What I was trying to accomplish is optional HTTP authentication: that is, a resource logs you in if your credentials are correct, but if they aren’t present then it just lets you go on as an anonymous user. This is essential if you are developing, say, a web shop: if you want to offer personalized item selections you need to request login, but if you require authentication just to browse the site you’ve lost a good %LARGE_NUMBER% of your customers right there.
However, as fuzzyman rightly pointed out, most browers do not even bother to send the Authorization header unless they actually get a 401 on a page, even if credentials are explicitly provided (as in my use of XMLHTTPRequest). The solution to this is to “fool” the browser into thinking your site requires authentication by creating a dummy action that just requires authentication. The slight complication is that this action must be in the root directory! If you attempt to create the dummy action in a subdirectory, the browser may only send the authentication information thus forced into it when paths are accessed that appear to be in that directory. This means, for example, that if you have the authenicated action “/sessions/secret” then authorization info will be sent for “/sessions/foo”, but will not for “/products/list”. Making an action like “/secret” works around this, although it is slightly ugly.
So then in your user creation view and login view you will have a script block which forces the logged in / newly created user to login. This example is for user creation (hence @user.password) and uses a modified version Prototype (the de-facto Rails JS library) to perform the request. I had to modify Prototype to add support for using the username and password parameters of the underling XMLHTTPRequest object, despite the fact that they are widely supported in practice.
Yes, this does send the username and password in plaintext over the wire :-). However, this is OK since they are sent in the clear during signup anyway. I’m also currently using Basic authentication, which means that upon ANY login the username/password are vulnerable (albeit after Base64 decoding): I will probably change this at some point, but the Digest scheme I should be using requires a bit of server side state to prevent replay attacks so is a little tricker to implement.
Right, I should probably try and get some revision done now :-). If only I could answer on Ruby on Rails instead of complexity theory…