[bug 93853][merge] Better message about inability to place tags in empty branch

[John Arbash Meinel]

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Martin Pool wrote:
> I intend to fix this properly, but would like to merge Tim's patch
> below as a stopgap.  In merging it I'd add a comment with the bug
> number.
> 
> ---------- Forwarded message ----------
> From: Tim Hatch <tim at timhatch.com>
> Date: Jul 27, 2007 5:28 PM
> Subject: [Bug 93853] Re: Tag command breaks badly with empty repository
> To: mbp at sourcefrog.net
> 
> 
> I also agree with John, having just hit this.  Something along the lines
> of:
> 
> === modified file 'bzrlib/builtins.py'
> --- bzrlib/builtins.py  2007-07-23 19:37:15 +0000
> +++ bzrlib/builtins.py  2007-07-27 22:24:01 +0000
> @@ -3847,6 +3847,10 @@
>                      revision_id = revision[0].in_history(branch).rev_id
>                  else:
>                      revision_id = branch.last_revision()
> +                if not revision_id:
> +                    raise errors.BzrCommandError(
> +                            "Tags can not be placed on an empty branch, "
> +                            "try making a commit first")
>                  if (not force) and branch.tags.has_tag(tag_name):
>                      raise errors.TagAlreadyExists(tag_name)
>                  branch.tags.set_tag(tag_name, revision_id)
> 
> --
> Tag command breaks badly with empty repository
> https://bugs.launchpad.net/bugs/93853
> You received this bug notification because you are a member of Bazaar
> Developers, which is the registrant for Bazaar.


I think [merge] has to be the first entry, not the second.

I think we would want to do:

revision_id = ensure_null(revision_id)
if revision_id != _mod_revision.NULL_REVISION:
    raise errors.***

That way, when we upgrade branch.last_revision() to return NULL_REVISION, it
still works.

Also, this shouldn't be too hard to add as a blackbox test:

format = bzrlib.bzrdir.format_registry.get('dirstate-tags')
tree = self.make_branch_and_tree('.', format=format)
self.run_bzr_error(['Tags can not be placed on an empty branch'],
		   'tag')

Arguably we should check if the default format supports tags, and only if it
doesn't should we force 'dirstate-tags' format.

Otherwise, !approve

John
=:->
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