Re: git stash deletes/drops changes of

From: Thomas Rast <trast@inf.ethz.ch>
Date: May 23, 2013, 22:10 UTC
Message-ID: <87sj1d5ous.fsf@linux-k42r.v.cablecom.net>
In-Reply-To: <loom.20130523T185301-635@post.gmane.org>

Jim Greenleaf <james.a.greenleaf@gmail.com> writes:

Show 9 quoted lines

> Adeodato Simó <dato <at> net.com.org.es> writes:
>
>> I was unpleasantly surprised to discover yesterday that doing `git
>> stash` on a repository where I had previously run `git update-index
>> --assume-unchanged FOO` completely lost all changes I had in file FOO.
>
> I just ran into this today.
>
> Was a decision about this behavior reached in the intervening time?

When you mark a file assume-unchanged, git internally sets a flag that this file should not be considered when doing cache refreshes -- the file is always assumed to be up-to-date.

So while I haven't actually looked into all of the code, I imagine it goes something like this:

* git-stash uses git update-index --all on all modified files.  But it
  doesn't show up as modified, because you promised it isn't.

* Later it calls git reset --hard, which blows away the existing state.
  This would seem to ignore the assume-unchanged flag in this case, as
  otherwise it wouldn't overwrite it.

Whether the last behavior is a bug is in the eye of the beholder. In your case you apparently lost work. However, 'git reset --hard' in itself should discard all uncommitted work without asking any further questions (because it's --hard). So the bug is then in the sequence

  ask about uncommitted work
  save it elsewhere
  git reset --hard

assuming that this actually makes sure nothing gets lost. But the only thing that was lost was *files that you promised would not be changed*.

What's really unfortunate is that we caused this in the first place by Pasky's 6259ac6 (Documentation: How to ignore local changes in tracked files, 2008-07-18). It recommends exactly the --assume-unchanged strategy to ignore changes to tracked files.

And it's hard to disagree with its commit message:

    This is currently probably one of the top FAQs at #git and the
    --assume-unchanged switch is not widely known

Except that now the corresponding FAQ is that we have to actively dissuade people from using --assume-unchanged precisely because it keeps biting people.

So maybe it would be time to first make up our minds as to what --assume-unchanged should actually mean:

* Ignore changes to a tracked file, but treat them as valuable.  In
  this case we'd have to make sure that failures like git-stash's are
  handled properly.

* Ignore changes to a tracked file, as in "who cares if it was changed".

* A very specific optimization for users who know what they are doing.

-- 
Thomas Rast
trast@{inf,student}.ethz.ch