Is there any way to sync fewer merge commits when extracting a subfolder from a large repo?

[RalfJung]

I've asked this before in #952 and was told "no", but maybe it's worth asking again... as other projects that have a presence in the Rust compiler monorepo are considering josh, we're seeing initial josh syncs that add more than 10k commits from the parent repo, meaning that about 1/3 of the commits in that history are not actually from the subproject. In Miri we have accumulated at least around 3500 of these commits (it's hard to reliably find them all so this is a lower bound), which is more than a quarter of the commits in Miri. rust-analyzer seems to be doing better, "only" getting around 1500 commits in the initial sync, which is around 5% of the total commits in that history.

When adding an external repo as a subdirectory into a monorepo (what you want to do) Josh guarantees that splitting that subdirectory back out will yield the exact same sha1's like the original repo. (Most, if not all, other filtering tools do not make that guarantee)

These commits do not originate from the subrepo, so we don't need them to be preserved. But of course josh has no way of knowing that... maybe if we could tell it which part of the history is originally from the subrepo ("this is the subrepo HEAD, everything above this, if it exists in the parent repo, must be extracted perfectly"), it could be more "sloppy" on the remaining history? But I can see how that could be anything between tricky and complete nonsense...

Cc @flip1995 @lnicola

[christian-schilling]

Interesting 🤔

"It's hard to reliably find them" also means: "it's hard to implement a rule to skip them".

But I have been looking again at the history you posted in #952.
A lot of the merges look degenerate. the second parent is an ancestor of the first parent. I wonder if that would be a rule to use to filter them out.
Or maybe: If the second parent is an ancestor of the first and the diff with the first parent is empty, skip the commit.

The current rule is to keep a merge if the diff with either parent is non-empty. And to never change the number of parents a commit has.

[RalfJung]

"It's hard to reliably find them" also means: "it's hard to implement a rule to skip them".

Yeah, I understand. And obviously keeping the round-trips working is very important.

Even if you found a better heuristic now, I have no idea how we'd migrate to that given that we've already integrated all these merges into Miri's history. Though maybe it would be worth a single force-push to Miri...

[christian-schilling]

Even if you found a better heuristic now, I have no idea how we'd migrate to that given that we've already integrated all these merges into Miri's history. Though maybe it would be worth a single force-push to Miri...

This could also be implemented in a new filter, or as an option for the :linear filter.