GNU bug report logs - #36350
[2.2.5] ‘read-headers’ blocks, thereby breaking web servers

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Message #16 received at 36350 <at> debbugs.gnu.org (full text, mbox):

From: Mark H Weaver <mhw <at> netris.org>
To: Ludovic Courtès <ludo <at> gnu.org>
Cc: 36350 <at> debbugs.gnu.org
Subject: Re: bug#36350: [2.2.5] ‘read-headers’
 blocks, thereby breaking web servers
Date: Mon, 24 Jun 2019 11:36:41 -0400

Hi Ludovic,

Ludovic Courtès <ludo <at> gnu.org> writes:

>   ./meta/guile examples/web/hello.scm &
>   wget -O - http://localhost:8080
>
> You’ll notice that ‘wget’ hangs (never receives a response) because the
> server is actually stuck in a read(2) call that will never complete, in
> ‘read-headers’.
>
> Reverting 73cde5ed7218a090ecee888870908af5445796f0 solves the problem.
>
> AIUI, before that commit, ‘read-header-line’ would read exactly one
> line.  After this change, it calls ‘lookahead-char’, which can block,
> and that’s exactly what’s happening here.

Gah, indeed!

Also, I see now that there was already some existing code to handle HTTP
continuation lines, before my commit referenced above, although it
neglects to fold the whitespace after the CRLF into a single SP octet.
I'm not sure how I failed to notice that.

The commit should simply be reverted, I think.  Pushed as commit
e1225d013ed8673382d6d8f9300dd6b175c8b820 on the stable-2.2 branch.
I tried leaving the new test in place, but it failed due to the lack of
whitespace folding in the previous code.

I really messed up here, sorry.

> I also noticed that there are no unit tests for (web server), which we
> should probably address while we’re at it.  :-)

Yes, that would be great.  I won't be able to do it anytime soon,
though.

       Mark

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