[PATCH] zeroconf: fail nicely on IPv6 only system

[Jun Wu]

After reading the surrounding code, I believe 127.0.0.1 is fine. So this
patch looks good to me. Thanks!

The test failure where 127.0.0.1 does not exist is actually caused by other
code. I'll send a fix soon.

Excerpts from Simon Farnsworth's message of 2017-02-09 11:33:07 +0000:
> There's one chunk in test-paths.t that fails without this change on my 
> Facebook IPv6-only devserver, and succeeds with it. Beyond that, I have 
> no interest in this code at all.
> 
> I chose 127.0.0.1 as the replacement where the "external" address is 
> IPv6 because this code already uses 127.0.0.1 as an "error" indicator 
> (the last line of context in the patch below). I have zero interest in 
> fixing this code beyond making it not crash when test-paths.t invokes it 
> for me, so that I can stop doing `./run-tests.py -j50 -l ; hg email -r 
> REVS` (relying on me spotting that there have been "unexpected" test 
> failures, which has a tendency to go wrong towards the end of the 
> working day) and start doing `./run-tests.py -j50 -l && hg email -r 
> REVS` (relying on the machine not even invoking `hg email` if there are 
> test failures).
> 
> If someone else does the work to replace the use of 127.0.0.1 in the 
> existing error case with a different value, I'll happily redo this patch 
> to follow their lead, but I've already spent more time on this than I'm 
> willing to justify for an extension I've never used.
> 
> Simon
> 
> On 09/02/2017 04:24, Jun Wu wrote:
> > This does not work if the hardcoded 127.0.0.1 is not known to the system:
> >
> >     sudo ip addr del 127.0.0.1/8 dev lo # on Linux
> >
> > That could happen if the system is IPv6-only, or only has other addresses in
> > the 127./8 subnet like 127.0.0.2, or does not have a "lo" device. I think
> > the pathological environments are not that uncommon with all kinds of
> > container technologies.
> >
> > Maybe a better fix is to just skip the test when we detect "dumbip" is not
> > an IPv4 address.
> >
> > Excerpts from Simon Farnsworth's message of 2017-02-08 08:10:48 -0800:
> >> # HG changeset patch
> >> # User Simon Farnsworth <simonfar at fb.com>
> >> # Date 1486570121 28800
> >> #      Wed Feb 08 08:08:41 2017 -0800
> >> # Node ID a847eb00fcfeffce458e11b80ad38d4d4e7a700f
> >> # Parent  d50cda2a403786836d1f0d5c99401599dc4f43ec
> >> zeroconf: fail nicely on IPv6 only system
> >>
> >> zeroconf only knows how to deal with IPv4; I develop on a system where the only
> >> IPv4 address is 127.0.0.1.
> >>
> >> Teach zeroconf to ignore IPv6 addresses when looking for plausible IPv4
> >> connectivity.
> >>
> >> diff --git a/hgext/zeroconf/__init__.py b/hgext/zeroconf/__init__.py
> >> --- a/hgext/zeroconf/__init__.py
> >> +++ b/hgext/zeroconf/__init__.py
> >> @@ -64,7 +64,9 @@
> >>      # Generic method, sometimes gives useless results
> >>      try:
> >>          dumbip = socket.gethostbyaddr(socket.gethostname())[2][0]
> >> -        if not dumbip.startswith('127.') and ':' not in dumbip:
> >> +        if ':' in dumbip:
> >> +            dumbip = '127.0.0.1'
> >> +        if not dumbip.startswith('127.'):
> >>              return dumbip
> >>      except (socket.gaierror, socket.herror):
> >>          dumbip = '127.0.0.1'
>