Hi all,
I want to learn about how Git compares patches while doing a merge. For example, if a patch has been cherry-picked from branch A to branch B, and then downstream we do a "git merge" from A to B, how does Git know to skip the cherry-picked patch? It would have a different SHA-1, so what is the comparison algorithm/heuristic? What happens if the comment is different, but the actual patch is identical?
I searched around, but couldn't find information on this. I would appreciate it if someone could point me in the right direction.
Thanks!
-Senthil
On Fri, Mar 22, 2013 at 11:53:04AM -0700, Senthil Natarajan wrote:
I want to learn about how Git compares patches while doing a merge.
For example, if a patch has been cherry-picked from branch A to branch
B, and then downstream we do a "git merge" from A to B, how does Git
know to skip the cherry-picked patch?
It doesn't. Git's 3-way merge only looks at three things: where each
side of the merge ended, and what their common ancestor looked like.
So when you cherry-pick a commit, as long as the content in the file
ended up the same, there is no conflict. And it doesn't matter if it
happened by cherry-picking, or if you just happened to make a sequence
of commits that ended in the same state.
However, we do perform such detection during a rebase, in which we try
to skip patches that have already been applied upstream.
It would have a different SHA-1, so what is the comparison
algorithm/heuristic? What happens if the comment is different, but the
actual patch is identical?
Yes, the commit will have a different sha1. For that, we use the
"patch-id", which is basically a sha of the contents of the diff of the
commit against its parent. See the manual for git-patch-id, git-cherry,
and the --cherry-* options of "git log".
It will not find all duplicate commits (e.g., it will miss ones where
there was a conflict during cherry-pick, or even where the context is
slightly different). However, in many cases, rebase can also realize
while applying a patch that it has already been applied, and skip it
then.
-Peff