18n.t("lib.incoming_message_processor.failure_message.body", <<~TEXT, subject:, ref: error_info.dig(:extra, :ref)).to_s.gsub(/^ +/, "")
                     The message titled "%{subject}" could not be delivered.  The message was sent to an unknown mailbox address.  If you are trying to contact someone through Canvas you can try logging in to your account and sending them a message using the Inbox tool.

                     Thank you,
                     Canvas Support

                     Reference: %{ref}
                   TEXT
                 end

      [ndr_subject, ndr_body]
    end

    def valid_secure_id?(original_message_id, secure_id)
      options = {}
      options[:truncate] = 16 if secure_id.length == 16
      Canvas::Security.verify_hmac_sha1(secure_id, original_message_id, options)
    end

    def valid_user_and_context?(context, user)
      user && context && context.respond_to?(:reply_from)
    end

    def sent_from_channel(user, incoming_message)
      from_addresses = ((incoming_message.from || []) + (incoming_message.reply_to || [])).uniq
      user && from_addresses.lazy.map { |addr| user.communication_channels.active.email.by_path(addr).first }.first
    end

    def parse_tag(tag)
      match = tag.match(/^(\h+)-([0-9~]+)(?:-([0-9]+))?$/)
      [match[1], match[2], match[3]] if match
    end

    def get_original_message(original_message_id, timestamp)
      if timestamp
        Message.where(id: original_message_id).at_timestamp(timestamp).first
      else
        Message.where(id: original_message_id).first
      end
    end

    def get_ref_uuid
      SecureRandom.uuid
    end
  end
end