Re: git-clone causes out of memory

[Jeff King <peff@peff.net>]

On Fri, Oct 13, 2017 at 09:56:36AM -0400, Jeff King wrote:

> On Fri, Oct 13, 2017 at 09:55:15AM -0400, Derrick Stolee wrote:
> 
> > > We should be comparing an empty tree and d0/d0/d0/d0 (or however deep
> > > your pathspec goes). We should be able to see immediately that the entry
> > > is not present between the two and not bother descending. After all,
> > > we've set the QUICK flag in init_revisions(). So the real question is
> > > why QUICK is not kicking in.
> > 
> > I'm struggling to understand your meaning. We want to walk from root to
> > d0/d0/d0/d0, but there is no reason to walk beyond that tree. But maybe
> > that's what the QUICK flag is supposed to do.
> 
> Yes, that's exactly what it is for. When we see the first difference we
> should say "aha, the caller only wanted to know whether there was a
> difference, not what it was" and return immediately. See
> diff_can_quit_early().

Hmm. So this patch makes it go fast:

diff --git a/revision.c b/revision.c
index d167223e69..b52ea4e9d8 100644
--- a/revision.c
+++ b/revision.c
@@ -409,7 +409,7 @@ static void file_add_remove(struct diff_options *options,
 	int diff = addremove == '+' ? REV_TREE_NEW : REV_TREE_OLD;
 
 	tree_difference |= diff;
-	if (tree_difference == REV_TREE_DIFFERENT)
+	if (tree_difference & REV_TREE_DIFFERENT)
 		DIFF_OPT_SET(options, HAS_CHANGES);
 }
 

But that essentially makes the conditional a noop (since we know we set
either NEW or OLD above and DIFFERENT is the union of those flags).

I'm not sure I understand why file_add_remove() would ever want to avoid
setting HAS_CHANGES (certainly its companion file_change() always does).
This goes back to Junio's dd47aa3133 (try-to-simplify-commit: use
diff-tree --quiet machinery., 2007-03-14).

Maybe I am missing something, but AFAICT this was always buggy. But
since it only affects adds and deletes, maybe nobody noticed? I'm also
not sure if it only causes a slowdown, or if this could cause us to
erroneously mark something as TREESAME which isn't (I _do_ think people
would have noticed that).

-Peff