Bug#1014730: liburi-perl: Breaks apt-cacher with "Can't locate Regexp/IPv6.pm" error

[Mark Hindley]

Niko,

On Mon, Jul 11, 2022 at 09:26:12PM +0300, Niko Tyni wrote:
> [apt-cacher maintainers: the context here is that URI.pm introduced an
> optional dependency on Regexp::IPv6 by requiring it in an eval block,
> but the apt-cacher __DIE__ handler exits when the require fails.]

Thanks for including me.

> On Mon, Jul 11, 2022 at 05:35:17PM +0200, gregor herrmann wrote:
> 
> > So we have:
> > - do nothing
> > - patch URI to restart the default signal handler in the eval
> > - (reassign? and) do something in apt-cacher
> 
> I'm not really sure if there's a consensus on best practices around
> $SIG{__DIE__}.

I agree, although perlvar suggests it should be avoided completely:

  	 Having to even think about the $^S variable in your exception handlers
  	 is simply wrong.  $SIG{__DIE__} as currently implemented invites
  	 grievous and difficult to track down errors.  Avoid it and use an
  	 "END{}" or CORE::GLOBAL::die override instead.

I don't know when that was added. I don't remember reading it before.

> IMO apt-cacher is the one that should be fixed, rather
> than demanding that all the modules it uses need to reset and restore
> $SIG{__DIE__} before catching exceptions.
> 
> >From 'perldoc -f die' :
> 
>         You can arrange for a callback to be run just before the "die"
>         does its deed, by setting the $SIG{__DIE__} hook. The associated
>         handler is called with the exception as an argument, and can
>         change the exception, if it sees fit, by calling "die" again.
>         See "%SIG" in perlvar for details on setting %SIG entries, and
>         "eval" for some examples. Although this feature was to be run only
>         right before your program was to exit, this is not currently so:
>         the $SIG{__DIE__} hook is currently called even inside "eval"ed
>         blocks/strings! If one wants the hook to do nothing in such
>         situations, put
> 
>             die @_ if $^S;
> 
>         as the first line of the handler (see "$^S" in perlvar). Because
>         this promotes strange action at a distance, this counterintuitive
>         behavior may be fixed in a future release.
> 
> Corresponding untested patch against apt-cacher attached.

The problem with this approach is that errors from apt-cacher's own evals will
be skipped as well.

Mark