On Mon, Jun 26, 2017 at 12:54 AM, Junio C Hamano <gitster@xxxxxxxxx> wrote: > Orgad Shaneh <orgads@xxxxxxxxx> 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
