Re: RFE: support change-id generation natively

[Junio C Hamano <gitster@pobox.com>]

Martin Fick <mfick@codeaurora.org> writes:

> As a Gerrit maintainer, I would suspect that we would 
> welcome a way to track "changes" natively in git.

I would suspect that we would not mind "git commit --change-id" (and
probably "git commit-tree --change-id") option that can be used to
tell the command to add a new Change-Id: trailer at the end, if and
only if there is none in the log message to be recorded (this needs
to happen after the user possibly edits).  We may even want to
introduce commit.changeId boolean configuration variable if we did
so.

"git commit --amend", "git rebase", etc. can be left oblivious to
the "Change-Id:" trailer, as the default mode of operation you guys
want is to leave the existing one as-is, unless the end user really
wants to change it, I think.

It would be just the matter of updating commit_tree_extended() in
commit.c to:

 - detect the need to add a new Change-Id: trailer;

 - call hash_sha1_file() on the commit object buffer (assuming that
   a commit object that you can actually "git cat-file commit" using
   the change Id does not have to exist anywhere for Gerrit to
   work---otherwise you would need to call write_sha1_file()
   instead) before adding Change-Id: trailer;

 - add Change-Id: trailer to the buffer; and then finally

 - let the existing write_sha1_file() to write it out.

I would think.  You might have a funny chicken-and-egg problem with
the signed commit, though.  I didn't think that part through.