Improve documentation and add test

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

@@ -80,11 +80,8 @@ module ActiveRecord
     # * private methods that require being called in a +synchronize+ blocks
     #   are now explicitly documented
     class ConnectionPool
-      # Threadsafe, fair, LIFO queue.  Meant to be used by ConnectionPool
-      # with which it shares a Monitor.  But could be a generic Queue.
-      #
-      # The Queue in stdlib's 'thread' could replace this class except
-      # stdlib's doesn't support waiting with a timeout.
+      # Threadsafe, fair, FIFO queue.  Meant to be used by ConnectionPool
+      # with which it shares a Monitor.
       class Queue
         def initialize(lock = Monitor.new)
           @lock = lock
@@ -111,7 +108,7 @@ module ActiveRecord
         # Add +element+ to the queue.  Never blocks.
         def add(element)
           synchronize do
-            @queue.unshift element
+            @queue.push element
             @cond.signal
           end
         end
@@ -173,7 +170,7 @@ module ActiveRecord
 
           # Removes and returns the head of the queue if possible, or +nil+.
           def remove
-            @queue.shift
+            @queue.pop
           end
 
           # Remove and return the head the queue if the number of

activerecord/test/cases/connection_pool_test.rb

@@ -203,6 +203,14 @@ module ActiveRecord
         end.join
       end
 
+      def test_checkout_order_is_fifo
+        conn1 = @pool.checkout
+        conn2 = @pool.checkout
+        @pool.checkin conn1
+        @pool.checkin conn2
+        assert_equal [conn2, conn1], 2.times.map { @pool.checkout }
+      end
+
       # The connection pool is "fair" if threads waiting for
       # connections receive them in the order in which they began
       # waiting.  This ensures that we don't timeout one HTTP request