Add an Add to Cart form validate handler to ensure the product is still available

Nice. Here's what the current situation looks like when you add an item to the cart that's no longer available:

Patch that loads the product to check if it is valid.

Mighty slick and nice; works perfectly with and without javascript. However, in testing, I find that the form_set_error() doesn't work anyway, and there's no need to rebuild the form. Here's a simpler patch; see what you think.

Thanks, not sure why the form_set_error is not working but in all honesty I don't think there is a need to use it. I made one minor change to your code and set the message type to warning.

Works for me. This is such an edge case anyway, and @megensel's code works so nicely, that I think this is plenty good enough.

Please remember that setting #disabled to something doesn't prevent submission of that something. The Form API will protect against this and ignore values matching elements in #disabled state, but the form can still be submitted. The Form API doesn't prevent form submission even if there is no valid button submitted (which explains why it is way better to use button-level #validate and #submit rather then their form-level counterparts).

Here is a proper patch: we need allow the "Product not available" button to be pressed, and prevent the validation of the form in that case.