Setting extra fields at commit time (i.e. adding an --extra flag to the commit command)

[Angel Ezquerra]

Hi,

I find myself on the unfortunate position of having to find a way to
have mercurial co-exist with Microsoft's TFS. I have made a small
python script that is able to "pull" from TFS into a mercurial
repository. It does so by creating commits in a local mercurial
repository that correspond to the TFS revisions.

This works fine, but I think I should record the original TFS revision
id on the mercurial commit. I can do so by putting the information on
the commit message itself, but I'd like to also store that info in a
more formal way. I thought that the best place to do so would be to
add an entry on the extra revision field. This would let me use the
extra revsets function to find the commits that were imported from
TFS, for example.

The problem is that there is no way through the mercurial command line
to set entries in the extra dict. I could create a proper mercurial
extension rather than a simple script that calls the mercurial command
line instead but that has a couple of drawbacks:

1. I would now need to update the extension as mercurial evolves
(which is why we usually recommend that people use the mercurial
command line if they can).
2. I would have to call repo.commit, but commands.commit does a few
additional things (such as handling amend, etc).

I may end up doing this anyway, but I wonder if it would not be a good
idea to allow setting the extra field through the commit command
interface. The riskiest part is that giving unfettered access to that
field may interfere with existing uses of the extra field. Instead
perhaps we could create a "user namespace" within the extra field,
that is left for users to use (e.g. any extra fields passed to the
commit message would have their name automatically prepended with
"user.")

Regarding the interface for this new feature we could add an --extra
flag to the commit command. Ideally the user could specify --extra
multiple times to set multiple extra user fields, but I don't think
that is possible with the current mercurial command line specification
syntax. An alternative would be to accept a dict-like or json-like
syntax for the extra field. For example:

    hg commit -n "test" --extra "'field1': 'This is my first user
field', 'field2': 'This is a second user field'"

This would add two extra fields called 'user.field1' and
'user.field2'. A user could refer to then through a revset such as
"extra(user.field1)", etc. This syntax is a little heavy and ugly, but
I don't think that would be a huge problem for the kind of users that
would use this.

What do you guys think?

Angel