`pull` does not work when there are conflicts

[RalfJung]

To reproduce, run rustc-josh-sync pull when there are conflicts.

I would expect this to give me the chance to resolve the conflicts to complete the sync.

Instead, it reverts everything. Now it seems impossible for me to sync anything...?

incoming ref: abd9cba5acbcc35fb4cc59ce25580fa1624b5571
+ cd "/home/r/src/rust/miri" && "git" "merge" "FETCH_HEAD" "--no-verify" "--no-ff" "-m" "Merge ref \'733dab558992\' from rust-lang/rust\n\nPull recent changes from https://github.com/rust-lang/rust via Josh.\n\nUpstream ref: 733dab558992d902d6d17576de1da768094e2cf3\nFiltered ref: abd9cba5acbcc35fb4cc59ce25580fa1624b5571\n\nThis merge was created using https://github.com/rust-lang/josh-sync.\n"
Auto-merging src/helpers.rs
CONFLICT (content): Merge conflict in src/helpers.rs
Automatic merge failed; fix conflicts and then commit the result.
Reverting HEAD to 303dbdd54e986c9300ca69b7aeb4a31075b1f673
+ cd "/home/r/src/rust/miri" && "git" "reset" "--hard" "303dbdd54e986c9300ca69b7aeb4a31075b1f673"
Pull failure: FAILED to merge new commits, something went wrong

Caused by:
    Command `cd "/home/r/src/rust/miri" && "git" "merge" "FETCH_HEAD" "--no-verify" "--no-ff" "-m" "Merge ref \'733dab558992\' from rust-lang/rust\n\nPull recent changes from https://github.com/rust-lang/rust via Josh.\n\nUpstream ref: 733dab558992d902d6d17576de1da768094e2cf3\nFiltered ref: abd9cba5acbcc35fb4cc59ce25580fa1624b5571\n\nThis merge was created using https://github.com/rust-lang/josh-sync.\n"` failed with exit code Some(1)

[RalfJung]

I ended up creating rust-lang/miri#4500 partially by hand, but this is a clear regression compared to the old ./miri rustc-pull.

[Kobzol]

Right, so when a conflict happens, we should still finish the merge, push it and create a PR, and then let contributors finish the sync in that PR? I kind of assumed that if there's a conflict, people will just run rustc-josh-sync pull manually.

Do you want the PR to be created with the actual merge conflict markers? Or just have code from the rustc version?

[RalfJung]

I kind of assumed that if there's a conflict, people will just run rustc-josh-sync pull manually.

Yes, that's what I am doing. But it then undoes everything...

What we did with ./miri rustc-pull is:
if git merge fails, just exit with a non-zero exit code.

On CI, this meant the job failed and didn't create a PR.
Then I run the command locally again, and now I have a git repo in "merging" state where I can just resolve the conflict.

But what josh-sync does is it undoes everything so even when I run it locally, the entire invocation is a NOP! This gives me no way to actually resolve the conflicts.

[Kobzol]

Ah, I see, I didn't realize it's not possible to do it even locally. Will fix!