Re: git-clone causes out of memory

[Jeff King <peff@peff.net>]

On Fri, Oct 13, 2017 at 10:10:18AM -0400, Jeff King wrote:

> 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).

Answering my own question a little, there is a hint in the comment
a few lines above:

  /*
   * The goal is to get REV_TREE_NEW as the result only if the
   * diff consists of all '+' (and no other changes), REV_TREE_OLD
   * if the whole diff is removal of old data, and otherwise
   * REV_TREE_DIFFERENT (of course if the trees are the same we
   * want REV_TREE_SAME).
   * That means that once we get to REV_TREE_DIFFERENT, we do not
   * have to look any further.
   */

So my patch above is breaking that. But it's not clear from that comment
why we care about knowing the different between NEW, OLD, and DIFFERENT.

Grepping around for REV_TREE_NEW and REV_TREE_OLD, I think the answer is
in try_to_simplify_commit():

     case REV_TREE_NEW:
              if (revs->remove_empty_trees &&
                  rev_same_tree_as_empty(revs, p)) {
                      /* We are adding all the specified
                       * paths from this parent, so the
                       * history beyond this parent is not
                       * interesting.  Remove its parents
                       * (they are grandparents for us).
                       * IOW, we pretend this parent is a
                       * "root" commit.
                       */
                      if (parse_commit(p) < 0)
                              die("cannot simplify commit %s (invalid %s)",
                                  oid_to_hex(&commit->object.oid),
                                  oid_to_hex(&p->object.oid));
                      p->parents = NULL;
              }
    /* fallthrough */
    case REV_TREE_OLD:
    case REV_TREE_DIFFERENT:

So when --remove-empty is not in effect (and it's not by default), we
don't care about OLD vs NEW, and we should be able to optimize further.

-Peff