Fix race in ConnectionPool#checkout

After releasing monitor some connection(s) may appear in pool before monitor is re-aquired.
When this happens we'll wait for connection which is already available.

Signed-off-by: Michael Koziarski <michael@koziarski.com>

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

@@ -131,21 +131,20 @@ module ActiveRecord
       # Check-out a database connection from the pool.
       def checkout
         # Checkout an available connection
-        conn = @connection_mutex.synchronize do
-          if @checked_out.size < @connections.size
-            checkout_existing_connection
-          elsif @connections.size < @size
-            checkout_new_connection
-          end
-        end
-        return conn if conn
-
-        # No connections available; wait for one
         @connection_mutex.synchronize do
-          if @queue.wait(@timeout)
-            checkout_existing_connection
-          else
-            raise ConnectionTimeoutError, "could not obtain a database connection within #{@timeout} seconds.  The pool size is currently #{@size}, perhaps you need to increase it?"
+          loop do
+            conn = if @checked_out.size < @connections.size
+                     checkout_existing_connection
+                   elsif @connections.size < @size
+                     checkout_new_connection
+                   end
+            return conn if conn
+            # No connections available; wait for one
+            if @queue.wait(@timeout)
+              next
+            else
+              raise ConnectionTimeoutError, "could not obtain a database connection within #{@timeout} seconds.  The pool size is currently #{@size}, perhaps you need to increase it?"
+            end
           end
         end
       end
@@ -275,4 +274,4 @@ module ActiveRecord
       end
     end
   end
-end
+end