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@@ -12,6 +12,9 @@ module ActiveRecord
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         # The ConnectionSpecification for this pool
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         @spec = spec
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+
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+        # The mutex used to synchronize pool access
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+        @connection_mutex = Monitor.new
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       end
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       def active_connection_name #:nodoc:
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@@ -70,7 +73,7 @@ module ActiveRecord
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           # Verify the connection.
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           conn.verify!(verification_timeout)
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         else
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-          self.connection = spec
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+          self.set_connection spec
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           conn = active_connections[name]
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         end
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@@ -82,6 +85,7 @@ module ActiveRecord
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         active_connections[active_connection_name] ? true : false
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       end
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+      # Disconnect all connections in the pool.
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       def disconnect!
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         clear_cache!(@active_connections) do |name, conn|
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           conn.disconnect!
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@@ -89,16 +93,20 @@ module ActiveRecord
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       end
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       # Set the connection for the class.
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-      def connection=(spec) #:nodoc:
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+      def set_connection(spec) #:nodoc:
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         if spec.kind_of?(ActiveRecord::ConnectionAdapters::AbstractAdapter)
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           active_connections[active_connection_name] = spec
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         elsif spec.kind_of?(ActiveRecord::Base::ConnectionSpecification)
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-          self.connection = ActiveRecord::Base.send(spec.adapter_method, spec.config)
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+          self.set_connection ActiveRecord::Base.send(spec.adapter_method, spec.config)
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         else
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           raise ConnectionNotEstablished
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         end
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       end
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+      synchronize :active_connection, :connection, :clear_active_connections!,
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+        :clear_reloadable_connections!, :verify_active_connections!, :retrieve_connection,
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+        :connected?, :disconnect!, :set_connection, :with => :@connection_mutex
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+
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       private
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         def clear_cache!(cache, &block)
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           cache.each(&block) if block_given?

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@@ -27,6 +27,8 @@ module ActiveRecord
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     @@connection_pools = {}
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     class << self
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+      # Turning on allow_concurrency basically switches a null mutex for a real one, so that
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+      # multi-threaded access of the connection pools hash is synchronized.
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       def allow_concurrency=(flag)
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         if @@allow_concurrency != flag
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           if flag
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@@ -37,7 +39,7 @@ module ActiveRecord
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         end
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       end
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-      # for internal use only
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+      # for internal use only and for testing
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       def active_connections #:nodoc:
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         @@connection_pools.inject({}) do |hash,kv|
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           hash[kv.first] = kv.last.active_connection

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@@ -18,7 +18,7 @@ class Module
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       raise ArgumentError, "Synchronization needs a mutex. Supply an options hash with a :with key as the last argument (e.g. synchronize :hello, :with => :@mutex)."
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     end
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-    methods.each do |method|
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+    methods.flatten.each do |method|
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       aliased_method, punctuation = method.to_s.sub(/([?!=])$/, ''), $1
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       if instance_methods.include?("#{aliased_method}_without_synchronization#{punctuation}")
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         raise ArgumentError, "#{method} is already synchronized. Double synchronization is not currently supported."
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@@ -26,7 +26,7 @@ class Module
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       module_eval(<<-EOS, __FILE__, __LINE__)
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         def #{aliased_method}_with_synchronization#{punctuation}(*args, &block)
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           #{with}.synchronize do
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-            #{aliased_method}_without_synchronization#{punctuation}(*args,&block)
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+            #{aliased_method}_without_synchronization#{punctuation}(*args, &block)
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           end
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         end
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       EOS