bzrlib.bzrdir.format_registry.set_default() cannot be called twice

[John A Meinel]

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We currently use format_registry.set_default(name) to define what branch
format should be the default.

Someone asked on IRC how to switch the default to dirstate-tags, so my
first thought was to do this in a plugin:

import bzrlib.bzrdir

bzrlib.bzrdir.format_registry.set_default('dirstate-tags')

That failed because it is actually registering the format "default", and
that format has already been registered.

The workaround is to add

bzrlib.bzrdir.format_registry.remove('default')

before the set_default() call.

To me, it seems more reasonable to either:

1) have 'set_default' check if default is already registered and remove it.
2) Have 'set_default' use the default logic that Registry already has.
(you can use registry.get(None) to get the default object, rather than
using a key like 'default').
This would mean you don't have 'default' registered

 affects /products/bzr
 importance low
 status confirmed
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