What order do before filters occur in?
The accepted answer is no longer true as of Rails 4.2.6. It was probably changed earlier, but that's the version I'm on right now.
Parent filters are now called before child filters, so the ordering will now be:
:set_user:set_post
And, if you think about it, that ordering really makes more sense. As previously said, if you need the child to call the filter before the parent, you need to use prepend_before_action.
I suggest taking a look at the source code and API Docs on filters.
The default ordering should be
:set_post:set_user
I think if you wanted to push :set_user to the top of the stack you could change the line in A to
prepend_before_action :set_user
Also worth pointing out, this isn't the only question on the topic; there are others here on SO.
As for your specific situation, it looks like you'll need to change A as I mentioned above in order to have @user be assigned by the time set_post in B runs.
As of 4.2.6 (probably changed in an earlier version), the ordering is now parent before child:
:set_user:set_post