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From: Alban Gruin <alban.gruin@gmail.com>
To: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Elijah Newren <newren@gmail.com>, git@vger.kernel.org
Subject: Re: Comparing rebase --am with --interactive via p3400
Date: Wed, 1 Apr 2020 13:33:27 +0200
Message-ID: <b380123e-dbcd-7e92-67ab-e6cbf0cec061@gmail.com> (raw)
In-Reply-To: <nycvar.QRO.7.76.6.2001312216410.46@tvgsbejvaqbjf.bet>

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


  reply	other threads:[~2020-04-01 11:33 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-01  6:04 Johannes Schindelin
2019-02-01  7:22 ` Johannes Schindelin
2019-02-01  9:26 ` Elijah Newren
2019-12-27 21:11 ` Alban Gruin
2019-12-27 22:45   ` Elijah Newren
2019-12-29 17:25     ` Alban Gruin
2020-01-02 20:17       ` Johannes Schindelin
2020-01-31 21:23   ` Johannes Schindelin
2020-04-01 11:33     ` Alban Gruin [this message]
2020-04-01 14:00       ` Phillip Wood
2020-04-04 20:33         ` Johannes Schindelin

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