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From: Kyle Zhao <>
Subject: Re: [PATCH v2] merge: avoid write merge state when unable to write index
Date: Fri, 17 May 2024 11:59:23 +0800	[thread overview]
Message-ID: <> (raw)

> "Currently, " is superfluous in this project, as by convention our
> proposed log message begins with an observation of the current
> status (and "what's wrong in it" follows) in the present tense.


> My guess, from reading the tests, of the situation this patch
> attempts to handle is something like this?
>    When running a merge while the index is locked (presumably by
>   another process), the merge state is written in SUCH and SUCH
>    files, the index is not updated, and then the merge fails.  This
>    leaves the resulting state inconsistent.
>> If the user exec "git commit" instead of "git merge --abort" after this,
>> a merge commit will be generated and all modifications from the source
>> branch will be lost.
> I do not think this is accurate description of the "an user action
> can make it worse".  In reality, if the user runs "git commit", a
> safety kicks in and they get:
>    fatal: Unable to create '.../.git/index.lock': file exists.
> In fact, your "How to reproduce" below the three-dash line removes
> the stale index.lock file before running "git commit".
>    From this state, if the index.lock file gets removed and the
>    user runs "git commit", a merge commit is created, recording the
>    tree from the inconsistent state the merge left.
> may be a better description of the breakage.
> But stepping back a bit, I do not think this extra paragraph is
> needed at all.  If there were a competing process holding the
> index.lock, it were in the process of updating the index, possibly
> even to create a new commit.  If that process were indeed running
> the "git commit" command, MERGE_HEAD and other state files we write
> on our side will be taken into account by them and cause them to
> record a merge, even though they may have been trying to record
> something entirely different.  So regardless of what _this_ user,
> whose merge failed due to a competing process that held the
> index.lock file, does after the merge failure, the recorded history
> can be hosed by the other process.
> In any case, to prevent the other "git commit" process from using
> "our" MERGE_HEAD and other state files to record a wrong contents,
> the right fix is to make sure everybody who takes the lock on the
> index file does *not* create any other state files to be read by
> others before it successfully takes the lock.  That will also
> prevent "git commit" we run after a failed merge (and removing the
> lockfile) from recording an incorrect merge.

Yes, your understanding is very correct.

In fact, there are many other situations besides the example I give
that will be affected.

> I do not offhand know if returning 2 (aka "I am not equipped to
> handle this merg e at all") is a good way to do so, but if it is,
> then the patch given here is absolutely the right thing to do.

In the latest version I modified it to:
    die(_("Unable to write index."));

Maybe it's better. (Refer to the code elsewhere)

> Do not use "touch" when the timestamp is not the thing we care
> about.  In this case, you want that file to _exist_, and the right
> way to do so is
> >.git/index.lock &&
> > which already appears in t7400 (it uses a no-op ":" command, but
> > that is not needed).



             reply	other threads:[~2024-05-17  3:59 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-05-17  3:59 Kyle Zhao [this message]
  -- strict thread matches above, loose matches on Subject: below --
2024-05-16  5:08 [PATCH] merge: avoid write merge state when unable to write index Kyle Zhao via GitGitGadget
2024-05-16  5:18 ` [PATCH v2] " Kyle Zhao via GitGitGadget
2024-05-16 16:18   ` Junio C Hamano

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