Use an advisory lock that is released on COMMIT/ROLLBACK

for MySQL this seems to be the only available workaround that is not destroying running transactions.

Note that TransactionalLock is a self-developed gem, that will emulate PostgreSQL style advisory locks, that are automatically released at transaction end.

Gemfile

@@ -99,6 +99,8 @@ gem 'gon', '~> 4.0'
 # don't require by default, instead load on-demand when actually configured
 gem 'airbrake', '~> 4.1.0', require: false
 
+gem 'transactional_lock', git: 'https://github.com/finnlabs/transactional_lock.git', branch: 'master'
+
 group :production do
   # we use dalli as standard memcache client
   # requires memcached 1.4+

Gemfile.lock

@@ -28,6 +28,13 @@ GIT
   specs:
     rspec-example_disabler (0.0.1)
 
+GIT
+  remote: https://github.com/finnlabs/transactional_lock.git
+  revision: fe82192efa0346e49d19bbe82865280fdae79487
+  branch: master
+  specs:
+    transactional_lock (0.1.0)
+
 GIT
   remote: https://github.com/opf/openproject-translations.git
   revision: a2a7d246c6212c0867970ee44c505f97e5c96ee3
@@ -596,6 +603,7 @@ DEPENDENCIES
   sys-filesystem (~> 1.1.4)
   thin
   timecop (~> 0.7.1)
+  transactional_lock!
   unicorn
   warden (~> 1.2)
   warden-basic_auth (~> 0.2.1)

app/models/journal.rb

@@ -63,11 +63,14 @@ class Journal < ActiveRecord::Base
   # Note for MySQL: THis method does not currently change anything (no locking at all)
   def self.with_write_lock
     if OpenProject::Database.mysql?
-      # N.B. there seems to be no acceptable locking available for MySQL:
-      # - table locks will break (i.e. COMMIT) an already running transaction.
-      # - advisory locks will do their job, except that when they end inside a transaction,
-      #   nobody will see the changes from inside the lock until the transaction finished.
+      Journal.transaction do
+        # MySQL is very weak when combining transactions and locks. Using an emulation layer to
+        # automatically release an advisory lock at the end of the transaction
+        # FIXME: still creates duplicates... because MySQL defaults to READ REPEATABLE isolation
+        # we need READ COMMITED, which is also the default for PostgreSQL
+        TransactionalLock::AdvisoryLock.new('journals.write_lock').acquire
         yield
+      end
     else
       Journal.transaction do
         ActiveRecord::Base.connection.execute("LOCK TABLE #{table_name} IN SHARE ROW EXCLUSIVE MODE")

config/initializers/transactional_lock.rb

@@ -0,0 +1,34 @@
+#-- encoding: UTF-8
+#-- copyright
+# OpenProject is a project management system.
+# Copyright (C) 2012-2015 the OpenProject Foundation (OPF)
+#
+# 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 doc/COPYRIGHT.rdoc for more details.
+#++
+
+OpenProject::Application.configure do
+  config.after_initialize do
+    TransactionalLock.initialize
+  end
+end