database connections are automatically established after forking.

Connection pools are 1:1 with pids.

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -328,16 +328,18 @@ module ActiveRecord
     # ActiveRecord::Base.connection_handler. Active Record models use this to
     # determine that connection pool that they should use.
     class ConnectionHandler
-      attr_reader :connection_pools
-
-      def initialize(pools = {})
+      def initialize(pools = Hash.new { |h,k| h[k] = {} })
         @connection_pools = pools
-        @class_to_pool    = {}
+        @class_to_pool    = Hash.new { |h,k| h[k] = {} }
+      end
+
+      def connection_pools
+        @connection_pools[$$]
       end
 
       def establish_connection(name, spec)
-        @connection_pools[spec] ||= ConnectionAdapters::ConnectionPool.new(spec)
-        @class_to_pool[name] = @connection_pools[spec]
+        set_pool_for_spec spec, ConnectionAdapters::ConnectionPool.new(spec)
+        set_class_to_pool name, connection_pools[spec]
       end
 
       # Returns true if there are any active connections among the connection
@@ -350,21 +352,21 @@ module ActiveRecord
       # and also returns connections to the pool cached by threads that are no
       # longer alive.
       def clear_active_connections!
-        @connection_pools.each_value {|pool| pool.release_connection }
+        connection_pools.each_value {|pool| pool.release_connection }
       end
 
       # Clears the cache which maps classes.
       def clear_reloadable_connections!
-        @connection_pools.each_value {|pool| pool.clear_reloadable_connections! }
+        connection_pools.each_value {|pool| pool.clear_reloadable_connections! }
       end
 
       def clear_all_connections!
-        @connection_pools.each_value {|pool| pool.disconnect! }
+        connection_pools.each_value {|pool| pool.disconnect! }
       end
 
       # Verify active connections.
       def verify_active_connections! #:nodoc:
-        @connection_pools.each_value {|pool| pool.verify_active_connections! }
+        connection_pools.each_value {|pool| pool.verify_active_connections! }
       end
 
       # Locate the connection of the nearest super class. This can be an
@@ -388,21 +390,53 @@ module ActiveRecord
       # can be used as an argument for establish_connection, for easily
       # re-establishing the connection.
       def remove_connection(klass)
-        pool = @class_to_pool.delete(klass.name)
+        pool = class_to_pool.delete(klass.name)
         return nil unless pool
 
-        @connection_pools.delete pool.spec
+        connection_pools.delete pool.spec
         pool.automatic_reconnect = false
         pool.disconnect!
         pool.spec.config
       end
 
       def retrieve_connection_pool(klass)
-        pool = @class_to_pool[klass.name]
+        pool = get_pool_for_class klass.name
         return pool if pool
         return nil if ActiveRecord::Model == klass
         retrieve_connection_pool klass.active_record_super
       end
+
+      private
+
+      def class_to_pool
+        @class_to_pool[$$]
+      end
+
+      def set_pool_for_spec(spec, pool)
+        @connection_pools[$$][spec] = pool
+      end
+
+      def set_class_to_pool(name, pool)
+        @class_to_pool[$$][name] = pool
+        pool
+      end
+
+      def get_pool_for_class(klass)
+        @class_to_pool[$$].fetch(klass) {
+          c_to_p = @class_to_pool.values.find { |class_to_pool|
+            class_to_pool[klass]
+          }
+
+          if c_to_p
+            pool = c_to_p[klass]
+            pool = ConnectionAdapters::ConnectionPool.new pool.spec
+            set_pool_for_spec pool.spec, pool
+            set_class_to_pool klass, pool
+          else
+            set_class_to_pool klass, nil
+          end
+        }
+      end
     end
 
     class ConnectionManagement

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -1238,7 +1238,11 @@ module ActiveRecord
             # prepared statements whose return value may have changed is
             # FEATURE_NOT_SUPPORTED.  Check here for more details:
             # http://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/cache/plancache.c#l573
-            code = e.result.result_error_field(PGresult::PG_DIAG_SQLSTATE)
+            begin
+              code = e.result.result_error_field(PGresult::PG_DIAG_SQLSTATE)
+            rescue
+              raise e
+            end
             if FEATURE_NOT_SUPPORTED == code
               @statements.delete sql_key(sql)
               retry

activerecord/test/cases/connection_management_test.rb

@@ -26,6 +26,27 @@ module ActiveRecord
         assert ActiveRecord::Base.connection_handler.active_connections?
       end
 
+      def test_connection_pool_per_pid
+        return skip('must support fork') unless Process.respond_to?(:fork)
+
+        object_id = ActiveRecord::Base.connection.object_id
+
+        rd, wr = IO.pipe
+
+        pid = fork {
+          rd.close
+          wr.write Marshal.dump ActiveRecord::Base.connection.object_id
+          wr.close
+          exit!
+        }
+
+        wr.close
+
+        Process.waitpid pid
+        assert_not_equal object_id, Marshal.load(rd.read)
+        rd.close
+      end
+
       def test_app_delegation
         manager = ConnectionManagement.new(@app)