Connection pool timeout in production mode with AR sessions

It would appear that in Production mode with ActiveRecord session store enabled there is a timeout (default 5 seconds) when the connection pool is full when using MySQL.

To reproduce:

Create a trivial rails app with 2.3RC1 with ActionController::Base.session_store = :active_record_store (I'm using a trivial, scaffold generated model/controller)

Run the app in production mode, create a record in the database and then, in the browser, hit "refresh" on the index action of the controller at 1 second intervals. Every 4-5 refreshes the browser will "stall" for 5 seconds although the log output shows no addtional time in the "View:" or "DB:" entries in the log (see below).

Increasing the size of "pool" in database.yml delays the onset of the problem.

Changing "timeout" from it's default of 5000 in database.yml appears to have no effect.

However, adding an entry for "wait_timeout" to database.yml and reducing it's default value(5) to 1 or 0.5 changes the "stall" time in the browser to 1 or 0.5 seconds.

The problem is not apparent in development mode (I believe the connection pool is cleared for each request).

The problem is not apparent if AR session store is disabled.

Monitoring the connections in MySQL Admin I can see that the number of open connections increases with each request until it reaches the value of "pool" at which point the problem starts - I assume that connections are not being closed at the end of each request.

Environment:

Server: MySQL server 5.0.45 on a networked RedHat Linux server (not localhost)

Client 1: CentOS 5.2, MySQL 5.0.45, mysql gem 2.7.3

Client 2: Windows Vista, MySQL 5.0.67, mysql gem 2.7.3

Identical results on both clients.

I have not tried to reproduce the problem with any other database adapter.

Sample log output from hitting "refresh" in the browser aprrox. 1 second after each page reload - note the 5+ second delay between the last an penultimate request:

Processing MoviesController#index (for 127.0.0.1 at 2009-02-10 10:02:58) [GET] Rendering template within layouts/movies Rendering movies/index Completed in 7ms (View: 2, DB: 6) | 200 OK [http://localhost/movies]

Processing MoviesController#index (for 127.0.0.1 at 2009-02-10 10:02:59) [GET] Rendering template within layouts/movies Rendering movies/index Completed in 7ms (View: 4, DB: 5) | 200 OK [http://localhost/movies]

Processing MoviesController#index (for 127.0.0.1 at 2009-02-10 10:03:01) [GET] Rendering template within layouts/movies Rendering movies/index Completed in 6ms (View: 3, DB: 10) | 200 OK [http://localhost/movies]

Processing MoviesController#index (for 127.0.0.1 at 2009-02-10 10:03:02) [GET] Rendering template within layouts/movies Rendering movies/index Completed in 8ms (View: 3, DB: 9) | 200 OK [http://localhost/movies]

Processing MoviesController#index (for 127.0.0.1 at 2009-02-10 10:03:03) [GET] Rendering template within layouts/movies Rendering movies/index Completed in 5ms (View: 2, DB: 52) | 200 OK [http://localhost/movies]

Processing MoviesController#index (for 127.0.0.1 at 2009-02-10 10:03:09) [GET] Rendering template within layouts/movies Rendering movies/index Completed in 6ms (View: 2, DB: 9) | 200 OK [http://localhost/movies]