I've been poking in the innards of the menu system the last few days. This is due to yet another client wanting to do something that goes completely against the grain of Drupal.
In this case, it's the way Drupal only shows you menu links you have access to. This to me seems perfectly reasonable good usability: why show you something you can't use? On the other hand, there is a long-standing feature in Comment module that shows anonymous users a 'Login or register to post comments' link on nodes, so 'incitements to action' or whatever the social media buzzword is do exist in Drupal.
So the challenge was to show a link that the anonymous user can't use, send them to login, and back to the link they wanted in the first place. The last part is just some hook_form_alter() work with form redirection (though it did allow me to discover that everything drupal_get_form() is passed is available to the alter hooks, just in a funny place). The access to the menu item is done by intercepting in hook_menu_item_alter() to save a twin of the menu item, and a checkbox in the menu edit form to trigger this. Even registering the path is easy: a custom menu access callback which takes as access arguments those of the original item plus its access callback and negates whatever the original item would return for access. The part I'm (so far) stuck on is getting hook_menu_alter() to know about what the admin user has done in hook_menu_item_alter(): the next job will probably involve creating a truly ugly query that uses %LIKE% to grab menu items based on their options array.
But that's not what I came here to tell you about today.
In my prodding around of the menu system, I found that menu router items have a 'block_callback' property, with its own database field and everything. Now get this: only one item in the entire {menu_router} table on D6 has this filled, and to boot, it has absolutely no effect. (It's the admin theme page, by the way.) This property is something to do with the way that the root admin page is made out of things that are sort of blocks, but not quite. If the property exists on the router item, then the callback is called to add (!!) to the content. (The admin theme page of course plays no part in the root admin page.)
So we had here a completely useless database field, probably left over from Drupal 5 or even earlier. By the way, in a standard Drupal 7 install, the database field is completely unused. I made a dummy patch to get the issue queue testbot confirm that we never get to this particular piece of code, and the superfluous field has now been removed. It's amazing what you find!
By the way, if anyone fancies some bikeshedding, I still don't have a name better than 'menu_login' for this module. To your paintbrushes!