Enforce default isolation level of READ COMMITTED

This sets the session (== ActiveRecord connection) wide default transaction isolation level.

Specific reason: For advisory based locking over journals in MySQL to work correctly,
we need to ensure that the SELECT that determines the maximum version reads from the most recent committed data.
However, by default MySQL would use REPEATABLE READ, which snapshots the database at the first SELECT of a transaction.

Generic reason: Until now we did not expect any specific isolation level,
this might be bad as MySQL and PostgreSQL already use different defaults. It is also possible to reconfigure a DBMS for a different default, so we would either need to require our customers to configure a specified isolation level OR we set one for our own sessions... Now we know for sure that our transactions are using READ COMMITTED unless specified otherwise on the transaction.

config/initializers/enforce_isolation_level.rb

@@ -0,0 +1,60 @@
+#-- 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.
+#++
+
+# We need to ensure that we operate on a well-known TRANSACTION ISOLATION LEVEL
+# However, the default isolation level is different for MySQL and PostgreSQL and it is also
+# possible (at least for MySQL) to globally override the default for your DBMS installation.
+# Therefore we want to ensure that the isolation level is consistent on a session basis.
+# We chose READ COMMITTED as our expected default isolation level, this is the default of
+# PostgreSQL.
+module ConnectionIsolationLevel
+  module ConnectionPoolPatch
+    def new_connection
+      connection = super
+      ConnectionIsolationLevel.set_connection_isolation_level connection
+      connection
+    end
+  end
+
+  def self.set_connection_isolation_level(connection)
+    isolation_level = 'ISOLATION LEVEL READ COMMITTED'
+    if OpenProject::Database.mysql?
+      connection.execute("SET SESSION TRANSACTION #{isolation_level}")
+    else
+      connection.execute("SET SESSION CHARACTERISTICS AS TRANSACTION #{isolation_level}")
+    end
+  end
+end
+
+ActiveRecord::ConnectionAdapters::ConnectionPool.send(:prepend,
+                                                      ConnectionIsolationLevel::ConnectionPoolPatch)
+
+# in case the existing connection was created before our patch
+# N.B.: this assumes that our process only has this single thread, which is at least true today...
+ConnectionIsolationLevel.set_connection_isolation_level ActiveRecord::Base.connection