SIGPIPE from syntax highlighting filter if highlight is not available

[John Keeping]

On Thu, Sep 29, 2016 at 04:38:41PM +0200, Vadim Zeitlin wrote:
>  As this is the first time I'm posting here, let me start by thanking you
> for developing cgit! I'm using it since quite some time under Debian
> (currently Jessie) and it works very well but recently I've mistakenly
> removed highlight package because I thought it was redundant with other
> similar tools and completely forgot that cgit depended on it.
> 
>  Since then, viewing any files in cgit didn't work, which was probably only
> to be expected, but viewing sufficiently large files resulted in 500 server
> error due to, after debugging, SIGPIPE received but not handled by cgit
> when reading from the filter subprocess.
> 
>  I'm not sure what exactly should be done here: maybe nothing, and people
> should be just more careful with uninstalling dependencies. But I think
> ideally cgit should give an error indicating that filter execution failed
> and I'd at least expect it not to just die without any further information.

If we ignore SIGPIPE we should just hit the die_errno() in
html.c::html_raw() which will say "write error on html output".  That's
not really as useful as it could be, since that code doesn't care
whether or not it's writing via a filter.

I'm not sure if we want to start keeping track of whether we're writing
through a filter, but some form of error message rather than a silent
failure does seem like an improvment.  I think we just need to ignore
SIGPIPE to make that happen.