The problem is the following: when I try to login to
https://aur.archlinux.org with `url-retrieve-synchronously' (see the
code below), Emacs freezes (C-g helps) because callback passed to
`url-retrieve' is never called.

AFAIU this happens because `url-http-parse-headers' returns nil if
response status is 301/302/307 and method is not "HEAD" or "GET" (see
<http://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/url/url-http.el#n574>).
At least the problem goes away when I add:

  (setq success t)

after

  (or `moved-permanently `found `temporary-redirect)

in `url-http-parse-headers'.

Would it be OK to allow such requests?

Recipe is simple but you need to have an account on
<https://aur.archlinux.org> for that:

1. emacs -Q
2. Evaluate (setq url-debug t)
3. Evaluate the following code: