How do you figure out the previous tag in such a workfow?

Question

I have a project were I use a rather uncommon tag strategy. We end up with tags being leafs so our history looks like this:

  0.3.0                        0.3.0
|/                               |
| 0.2.0      rather than       0.2.0
|/                               |
| 0.1.0                        0.1.0
|/                               |

The reason why I do this is that our tags should include the dist output whereas during development we don't want to commit such files into version control. So when I run the build, the build tool automatically branches off, adds the build artifacts (dist folder) with a generated commit and then creates the tag there.

This workflow might look odd at first glance but so far I have found it to be quite convenient as we need to have the dist folder in our tags for a downstream deployment process.

Now the problem is, I want to generate release notes automatically, just the problem is, how do I figure out the previous tag in such a scenario? I'm aware of the answers given here but with tags being leafs, this doesn't work that way.

Answer 1

(edit: original rewarded answer down below, here's a much simpler way that tags merge bases so git describe can find them.

doit () 
{ 
    cleanup="`mktemp -t`";
    git for-each-ref refs/tags/"$1" --format='
           echo git tag -d base/%(refname:short) >> '"$cleanup"'
           git tag base/%(refname:short) $(git merge-base HEAD %(refname:short))
        ' | sh -x;
    result=`git describe --tags`;
    sh -x "$cleanup";
    rm "$cleanup"
    echo ${result#base/}
}
doit "0.*"

)

---

Use latest.awk from this answer this way:

doit ()
{
        git rev-list --topo-order --first-parent --children --tags --format=%d \
        | awk -f path/to/latest.awk \
        | sed -n /$1"/,$ { /"$1"/! {p;q} }"
}

Testing:

~/sandbox/15$ git lgdo --topo-order --tags
* ecb363d (tag: tag4) tag4
* cd9f402 master
| * 26aa94b (tag: tag3) tag3
|/
* a1b6c1b master
| * 8866091(tag: tag2)tag2
|/
* b5d5283 master
| * 29a4e54(tag: tag1)tag1
|/
* cfcd7dc master
* 6120ab4 (tag: empty)
~/sandbox/15$ doit tag2
29a4e54debae973dfc3955d6663f14d6ade73df9 (tag: tag1)
~/sandbox/15$

(edit: and

~/sandbox/15$ git checkout tag4
HEAD is now at ecb363d...  tag4
~/sandbox/15$ git describe --tags
tag4
~/sandbox/15$ doit `git describe --tags`
26aa94bad1e37602791c354823cb4a84ff6fc437  (tag: tag3)
~/sandbox/15$ 

)

---

(git lgdo being an alias for git log --graph --decorate --oneline, and the "master"s in there being commit-message artifacts of a make-me-some-commits helper)