bug#526: Emacs 22, url package, problems with cache

[Alexander Pohoyda]

The url-automatic-caching variable is nil by default, so cache is not written 
(even the cache directory is not created).  However, if I manually provide 
"If-Modified-Since" header in HTTP request and the server responds with "Not 
Modified 304", the cache file is expected to exists and is accessed -- which 
leads to error.

Possible solution -- before accessing the cache file, check whether it exists 
(url-is-cached).
Alternatively, do not expect a cache to exist if url-automatic-caching is set 
to nil.

-- 
Alexander Pohoyda <alexander.pohoyda@gmx.net>
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