bug#8827: Problem in url-cache

[Nick Dokos <nicholas.dokos@hp.com>]

I was chasing down some problems I was having with Julien Danjou's
google-weather.  The blow-by-blow description can be found in the
following threads on the emacs and orgmode mailing lists:

  http://thread.gmane.org/gmane.emacs.help/81302
  http://thread.gmane.org/gmane.emacs.orgmode/42565

This bug report (and possible patch) concern one aspect of this:
google-weather uses the url package to retrieve icons in GIF format (but
the problem may arise from any kind of binary data) from the server and
caches them locally for 12 hours by default. It does that by calling
url-store-in-cache:

(defun url-store-in-cache (&optional buff)
  "Store buffer BUFF in the cache."
    (with-current-buffer (get-buffer (or buff (current-buffer)))
      (let ((fname (url-cache-create-filename (url-view-url t))))
        (if (url-cache-prepare fname)
            (let ((coding-system-for-write 'binary))
              (write-region (point-min) (point-max) fname nil 5))))))

While the cache is valid, it then fetches the cached data by calling
url-cache-extract:

(defun url-cache-extract (fnam)
  "Extract FNAM from the local disk cache."
  (erase-buffer)
  (insert-file-contents-literally fnam))

But it seems that in a multibyte context, what url-store-in-cache
stores and what url-cache-extract retrieves are not identical:
the former stores a sequence of bytes as they come from the network
and the latter may combine individual bytes into multibyte entities.

My workaround for this problem is

--8<---------------cut here---------------start------------->8---
diff --git a/lisp/url/url-cache.el b/lisp/url/url-cache.el
index 1615920..80d7702 100644
--- a/lisp/url/url-cache.el
+++ b/lisp/url/url-cache.el
@@ -192,6 +192,7 @@ Very fast if you have an `md5' primitive function, suitably fast otherwise."
 (defun url-cache-extract (fnam)
   "Extract FNAM from the local disk cache."
   (erase-buffer)
+  (set-buffer-multibyte nil)
   (insert-file-contents-literally fnam))
 
 (defun url-cache-expired (url &optional expire-time)
--8<---------------cut here---------------end--------------->8---

It seems to work satisfactorily for google-weather. I don't know whether
it causes problems in other ways.

Thanks,
Nick