bug#44822: 27.1; Regression in `ffap-read-file-or-url'

[Eli Zaretskii]

> Date: Mon, 23 Nov 2020 09:23:37 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> 
> `ffap-read-file-or-url' now reads a URL as a file name, truncating to
> remove a prefix such as http:/.  Prior to Emacs 27, it correctly
> returned the URL.
> 
> If point is not on a URL, then the guess is nil.  The user is prompted
> (as before) with the default directory as default.  If the user rejects
> that default value and replaces it by a URL (e.g. yanking), then
> `ffap-read-file-or-url' should just return that URL, as it has always
> done.  In Emacs 27, it instead tries to handle it as a file name,
> removing the prefix up to the first `/' before a non-/ char.
> 
> emacs -Q
> 
> ;; With point not on a URL or file name:
> 
> (ffap-read-file-or-url "URL: " nil)
> 
> ;; User is prompted, with the default-directory as default:
> 
> URL: /my/default/dir/
> 
> ;; User pastes a URL after that or replaces that with a URL.
> 
> URL: /my/default/dir/http://foobar.com RET
> ;; or
> URL: http://foobar.com RET
> 
> ;; This is returned: /foobar.com
> 
> In Emacs 26.3 the URL entered by the user is returned correctly.
> 
> A main use of `read-file-or-url' is to prompt for and read a URL.  If a
> user enters a URL when FFAP has not been able to guess a URL, FFAP now
> just treats the input as a filename.  It treats prefix `http:/' the same
> way it would treat prefix `c:' on MS Windows.
> 
> Reading a URL is maybe the main use case of `ffap-read-file-or-url'.  (There 
> is no `ffap-read-url'.)  It's now broken.

Thierry and Stefan, these changes seem to have been done by you.  Can
you please take a look at this issue?  Would it be possible to fix
this for Emacs 27.2?