Subject: Re: git stash deletes/drops changes of

From: Thomas Rast <[email-masked]>
Date: Fri, 24 May 2013 00:10:51 +0200

Jim Greenleaf <[email-masked]> writes:

> 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