Re: Getting first tag per branch for a commit

Orgad Shaneh <orgads@gmail.com> writes:

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".

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".