Add primary key to signup_log table

I'm not convinced this is a bug, but I agree something like this would probably be worth adding. For example, it'd provide most of the solution for #325237: Allow users (even anonymous) to cancel signup via secure URL.

That said, please don't write your external module to be insecure by taking URLs like http://example.com/cancel-signup/sid. That provides all sorts of potential for abuse and CSRF. If sid is a monotonically increasing integer, it's trivial to guess the sid for other signups and cancel other people's signups, etc. Bad, bad. Instead, you want to use a "signup token". Something like:

function signup_get_token($sid) {
  $private_key = drupal_get_private_key();
  return md5('signup_token' . $sid . $private_key);
}

function signup_valid_token($token, $sid) {
  $private_key = drupal_get_private_key();
  return $token == md5('signup_token' . $sid . $private_key);
}

Then, in the signup confirmation email, you'll know the sid, so you can generate the right token and create a URL like http://example.com/cancel-signup/sid/token -- the menu handler for that URL can then validate the token based on the sid and either allow or deny the request.

In fact, I'd rather this functionality (a secure way to cancel a signup via a given URL) was in signup itself instead of an external module -- so after I add the numeric sid here, let's move the rest of this discussion over to #325237 to provide a secure signup-cancel URL.

This patch had a few fatal flaws:

A) 'serial' fields (e.g. auto_increment in MySQL) cannot have a DEFAULT value of 1.

B) The DB update should use db_add_field() instead of trying to manually alter the tables to add the new field.

I fixed both, renumbered the update (6200), tested the upgrade path and a new install, and this is all working now. Earlier tonight I created the DRUPAL-6--1 branch for the 6.x-1.* stable series, and I just created a 6.x-2.x-dev release node pointing to HEAD. So, I committed the attached patch to HEAD as the first new feature for 6.x-2.*. Thanks!

FYI: After further consideration, I just backported this (and related changes) into DRUPAL-6--1: http://drupal.org/cvs?commit=159750