Annotated branch ≈ annotated tag?

On 09/27/2011 11:58 PM, Jeff King wrote:

I don't think that branch descriptions should be local-only. They would be a good way to share information with others about work in progress.

It seems to me that an annotated branch is very much like an (unsigned) annotated tag, except that it is movable and disposable like a normal branch. What would be the ramifications of using an annotated-tag-like object to record metainformation about a branch? (Let's just call it an "annotation object" for this discussion.)

* The branch would point not at a commit but at an annotation object
that points at a commit.

* Obviously, a new annotation object would have to be written every time
the branch is updated.

  * Presumably, by default, the old description would be copied from the
old annotation object to the new one; the committer would be set to
those of the user doing the update.

  * The old annotation object would become unreachable after every
branch update, though locally it would still be reachable via the
branch's reflog [1].

* Creating a new branch from an annotated branch should perhaps open an
editor to allow the user to set a new comment.  If the user deletes the
whole comment in the editor, then an unannotated branch is created
instead of an annotated branch.

* We would need rules for merging annotation objects:

  * There is no such thing as a fast-forward merge to an annotated
branch, because the merged-from branch, even if annotated, can never
have the merged-to annotation object in its history.  So proceed to the
following rules.

  * If both branches are unannotated, then the result should be an
unannotated branch (like today).

  * If both branches have the same comment, the comment should be
carried over to the result without prompting.

  * If the merge-to branch is annotated and the merge-from branch is
not, keep the merge-to branch's annotation.

  * If the merge-to branch is unannotated and the merge-from branch is
annotated, the result should be a conflict.  This cannot be resolved
automatically because there are two likely scenarios:

    * A remote branch is being merged into a remote-tracking branch.  In
this case somebody upstream probably added a comment to the branch, and
one would like to preserve this comment in a local annotated branch.

    * A feature branch is being merged back to mainline.  In such a case
one would *not* like to carry over the branch annotation (which
presumably describes the feature that was developed on the branch),
though it would often be convenient to integrate the merged-from branch
annotation into the log message of the merge commit.

    I'm not sure how to let the user distinguish between the two cases
above, but it probably involves $EDITOR.

  * If the merge-to branch and the merge-from branch have conflicting
annotations, there are two possibilities much like in the previous case.

* Annotated tag objects include the name of the tag that is being
annotated.  Annotated branch objects should *not* include this
information, because it does not carry across from one repo to another
when branches are renamed.  (Perhaps the presence/absence of the tag
name could be what distinguishes an (unmovable) annotated tag object
from a (movable) annotated branch object.)

It would even be possible to allow signatures in the annotated objects; these could play the role of push certificates. Whenever a signed annotated branch is updated, the signature would go away (it would probably be too cumbersome to prompt the user to generate it a new signature for every branch update). It should be easy to add a signature to an existing annotated or unannotated branch (writing a new annotation object, of course).

ISTM that the semantics would be very close to what is desired, and would satisfy a few needs: a place to describe work-in-progress in a sharable way, branch descriptions that can be used for constructing pull requests and merge commit messages, and (optionally) a way to implement signed pushes. It would surely be a lot of work to implement, but being based on annotated tags would mean that a lot of code, documentation, and training would be common to the two concepts.

Michael

[1] If the retention of annotation history were considered a requirement, the annotation object could record as a "parent" the object name of the annotation object that it is succeeding. But I don't think that this is a good idea; it would make branches too heavyweight and every branch update would be recorded permanently, both of which are contrary to the git philosophy.