Zero-length files cached by nginx

[ludo@gnu.org (Ludovic Courtès)]

John Darrington <john@darrington.wattle.id.au> skribis:

> and if I try to manually download https://mirror.hydra.gnu.org/nar/ciqw5z470c8ihl1kfswj1j3ix6hs092d-module-import
>
> I get a zero length file. :(

Indeed:

--8<---------------cut here---------------start------------->8---
$ wget -q -O - https://mirror.hydra.gnu.org/nar/ciqw5z470c8ihl1kfswj1j3ix6hs092d-module-import |wc -c
0
$ wget -q -O - https://hydra.gnu.org/nar/ciqw5z470c8ihl1kfswj1j3ix6hs092d-module-import |wc -c
0
--8<---------------cut here---------------end--------------->8---

So the problem is that nginx on hydra.gnu.org cached a zero-length file,
and then nginx on mirror.hydra.gnu.org cached that too.

On hydra.gnu.org, we have:

--8<---------------cut here---------------start------------->8---
/var/cache/nginx/nar# cat 09/87d4a93959df3b0c82baed615b578109
X��������:kX`��
KEY: http://127.0.0.1:3000/nar/ciqw5z470c8ihl1kfswj1j3ix6hs092d-module-import
HTTP/1.1 200 OK
Content-Type: application/x-nix-archive
Set-Cookie: hydra_session=437ad1277e047114d0a49212805fd1867c0cf914; path=/; expires=Tue, 10-Jan-2017 05:43:20 GMT; HttpOnly
Transfer-Encoding: chunked
Date: Tue, 03 Jan 2017 05:43:43 GMT
Connection: close

--8<---------------cut here---------------end--------------->8---

Hydra (the software) produces HTTP chunked-encoded nars, as can be seen
above.

Rereading <https://tools.ietf.org/html/rfc2616#section-3.6.1>, it seems
that HTTP clients can detect truncated chunks, but they cannot detect
missing chunks since the total number of chunks is unknown and the
trailer is optional.

Presumably, what happened is that hydra.gnu.org was loaded and Hydra
dropped the connection, and then nginx cached what it got (i.e.,
nothing.)

Seems like the only way to fix it is by using ‘Content-Length’.  At the
very least, we should do that in ‘guix publish’.

For now, I’ve removed the faulty cached item from the nginx cache on
both machines.

Thanks,
Ludo’.