Re: [BUG] index corruption with git commit -p

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

On Sun, Sep 02, 2018 at 03:24:09AM -0400, Jeff King wrote:
> On Sun, Sep 02, 2018 at 09:12:04AM +0200, Duy Nguyen wrote:
> > diff --git a/builtin/commit.c b/builtin/commit.c
> > index 2be7bdb331..60f30b3780 100644
> > --- a/builtin/commit.c
> > +++ b/builtin/commit.c
> > @@ -432,6 +432,7 @@ static const char *prepare_index(int argc, const char **argv, const char *prefix
> >  		if (update_main_cache_tree(WRITE_TREE_SILENT) == 0) {
> >  			if (reopen_lock_file(&index_lock) < 0)
> >  				die(_("unable to write index file"));
> > +			ftruncate(index_lock.tempfile->fd, 0);
> >  			if (write_locked_index(&the_index, &index_lock, 0))
> >  				die(_("unable to update temporary index"));
> >  		} else
> 
> Doh, of course. I even thought about this issue and dug all the way into
> reopen_lock_file(), but for some reason temporarily forgot that O_WRONLY
> does not imply O_TRUNC.
> 
> Arguably this should be the default for reopen_lockfile(), as getting a
> write pointer into an existing file is not ever going to be useful for
> the way Git uses lockfiles. Opening with O_APPEND could conceivably be
> useful, but it's pretty unlikely (and certainly not helpful here, and
> this is the only caller). Alternatively, the function should just take
> open(2) flags.
> 
> At any rate, I think this perfectly explains the behavior we're seeing.

Yes, this certainly make sense.

For fun (and testing) I tried to take the problem in the other direction:
* why hasn't this be detected earlier (I'm reasonably be sure I did the
  same operation a few times already without seeing the corruption)?
* how easy it is to reproduce the problem?
* Is it enough to do an interactive commit with nothing needing interactive?

So I tried the following and some variants:
  > for i in $(seq -w 0 100); do echo $i > f$i; done
  > git add f* && git commit -m add f* && git rm -q f* && git commit -m rm -p

but I didn't succeed to recreate the problem. So I'm still wondering
what is special in my repository & tree that trigger the corruption.

Anyway, thanks to al of you for looking at this so quickly.

-- Luc