fix: define connection holder more thread-safe

lib/standby/connection_holder.rb

@@ -13,15 +13,19 @@ def activate(target)
   end
 
   class << self
+    MUTEX = Mutex.new
+
     def connection_holder(target)
       klass_name = "Standby#{target.to_s.camelize}ConnectionHolder"
-      standby_connections[klass_name] ||= begin
-        klass = Class.new(Standby::ConnectionHolder) do
-          self.abstract_class = true
+      standby_connections[klass_name] || MUTEX.synchronize do
+        standby_connections[klass_name] ||= begin
+          klass = Class.new(Standby::ConnectionHolder) do
+            self.abstract_class = true
+          end
+          Object.const_set(klass_name, klass)
+          klass.activate(target)
+          klass
         end
-        Object.const_set(klass_name, klass)
-        klass.activate(target)
-        klass
       end
     end
   end

spec/slavery_spec.rb

@@ -104,4 +104,27 @@ class ActiveRecord::Relation
     expect(User.on_standby(:two).where(nil).to_a.size).to be 0
     expect(User.on_standby.where(nil).to_a.size).to be 1
   end
+
+  it 'works concurrently when connection is not established beforehand' do
+    threads = 50.times.map do |i|
+      Thread.new do
+        begin
+          # Wrapping with "with_connection" as on_standby checks if we are
+          # inside transaction using default connection of ActiveRecord::Base.
+          ActiveRecord::Base.connection_pool.with_connection do
+            Standby.on_standby(:three) do
+              ActiveRecord::Base.connection.exec_query "SELECT 1;"
+            end
+          end
+          i
+        ensure
+          # Checking the used connection back into connection_pool.
+          StandbyStandbyThreeConnectionHolder.connection_pool.release_connection if StandbyStandbyThreeConnectionHolder.connection_pool.active_connection?
+        end
+      end
+    end
+
+    threads.map(&:join)
+    expect(threads.map(&:value)).to match_array threads.size.times.to_a
+  end
 end

spec/spec_helper.rb

@@ -6,9 +6,10 @@
 require 'standby'
 
 ActiveRecord::Base.configurations = {
-  'test'            =>  { 'adapter' => 'sqlite3', 'database' => 'test_db' },
-  'test_standby'      =>  { 'adapter' => 'sqlite3', 'database' => 'test_standby_one' },
-  'test_standby_two'  =>  { 'adapter' => 'sqlite3', 'database' => 'test_standby_two'},
+  'test'            =>  { 'adapter' => 'sqlite3', 'database' => 'test_db', 'pool' => 30 },
+  'test_standby'      =>  { 'adapter' => 'sqlite3', 'database' => 'test_standby_one', 'pool' => 30 },
+  'test_standby_two'  =>  { 'adapter' => 'sqlite3', 'database' => 'test_standby_two', 'pool' => 30 },
+  'test_standby_three' => { 'adapter' => 'postgresql', 'database' => 'test_standby_three', 'pool' => 10, 'checkout_timeout' => 1 },
   'test_standby_url'  =>  'postgres://root:@localhost:5432/test_standby'
 }
 

standby.gemspec

@@ -20,5 +20,6 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency 'activerecord', '>= 3.0.0'
 
   gem.add_development_dependency 'rspec'
-  gem.add_development_dependency 'sqlite3'
+  gem.add_development_dependency 'sqlite3', '~> 1.3', '< 1.4'
+  gem.add_development_dependency 'pg'
 end