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lstat-ing delayed-filter output, was Re: playing with MSan

On Wed, Oct 04, 2017 at 08:30:05PM +0100, Thomas Gummerer wrote:

> > So I dunno. This approach is a _lot_ more convenient than trying to
> > rebuild all the dependencies from scratch, and it runs way faster than
> > valgrind.  It did find the cases that led to the patches in this
> > series, and at least one more: if the lstat() at the end of
> > entry.c:write_entry() fails, we write nonsense into the cache_entry.
> Yeah valgrind found that one too, as I tried (and apparently failed :))
> to explain in the cover letter.  I just haven't found the time yet to
> actually try and go fix that one.

No, I just have poor memory. :)

The obvious fix is that we should check the return value of `lstat`, but
the bigger question is why and when that would fail.

The case triggered by t0021 is using the new "delayed" filter mechanism.
So at the time that write_entry() finishes, we don't actually have the
file in the filesystem. I think we need to recognize that we got delayed
and didn't actually check anything out, and skip that whole "if
(state->refresh_cache)" block. It's not clear to me, though, how we tell
the difference between the delayed and normal cases in that function.

But I think this lstat could also fail if we are checking out and
somebody else racily deletes our file. This is presumably sufficiently
rare that I actually wonder if we should just bail with an error, so
that the user knows something funny is going on.

+cc Lars for thoughts no the delayed-filter case.