Synchronize the ConnectionPool#release method to avoid thread safety issues [#6464]

Fixes #6464

Synchronize the contents of the release method in ConnectionPool due to
errors when running in high concurrency environments.

    Detected invalid hash contents due to unsynchronized modifications
with concurrent users
    org/jruby/RubyHash.java:1356:in `keys'
/usr/local/rvm/gems/jruby-1.6.7@new_import/gems/activerecord-3.2.3/lib/a
ctive_record/connection_adapters/abstract/connection_pool.rb:294:in
`release'

/usr/local/rvm/gems/jruby-1.6.7@new_import/gems/activerecord-3.2.3/lib/a
ctive_record/connection_adapters/abstract/connection_pool.rb:282:in
`checkin'

1 files changed, 12 insertions, 10 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index f44cba3978..d4649102df 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -290,17 +290,19 @@ connection.  For example: ActiveRecord::Base.connection.close
       private
 
       def release(conn)
-        thread_id = nil
-
-        if @reserved_connections[current_connection_id] == conn
-          thread_id = current_connection_id
-        else
-          thread_id = @reserved_connections.keys.find { |k|
-            @reserved_connections[k] == conn
-          }
-        end
+        synchronize do
+          thread_id = nil
+
+          if @reserved_connections[current_connection_id] == conn
+            thread_id = current_connection_id
+          else
+            thread_id = @reserved_connections.keys.find { |k|
+              @reserved_connections[k] == conn
+            }
+          end
 
-        @reserved_connections.delete thread_id if thread_id
+          @reserved_connections.delete thread_id if thread_id
+        end
       end
 
       def new_connection