open() with pipe breaks AnyEvent STDIN poll

[Nick Andrew]

On Wed, Nov 26, 2014 at 01:26:47PM +0100, Marc Lehmann wrote:
> On Wed, Nov 26, 2014 at 06:10:43PM +1100, Nick Andrew <nick at nick-andrew.net> wrote:
> > I found a problem where a working AnyEvent script broke badly because
> > of some code in a 3rd party module called from the script. I isolated
> > it to the call to open(CMD, "command |").
> 
> The callback modifies $_, which is a global variable, without restoring it,
> so code outside the watcher will have its loop variable destroyed.

Ok. I wasn't aware that $_ is a global variable; it didn't even enter my
thinking as you can see; I thought the problem was with file handles and
STDIN; the while (<FH>) loop didn't even occur to me as the source of
the problem.

> This is, strictly speaking, a bug in the 3rd party module - it shouldn't
> trash variables in use by the caller. The workaround is easy though, just add
> a local $_ before you call it.

I added this and the script resumed working as expected:

	cb => sub {
		local $_;
		call_into_third_party_module();
		[...]

> > which case it deserves a big fat warning somewhere in the AnyEvent documentation)?
> 
> There is one in bold letters in the WATCHERS section (and the perlop
> docs also mention that $_ isn't localised by while and you would need an
> explicit local $_ for that):

I guess the problem here is that I wasn't even aware that $_ was being
changed by something; it took me about 6 hours of debugging to isolate
the problem down to the open() ... while ... cause it's not my 3rd
party code.

> It is probably useful to report the bug to the 3rd party lib, as modifying
> the caller's $_ variable is likely going to break other code as well and
> is usually hard to diagnose (outside of AnyEvent's strict mode :).

I'll report it but unsure of my ability to get the code changed.

Would it be reasonable for AnyEvent to change to not use $_ within
the event loop, to prevent a whole class of problems and protect its
own data structures?

Nick.
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