Urgent Bugfix: workaround therubyracer corruption when it is run within a fiber

lib/jobs.rb

@@ -61,11 +61,40 @@ module Jobs
 
       dbs.each do |db|
         begin
-          RailsMultisite::ConnectionManagement.establish_connection(db: db)
-          I18n.locale = SiteSetting.default_locale
-          execute(opts)
-        ensure
-          ActiveRecord::Base.connection_handler.clear_active_connections!
+          thread_exception = nil
+          # NOTE: This looks odd, in fact it looks crazy but there is a reason
+          #  A bug in therubyracer means that under certain conditions running in a fiber
+          #  can cause the whole v8 context to corrupt so much that it will hang sidekiq
+          #
+          #  If you are brave and want to try to fix this either in celluloid or therubyracer, the repro is:
+          #
+          #  1. Create a big Discourse db: (you can start from script/profile_db_generator.rb)
+          #  2. Queue a ton of jobs, eg: User.pluck(:id).each{|id| Jobs.enqueue(:user_email, type: :digest, user_id: id)};
+          #  3. Run sidekiq
+          #
+          #  The issue only happens in Ruby 2.0 for some reason, you start getting V8::Error with no context
+          #
+          #  See: https://github.com/cowboyd/therubyracer/issues/206
+          #
+          #  The restricted stack space of fibers opens a bunch of risks up, by avoiding them altogether
+          #   we can mitigate giving up a very marginal amount of throughput
+          #
+          #  Ideally we could just tell sidekiq to avoid fibers
+
+          t = Thread.new do
+            begin
+              RailsMultisite::ConnectionManagement.establish_connection(db: db)
+              I18n.locale = SiteSetting.default_locale
+              execute(opts)
+            rescue => e
+              thread_exception = e
+            ensure
+              ActiveRecord::Base.connection_handler.clear_active_connections!
+            end
+          end
+          t.join
+
+          raise thread_exception if thread_exception
         end
       end