Hi Johannes, Sorry for the late answer, I was really busy for the last months. Le 31/01/2020 à 22:23, Johannes Schindelin a écrit : > Hi Alban, > -%<- > > Indeed. I offered these insights in #git-devel (slightly edited): > > This `discard_index()` is in an awfully central location. I am rather > certain that it would cause problems to just remove it. > > Looking at `do_merge()`: it explicitly discards and re-reads the index if > we had to spawn a `git merge` process (which we do if a strategy other > than `recursive` was specified, or if it is an octopus merge). But I am > wary of other code paths that might not be as careful. > > I see that `do_exec()` is similarly careful. > I have to admit that the index is not my area of expertise in git, so sorry if my question is stupid: isn't there a less heavy way to find unstaged or uncommitted changes than discarding and then reloading the index? > One thing I cannot fail to notice: we do not re-read a changed index > after running the `prepare-commit-msg` hook, or for that matter, any other > hook. That could even be an old regression from the conversion of the > interactive rebase to using the sequencer rather than a shell script. > > Further, `reset_merge()` seems to spawn `git reset --merge` without > bothering to re-read the possibly modified index. Its callees are > `rollback_single_pick()`, `skip_single_pick()` and `sequencer_rollback()`, > none of which seem to be careful, either, about checking whether the index > was modified in the meantime. > > Technically, the in-memory index should also be discarded > in `apply_autostash()`, but so far, we do not have any callers of that > function, I don't think, that wants to do anything but release resources > and exit. > > The `run_git_checkout()` function discards, as intended. I > am not quite sure whether it needs to, though, unless the `.git/index` > file _was_ modified (it _is_ possible, after all, to run `git rebase -i > HEAD`, and I do have a use case for that where one of my scripts generates > a todo script, sort of a `git cherry-pick --rebase-merges`, because > `cherry-pick` does not support that mode). > > The `continue_single_pick()` function spawns a `git > commit` which could potentially modify the index through a hook, but the > first call site does not care and the second one guards against that > (erroring out...). > > My biggest concern is with the `run_git_commit()` function: it does not > re-read a potentially-modified index (think of hooks). Thank you for your analysis. > > We will need to be very careful with this `discard_index()`, I think, and > in my opinion there is a great opportunity here for cleaning up a little: > rather than discarding and re-reading the in-memory index without seeing > whether the on-disk index has changed at all appears a bit wasteful to me. > > This could be refactored into a function that only discards and re-reads > the index if the mtime of `.git/index` changed. That function could then > also be taught to detect when the in-memory index has unwritten changes: > that would constitute a bug. > Hmm, checking if the mtime of the index to see if it changed isn't racy? Sub-second changes should happen, and to quote a comment in is_racy_stat(), "nanosecond timestamped files can also be racy" -- even if it should not really happen in the case of rebase… > Ciao, > Dscho > Alban
