Hang when running 'bzr remove-pipe'

  affects bzr-pipeline

This bug was recently filed against the Debian package for bzr-pipeline
in Debian bug 574682 by Ben Finney. Quoting from his bug report:

I'm trying to get my head around how the ‘pipeline’ plugin works, and
have found a bug which causes Bazaar to hang.

Bumbling with adding a new pipe before the current one:

    $ bzr add-pipe --before
    bzr: ERROR: command 'add-pipe' requires argument PIPE

Oh okay, it wants to know *which* pipe the new one should go before.
Let's try adding it after the previous one (oblivious to the fact that
I've not decided on a name for the new pipe):

    $ bzr add-pipe --after :prev
    Tree is up to date at revision 2.
    Created and switched to pipe ":prev".

No, that's not what I meant; it created a pipe *named* “:prev”. That's
okay, I'll just remove the pipe:

    $ bzr remove-pipe
    [… hang indefinitely …]

Here's the entries from ‘$HOME/.bzr.log’ for this activity:

=====
Sat 2010-03-20 16:52:18 +1100
0.067 bazaar version: 2.1.0
0.067 bzr arguments: [u'add-pipe', u'--before']
0.089 looking for plugins in /home/bignose/.bazaar/plugins
0.089 looking for plugins
in /usr/lib/python2.5/site-packages/bzrlib/plugins
0.296 Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 853,
in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 1055,
in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 651,
in run_argv_aliases
    cmdargs = _match_argform(self.name(), self.takes_args, args)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 793,
in _match_argform
    % (cmd, argname.upper()))
BzrCommandError: command 'add-pipe' requires argument PIPE

0.296 Transferred: 0KiB (0.0K/s r:0K w:0K)
0.296 return code 3

Sat 2010-03-20 16:52:30 +1100
0.054 bazaar version: 2.1.0
0.055 bzr arguments: [u'add-pipe', u'--after', u':prev']
0.076 looking for plugins in /home/bignose/.bazaar/plugins
0.077 looking for plugins
in /usr/lib/python2.5/site-packages/bzrlib/plugins
0.280 encoding stdout as sys.stdout encoding 'UTF-8'
0.293 bzr-hg: using Mercurial 1.4.1
0.335 opening working tree
'/home/bignose/Projects/debian/subtitleeditor/patch-pipeline/debian-patches'
0.457 opening working tree
'/home/bignose/Projects/debian/subtitleeditor/patch-pipeline/debian-patches'
0.553 opening working tree
'/home/bignose/Projects/debian/subtitleeditor/patch-pipeline/debian-patches'
0.592 opening working tree
'/home/bignose/Projects/debian/subtitleeditor/patch-pipeline/debian-patches'
[ 2638] 2010-03-20 16:52:30.968 INFO: Tree is up to date at revision 2.
[ 2638] 2010-03-20 16:52:30.970 INFO: Created and switched to pipe
":prev".
0.600 Transferred: 0KiB (0.0K/s r:0K w:0K)
0.600 return code 0

Sat 2010-03-20 16:52:46 +1100
0.063 bazaar version: 2.1.0
0.063 bzr arguments: [u'remove-pipe']
0.090 looking for plugins in /home/bignose/.bazaar/plugins
0.090 looking for plugins
in /usr/lib/python2.5/site-packages/bzrlib/plugins
0.343 encoding stdout as sys.stdout encoding 'UTF-8'
0.359 bzr-hg: using Mercurial 1.4.1
0.414 opening working tree
'/home/bignose/Projects/debian/subtitleeditor/patch-pipeline/debian-patches'
=====

No further entries, and the process is still hung.

Presumably it's hanging because the pipe is named “:prev”. If that's
the case, then those special names should be reserved, and attempting
to work with (any operation, not just create) a pipe with those
reserved names should raise an error.