Re: Getting first tag per branch for a commit

[Orgad Shaneh <orgads@gmail.com>]

On Mon, Jun 26, 2017 at 12:54 AM, Junio C Hamano <gitster@pobox.com> wrote:
> Orgad Shaneh <orgads@gmail.com> writes:
>
>> What I'd like to have is a way to tell the first tag per branch (or
>> per merge) that the commit appeared on.
>
>> I think that this can be done by filtering out tags that are connected
>> to already listed tags by first-parent link.
>
> Yes.  When one tag can be reached by another tag, then the former is
> definitely an earlier tag than the latter.
>
> A trivial way to compute it would require O(n^2) invocations of "git
> merge-base --is-ancestor".  Alternatively, I think you can perhaps
> use "git merge-base --independent".

I think this feature needs to be implemented in Git (by a flag to git describe).
O(n^2) is way too much when you have 20,000 tags.

Unfortunately, I don't feel qualified for implementing it myself. Does anyone
else volunteer? :)

> Having said that, one thing to keep in mind is that a single "first
> tag" may not exist at all.
>
> Consider this topology:
>
>           o---X-------.                topic
>          /     \       \
>  ---o---o---o-------o---N---S---o---   maint
>      \           \   \           \
>       o---o---o---M---o---o---T---o--- master
>
> where a topic branch was forked from the maintenance track, which is
> periodically merged to the master branch.  That topic branch has the
> commit of interest, X, which first gets merged to the master branch
> at merge M, which eventually gets tagged as T (i.e. a new feature
> release).  But (wall-clock-wise) after merge M is made and the
> change is tested in the context of the master branch, but before the
> release T happens, the topic may be merged down to the maintenance
> track at merge N.  Then eventually the tip of the maintenance track
> is tagged as S (i.e. a maintenance release).
>
> Topologically, T and S cannot be compared and they both contain X,
> so the question "what is the first tag on 'master' that has commit
> X?" does not have a single unique answer.  Both S and T are eligible.
>
> You could define various heuristics to tiebreak among these tags.
> You may be tempted to compare timestamps of S and T.  If they were
> equal, then you might want to compare timestamps of M and N.
>
> But you'd need to accept that fundamentally there may not be a
> single "first tag".

I accept that. Anyway, this looks to me like a corner case, and I don't mind
having several tags from the same branch for this case.

- Orgad