ETag support broken on hackage.haskell.org

[Gintautas Miliauskas]

Hi,

first of all, a shout out to all ZuriHac2014 folks, thanks for the gentle
introduction to the deeper trenches of Haskell infrastructure!

I spent a bit of time looking into why "cabal update" always redownloads
the entire package list even though the code has explicit support for
etags, and it looks like etags are broken on the server side. The server
does serve "Etag" headers, but does not react to If-None-Match:

$ curl -I --header 'If-None-Match: "1ea6c9a3168b9c186d5bbafd065ce5ec"'
hackage.haskell.org/packages/index.tar.gz
HTTP/1.1 200 OK
Server: nginx/1.6.0
Date: Mon, 09 Jun 2014 23:22:49 GMT
Content-Type: application/x-gzip
Content-Length: 7193838
Connection: keep-alive
Content-MD5: 1ea6c9a3168b9c186d5bbafd065ce5ec
ETag: "1ea6c9a3168b9c186d5bbafd065ce5ec"
Last-modified: Mon, 09 Jun 2014 23:10:44 GMT

In this case the server is supposed to return a 304 code because the ETag
matches, but it returns the usual 200.

Here's a well-behaved server for comparison:

$ curl -I --header 'If-None-Match: f89ef04efbcb1bbd413c9170523beeb1'
http://upload.wikimedia.org/
HTTP/1.1 304 Not Modified
Last-Modified: Thu, 03 Oct 2013 22:29:34 GMT
Etag: f89ef04efbcb1bbd413c9170523beeb1
X-Timestamp: 1380839374.33426
X-Object-Meta-Mtime: 1366745378.0
Content-Type: text/html
X-Varnish: 2552484147 2541352860, 2937266545 2853346814, 523731362
Via: 1.1 varnish, 1.1 varnish, 1.1 varnish
Accept-Ranges: bytes
Date: Mon, 09 Jun 2014 23:29:36 GMT
Age: 38954
Connection: keep-alive
X-Cache: cp1064 hit (336), cp3010 hit (1791), cp3007 frontend miss (0)
Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: Age, Content-Length, Date, X-Cache, X-Varnish

Note the 304 return code.

I do not know about the configuration in use on the server, but something
seems to be wrong there. There are some nginx bug reports [1] about similar
issues, not sure if they are relevant here.

[1] https://github.com/rgrove/rawgit/issues/13

Best regards,
-- 
Gintautas Miliauskas
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