Subject: Re: [BUG] git stash pop --quiet deletes files in git 2.24.0

From: Thomas Gummerer <[email-masked]>
Date: Wed, 13 Nov 2019 11:15:39 +0000

On 11/12, Junio C Hamano wrote:
> Thomas Gummerer <[email-masked]> writes:
> 
> >> > From what you are saying above, and from my testing I think this
> >> > refresh is actually unnecessary, and we could just remove it outright.
> >> 
> >> Perhaps.  But later it will bite us when somebody wants to rewrite
> >> the "status at the end" part in C.
> >
> > Hmm, wouldn't the not re-reading the index part bite us there, rather
> > than the not refreshing the index?
> 
> Yes.  Just removing the refresh-and-write that caused us to write
> out incorrect data would "fix" the bug, while leaving the bug of not
> re-reading to bite us later.
> 
> > Below is the patch that I believe has the least chances of biting us
> > in the future, with the appropriate updated tests.  I had considered
> > leaving the 'refresh_and_write_cache()' call there, but as I was
> > writing the commit message I had a harder and harder time justifying
> > that, so it's gone now, which I think is the right thing to do.
> > Leaving it there would be okay as well, however I don't think it would
> > have any benefit.
> >
> > --- >8 ---
> > Subject: [PATCH] stash: make sure we have a valid index before writing it
> >
> > In 'do_apply_stash()' we refresh the index in the end.  Since
> > 34933d0eff ("stash: make sure to write refreshed cache", 2019-09-11),
> > we also write that refreshed index when --quiet is given to 'git stash
> > apply'.
> >
> > However if '--index' is not given to 'git stash apply', we also
> > discard the index in the else clause just before.  This leads to
> > writing the discarded index, which means we essentially write an empty
> > index file.  This is obviously not correct, or the behaviour the user
> > wanted.  We should not modify the users index without being asked to
> > do so.
> >
> > Make sure to re-read the index after discarding the current in-core
> > index, to avoid dealing with outdated information.
> 
> Yup.  The "!has_index" codepath calls update_index() that turns the
> on-disk index into the desired shape (would it help explaining that
> in the previous paragraph, by the way?) so all we need to do is to
> read it back into core.  Makes sense.

Will add some more explanation about that.

> > We could also drop the 'discard_cache()' + 'read_cache()', however
> > that would make it easy to fall into the same trap as 34933d0eff did,
> > so it's better to avoid that.
> 
> This is the discarded alternative of the main fix we saw earlier.
> Perhaps it may make the flow of thought easier to follow if we moved
> it up before talking about "refresh-and-write can be thrown away"?

Thanks, will move.

> > Signed-off-by: Thomas Gummerer <[email-masked]>
> > ---
> >  builtin/stash.c  | 6 ++----
> >  t/t3903-stash.sh | 5 ++++-
> >  2 files changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/builtin/stash.c b/builtin/stash.c
> > index ab30d1e920..d00567285f 100644
> > --- a/builtin/stash.c
> > +++ b/builtin/stash.c
> > @@ -482,12 +482,10 @@ static int do_apply_stash(const char *prefix, struct stash_info *info,
> >  			return -1;
> >  
> >  		discard_cache();
> > +		read_cache();
> 
> A comment
> 
>     /* read back the result of update_index() back from the disk */
> 
> before discard_cache() may be warranted?

Yeah that makes sense, will add.

> >  	}
> >  
> > -	if (quiet) {
> > -		if (refresh_and_write_cache(REFRESH_QUIET, 0, 0))
> > -			warning("could not refresh index");
> > -	} else {
> 
> OK.
> 
> > +	if (!quiet) {
> >  		struct child_process cp = CHILD_PROCESS_INIT;
> >  
> >  		/*
> > diff --git a/t/t3903-stash.sh b/t/t3903-stash.sh
> > index 392954d6dd..b1c973e3d9 100755
> > --- a/t/t3903-stash.sh
> > +++ b/t/t3903-stash.sh
> > @@ -232,8 +232,9 @@ test_expect_success 'save -q is quiet' '
> >  	test_must_be_empty output.out
> >  '
> >  
> > -test_expect_success 'pop -q is quiet' '
> > +test_expect_success 'pop -q works and is quiet' '
> >  	git stash pop -q >output.out 2>&1 &&
> > +	test bar = "$(git show :file)" &&
> 
> Ah, this is to ensure that we didn't lose the "file" from the index?
> 
> Denton is on the quest of removing "$(git command substitution)"
> used in a way that might hide the error from git invocation in a
> separate thread [*1*].  This may want to become
> 
> 	git rev-parse --verify :file &&
> 
> or
> 
> 	git show :file >actual && echo bar >expect &&
> 	test_cmp expect actual &&
> 
> perhaps?

Hmm I just copy-pasted this from somewhere else in this test file.
I'll add a preparatory patch getting rid of "$(git command substitution)"
as I don't believe Denton got to t3903 yet.

There's some more opportunities for modernization of this test file,
but I refrained from doing that to not blow up this bug fix series too
much.

> >  	test_must_be_empty output.out
> >  '
> >  
> > @@ -242,6 +243,8 @@ test_expect_success 'pop -q --index works and is quiet' '
> >  	git add file &&
> >  	git stash save --quiet &&
> >  	git stash pop -q --index >output.out 2>&1 &&
> > +	git diff-files file2 >file2.diff &&
> > +	test_must_be_empty file2.diff &&
> >  	test foo = "$(git show :file)" &&
> >  	test_must_be_empty output.out
> >  '
> 
> Dittto.
> 
> Thanks.
> 
> 
> [Reference]
> 
> *1* <2f[email-masked]>
> 
> Note that "var=$(git subcmd)" is special and will signal us a failure
> of the git invocation.