Re: Getting an actuallt useful merge base?

On Thu, Feb 25, 2021 at 02:40:59AM +0000, brian m. carlson wrote:

On 2021-02-24 at 17:58:34, Michal Suchánek wrote:

quoted

Hello,

I find the results of git merge-base A B quite useless.

Suppose you have a repository with file sets

S and T

where S are sources which are developed in mainline and number of stable
versions, and feature branches, and T are build tools (such as autoconf
tests or whatever) that are largely independent of the source version.

Because of the independence of T from S T are developed in a separate
branch t which is merged into all branches developing S as needed.

Fixes to S may affect more than one version, and depending on the
situation it might be useful to apply fixes to S to mutiple
stable/feature branche at once. For that one would need a merge base of
the branches in question.

However, merge-base almost always give a commit on branch t which is the
merge base of files in set T and does not contain files in set S at all.
In other words it is merge base only for files from set T and not set S.
Can I get merge base that is merge base for all files that have common
history between two branches?

The merge base is determined by the history.  In your case, I imagine
you have a history like this:

 A -- B -- C -- D -- E -- F -- G (S)
        _/        _/        _/
 H -- I -- J -- K -- L -- M -- N (T)

Here, the merge base of N and G is M, and the merge base of F and M is
K.  Those are the most recent common ancestors, which are typically
chosen as the merge base.

In your case, you probably want to cherry-pick a commit, or maybe rebase
a small set of commits onto another set.  That would probably work
better than trying to merge.  It's possible that there's something about
this case that I'm missing where it wouldn't work properly, but it's
definitely the approach I would try.

It's like this

T
----o----o----o----o----o----o----o----o----o----o----o----o---(t)---o----o----
     \             \     \                                      \\\
      \             \     \                                      \\\
       \             \     \                                      \\\
        \        o----o----o\̶---o---(s)---o----o----o----o----o----o\̶\̶-(a)
         \      /            \      /                                \\
S+T  o----o----o----o----o----o----o----o----o----o----o----o----o----o\̶--(b)
    /                                       /                           \
---o----o----o----o----o----o----o----o----o----o----o----o----o----o----o---(m)

So (t) is common ancestor for (a) and (b) that merge-base reports but it is
only ancestor for files in set T, and does not have files from set S at all.
The common ancestor I am insterested in is (s) which is merge base for both
sets of files.

Thanks

Michal