error reply for failed incoming email

app/mailers/user_mailer.rb

@@ -224,8 +224,40 @@ def activation_limit_reached(user_email, admin)
     send_mail(admin, t("mail_user_activation_limit_reached.subject"))
   end
 
+  ##
+  # E-Mail sent to a user when they tried sending an email to OpenProject to create or update
+  # a work package, or forum message for instance.
+  #
+  # @param [User] user User who sent the email
+  # @param [Mail] mail Sent email
+  # @param [Array<String>] List of logs collected during processing of the email
+  def incoming_email_error(user, mail, logs)
+    @user = user
+    @mail = mail
+    @logs = logs
+    @received_at = DateTime.now
+    @incoming_text = incoming_email_text mail
+    @quote = incoming_email_quote mail
+
+    headers['References'] = ["<#{mail.message_id}>"]
+    headers['In-Reply-To'] = ["<#{mail.message_id}>"]
+
+    send_mail user, mail.subject.present? ? "Re: #{mail.subject}" : I18n.t("mail_subject_incoming_email_error")
+  end
+
   private
 
+  def incoming_email_text(mail)
+    mail.text_part.present? ? mail.text_part.body.to_s : mail.body.to_s
+  end
+
+  def incoming_email_quote(mail)
+    quote = incoming_email_text(mail)
+    quoted = String(quote).lines.join("> ")
+
+    "> #{quoted}"
+  end
+
   def open_project_wiki_headers(wiki_content)
     open_project_headers 'Project' => wiki_content.project.identifier,
                          'Wiki-Page-Id' => wiki_content.page.id,

app/models/mail_handler.rb

@@ -34,7 +34,12 @@ class UnauthorizedAction < StandardError; end
 
   class MissingInformation < StandardError; end
 
-  attr_reader :email, :sender_email, :user, :options
+  attr_reader :email, :sender_email, :user, :options, :logs
+
+  def initialize
+    @result = false
+    @logs = []
+  end
 
   ##
   # Code copied from base class and extended with optional options parameter
@@ -70,7 +75,8 @@ def receive(email)
     @sender_email = email.from.to_a.first.to_s.strip
     # Ignore emails received from the application emission address to avoid hell cycles
     if sender_email.downcase == Setting.mail_from.to_s.strip.downcase
-      log "ignoring email from emission address [#{sender_email}]"
+      log "ignoring email from emission address [#{sender_email}]", report: false
+      # don't report back errors to ourselves
       return false
     end
     # Ignore auto generated emails
@@ -79,7 +85,8 @@ def receive(email)
       if value
         value = value.to_s.downcase
         if (ignored_value.is_a?(Regexp) && value.match(ignored_value)) || value == ignored_value
-          log "ignoring email with #{key}:#{value} header"
+          log "ignoring email with #{key}:#{value} header", report: false
+          # no point reporting back in case of auto-generated emails
           return false
         end
       end
@@ -112,6 +119,8 @@ def receive(email)
     end
     User.current = @user
     dispatch
+  ensure
+    report_errors if !@result && Setting.report_incoming_email_errors?
   end
 
   def options=(value)
@@ -151,14 +160,11 @@ def options=(value)
   # Relying on the subject of the mail, which had been implemented before, is brittle as it relies on the user not altering
   # the subject. Additionally, the subject structure might change, e.g. via localization changes.
   def dispatch
-    if (m, object_id = dispatch_target_from_header)
-      m.call(object_id)
-    else
-      dispatch_to_default
-    end
+    m, object_id = dispatch_target_from_header
+
+    @result = m ? m.call(object_id) : dispatch_to_default
   rescue ActiveRecord::RecordInvalid => e
-    # TODO: send a email to the user
-    logger&.error e.message
+    log "could not save record: #{e.message}", :error
     false
   rescue MissingInformation => e
     log "missing information from #{user}: #{e.message}", :error
@@ -597,11 +603,19 @@ def wp_done_ratio_from_keyword
     get_keyword(:done_ratio, override: true, format: '(\d|10)?0')
   end
 
-  def log(message, level = :info)
+  def log(message, level = :info, report: true)
+    logs << "#{level}: #{message}" if report
+
     message = "MailHandler: #{message}"
     logger.public_send(level, message)
   end
 
+  def report_errors
+    return if logs.empty?
+
+    UserMailer.incoming_email_error(user, email, logs).deliver_later
+  end
+
   def work_package_create_contract_class
     if options[:no_permission_check]
       CreateWorkPackageWithoutAuthorizationsContract

app/views/user_mailer/incoming_email_error.html.erb

@@ -0,0 +1,50 @@
+<%#-- copyright
+OpenProject is an open source project management software.
+Copyright (C) 2012-2022 the OpenProject GmbH
+
+This program is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public License version 3.
+
+OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+Copyright (C) 2006-2013 Jean-Philippe Lang
+Copyright (C) 2010-2013 the ChiliProject Team
+
+This program is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public License
+as published by the Free Software Foundation; either version 2
+of the License, or (at your option) any later version.
+
+This program 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 General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+
+See COPYRIGHT and LICENSE files for more details.
+
+++#%>
+
+<h2><%= t(:mail_body_incoming_email_error) %></h2>
+
+<p>
+  <%= t(:mail_body_incoming_email_error_logs) %>:
+</p>
+
+<code>
+<%= @logs.join("\n") %>
+</code>
+
+<p>
+  <%= t(
+    :mail_body_incoming_email_error_in_reply_to,
+    received_at: format_time(@received_at),
+    from_email: @mail.from.first
+  )%>:
+</p>
+
+<blockquote>
+  <%= @incoming_text %>
+</blockquote>

app/views/user_mailer/incoming_email_error.text.erb

@@ -0,0 +1,42 @@
+<%#-- copyright
+OpenProject is an open source project management software.
+Copyright (C) 2012-2022 the OpenProject GmbH
+
+This program is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public License version 3.
+
+OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+Copyright (C) 2006-2013 Jean-Philippe Lang
+Copyright (C) 2010-2013 the ChiliProject Team
+
+This program is free software; you can redistribute it and/or
+modify it under the terms of the GNU General Public License
+as published by the Free Software Foundation; either version 2
+of the License, or (at your option) any later version.
+
+This program 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 General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+
+See COPYRIGHT and LICENSE files for more details.
+
+++#%>
+
+<%= t(:mail_body_incoming_email_error) %>
+
+<%= t(:mail_body_incoming_email_error_logs) %>:
+
+<%= @logs.join("\n") %>
+
+<%= t(
+  :mail_body_incoming_email_error_in_reply_to,
+  received_at: format_time(@received_at),
+  from_email: @mail.from.first
+)%>:
+
+<%= @quote %>

config/constants/settings/definitions.rb

@@ -683,6 +683,8 @@
       },
       writable: false
 
+  add :report_incoming_email_errors, default: true
+
   add :repositories_automatic_managed_vendor,
       default: nil,
       format: :string,

config/locales/en.yml

@@ -2070,6 +2070,9 @@ en:
   mail_body_backup_token_info: The previous token is no longer valid.
   mail_body_backup_waiting_period: The new token will be enabled in %{hours} hours.
   mail_body_backup_token_warning: If this wasn't you, login to OpenProject immediately and reset it again.
+  mail_body_incoming_email_error: The email you sent to OpenProject could not be processed.
+  mail_body_incoming_email_error_in_reply_to: "At %{received_at} %{from_email} wrote"
+  mail_body_incoming_email_error_logs: "Logs"
   mail_body_lost_password: "To change your password, click on the following link:"
   mail_body_register: "Welcome to %{app_title}. Please activate your account by clicking on this link:"
   mail_body_register_header_title: "Project member invitation email"
@@ -2088,6 +2091,7 @@ en:
   mail_subject_account_activation_request: "%{value} account activation request"
   mail_subject_backup_ready: "Your backup is ready"
   mail_subject_backup_token_reset: "Backup token reset"
+  mail_subject_incoming_email_error: "An email you sent to OpenProject could not be processed"
   mail_subject_lost_password: "Your %{value} password"
   mail_subject_register: "Your %{value} account activation"
   mail_subject_reminder: "%{count} work package(s) due in the next %{days} days"

spec/mailers/user_mailer_spec.rb

@@ -230,6 +230,57 @@
     end
   end
 
+  describe '#incoming_email_error' do
+    let(:logs) { ['info: foo', 'error: bar'] }
+    let(:recipient) { user }
+    let(:current_time) { "2022-11-03 9:15".to_time }
+    let(:incoming_email) do
+      Mail.new(subject: subject, message_id: message_id, body: body, from: from_email )
+    end
+
+    let(:subject) { 'New work package 42' }
+    let(:message_id) { '<000501c8d452$a95cd7e0$0a00a8c0@osiris>' }
+    let(:from_email) { 'l.lustig@openproject.com' }
+    let(:body) { "Project: demo-project" }
+
+    let(:outgoing_email) { deliveries.first }
+
+    before do
+      described_class
+        .incoming_email_error(user, incoming_email, logs)
+        .deliver_now
+    end
+
+    it_behaves_like 'mail is sent' do
+      it "references the incoming email's subject in its own" do
+        expect(outgoing_email.subject).to eql "Re: #{subject}"
+      end
+
+      it "it's a reply to the incoming email" do
+        expect(message_id).to include outgoing_email.in_reply_to
+        expect(message_id).to include outgoing_email.references
+      end
+
+      it "contains the incoming email's quoted content" do
+        expect(html_body).to include body
+      end
+
+      it 'contains the date the mail was received' do
+        expect(html_body).to include "11/03/2022 09:15 AM"
+      end
+
+      it 'contains the email address from which the email was sent' do
+        expect(html_body).to include from_email
+      end
+
+      it 'contains the logs' do
+        logs.each do |log|
+          expect(html_body).to include log
+        end
+      end
+    end
+  end
+
   describe '#news_added' do
     let(:news) { build_stubbed(:news) }