Cannot open a file in a directory under control of Mecurial

[Kimikazu]

This problem happened just after the upgrade to Ubuntu 13.04.

When trying to open a file in a directory under control of Mercurial, i.e. a directory which has .hg directory, emacs stops in its staring process.
When this symptom occurs, in the minibuffer, it says:
Loading vc-hg...done

And the following is dumped to stderr:
(emacs:5305): GLib-WARNING **: GChildWatchSource: Exit status of a child process was requested but ECHILD was received by waitpid(). Most likely the process is ignoring SIGCHLD, or some other thread is invoking waitpid() with a nonpositive first argument; either behavior can break applications that use g_child_watch_add()/g_spawn_sync() either directly or indirectly.

Even when opening an unregistered file in a Mercurial-controlled directory, the same symptom happens.
It works when started alone (to open *scratch* buffer) and it's also OK to open a file not under control of Mercurial.

Additional information after the first post:
    - It gets fixed after uninstalling mercurial, and reproduces again after re-installing mercurial.
    - It occurs even when there's no ~/.emacs or ~/.emacs.d/init.el
    - It didn't reproduced on another PC. so it depends on the environments.
    - The same symptom reproduces in a git-controlled directory, so it seems to be a problem of vc-mode.
    - WORKAROUND: Cancel vc-mode in .emacs file by "(setq vc-handled-backends ())" then it works.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: emacs24 24.2+1-2ubuntu2
ProcVersionSignature: Ubuntu 3.8.0-19.29-generic 3.8.8
Uname: Linux 3.8.0-19-generic x86_64
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Thu May 2 10:50:18 2013
InstallationDate: Installed on 2012-11-05 (177 days ago)
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release amd64 (20121017.5)
MarkForUpload: True
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: emacs24
UpgradeStatus: Upgraded to raring on 2013-05-02 (0 days ago)

[Rolf Schreiber (t-launchpad-schreiber-rolf-de)]

I guess my problem has the same root cause: since 13.04 I cannot run emacs as root anymore.
to be more precise:

emacs -q hangs immediately with the same message: GLib-WARNING **: GChildWatchSource: ...

emacs -Q works, but then running dired on any dir freezes again with the same message

opening git-controlled files freezes as well

running (setq vc-handled-backends ()) after the start does help for the git-controlled files, but not for the dired
so the root cause most likely isn't vc-mode but somewhere else.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.