error on unix filesystems that don't support symlinks

bzr may be on a unix system that knows about symlinks, but a filesystem that doesn't support them. We should, at minimum, give a clear error about this. Perhaps it would be better to fallback to putting placeholders instead of the symlinks.

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I'm trying to push a branch to a repository on a USB stick, but
getting an error that appears to be ultimately from the filesystem.

=====
$ bzr version
Bazaar (bzr) 1.5
 Python interpreter: /usr/bin/python 2.5.2
 Python standard library: /usr/lib/python2.5
 bzrlib: /usr/lib/python2.5/site-packages/bzrlib
 Bazaar configuration: /home/bignose/.bazaar
 Bazaar log file: /home/bignose/.bzr.log
[...]

$ bzr init-repository --trees /media/sdb1/bazaar/
$ bzr info /media/sdb1/bazaar/
Shared repository with trees (format: pack-0.92)
Location:
 shared repository: /media/sdb1/bazaar

$ bzr init /media/sdb1/bazaar/roaming-profile/
$ bzr info /media/sdb1/bazaar/roaming-profile/
Repository tree (format: pack-0.92)
Location:
 shared repository: /media/sdb1/bazaar
 repository branch: /media/sdb1/bazaar/roaming-profile

$ bzr info .
Standalone tree (format: pack-0.92)
Location:
 branch root: .
[...]

$ bzr push /media/sdb1/bazaar/roaming-profile/
bzr: ERROR: [Errno 1] Operation not permitted
$ bzr push --verbose /media/sdb1/bazaar/roaming-profile/
bzr: ERROR: [Errno 1] Operation not permitted
=====

What can I do to find out what Bazaar is trying to do that fails here?
What can I do to fix it?

Here is the corresponding traceback from '$HOME/.bzr.log':

=====
0.254 encoding stdout as sys.stdout encoding 'UTF-8'
0.255 bzr arguments: [u'push', u'--verbose', u'/media/sdb1/bazaar/roaming-profile/']
0.255 looking for plugins in /home/bignose/.bazaar/plugins
0.255 looking for plugins in /usr/lib/python2.5/site-packages/bzrlib/plugins
0.256 Plugin name __init__ already loaded
0.256 Plugin name __init__ already loaded
0.316 encoding stdout as sys.stdout encoding 'UTF-8'
0.513 opening working tree '/media/sdb1/bazaar/roaming-profile'
0.582 Using fetch logic to copy between KnitPackRepository('file:///home/bignose/.roam/.bzr/repository/')(<RepositoryFormatKnitPack1>) and KnitPackRepository('file:///media/sdb1/bazaar/.bzr/repository/')(<RepositoryFormatKnitPack1>)
0.667 Traceback (most recent call last):
 File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 846, in run_bzr_catch_errors
   return run_bzr(argv)
 File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 797, in run_bzr
   ret = run(*run_argv)
 File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 499, in run_argv_aliases
   return self.run(**all_cmd_args)
 File "/usr/lib/python2.5/site-packages/bzrlib/builtins.py", line 865, in run
   tree_to.update()
 File "/usr/lib/python2.5/site-packages/bzrlib/workingtree.py", line 2093, in update
   return self._update_tree(old_tip, change_reporter)
 File "/usr/lib/python2.5/site-packages/bzrlib/mutabletree.py", line 52, in tree_write_locked
   return unbound(self, *args, **kwargs)
 File "/usr/lib/python2.5/site-packages/bzrlib/workingtree.py", line 2133, in _update_tree
   change_reporter=change_reporter)
 File "/usr/lib/python2.5/site-packages/bzrlib/merge.py", line 1196, in merge_inner
   return merger.do_merge()
 File "/usr/lib/python2.5/site-packages/bzrlib/merge.py", line 419, in do_merge
   merge.do_merge()
 File "/usr/lib/python2.5/site-packages/bzrlib/merge.py", line 519, in do_merge
   self._compute_transform()
 File "/usr/lib/python2.5/site-packages/bzrlib/merge.py", line 557, in _compute_transform
   file_status = self.merge_contents(file_id)
 File "/usr/lib/python2.5/site-packages/bzrlib/merge.py", line 827, in merge_contents
   self.other_tree, trans_id)
 File "/usr/lib/python2.5/site-packages/bzrlib/transform.py", line 1823, in create_by_entry
   tt.create_symlink(tree.get_symlink_target(entry.file_id), trans_id)
 File "/usr/lib/python2.5/site-packages/bzrlib/transform.py", line 388, in create_symlink
   os.symlink(target, self._limbo_name(trans_id))
OSError: [Errno 1] Operation not permitted
=====

That gives a pretty big clue: This repository is on a FAT16
filesystem, and I'm asking it to update a working tree containing
symlinks.

Should I expect that to work?

If it is to fail, I would at least expect a more sensible error
message, indicating what operation is failing, and exactly which file
in the working tree is being updated at the point of the failure.