Re: [RFC PATCH 1/3] list-objects: support for skipping tree traversal

[Matthew DeVore <matvore@google.com>]

On Sun, Oct 14, 2018 at 4:15 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> This step looks more like "ow, we could have done the tree:0 support
> that is in 'next' better" than a part of "here is a series to do
> tree:N for non zero value of N".
>
> If that is the case, I'd prefer to see this step polished enough
> before [2-3/3] of this RFC is worked on, so that we can merge the
> tree:0 (but not yet tree:N) support that is solid down to 'master'
> soonish.
That's fair. So I will prioritize this patch above the rest of the
patchset and send it separately in the future.

> OK, so "not collecting omits" is synonymous to "N==0, we aren't
> doing tree at any level", and at this point in the series before the
> support for N>0 is introduced, we'd always take this "else" clause
> because tree:0 is the only thing we support?
Actually "not collecting omits" and depth==0 are orthogonal concepts.
"Collect omits" is the logic needed to implement the
--filter-print-omitted flag on git rev-list. You can do this when
depth==0 (it will actually print all the trees and blobs recursively).
"Collect omits" is tested *with* tree:0 in test t6112 in the test
labeled 'verify tree:0 includes trees in "filtered" output'

IOW, both branches here are important even in this patch. If we are
collecting omits, then we can't skip the tree because omits collecting
is recursive. Otherwise, we *can* skip the tree.

But maybe printing omits should not be recursive? The decision was
never discussed. The code to not be recursive is simpler, because we
don't need this if/else. Recursiveness is counter-intuitive since we
would "skip" a tree and at the same time print its contents.

>
> Style: our modern style is to use {} around the body which is a
> single statement on the else clause when the body of the
> corresponding if clause needs {} around (and vice versa), so
>
Fixed, and I didn't realize I was supposed to be hugging "else" with
the curly braces. What you say is what CodingGuidelines says to do.
Thanks for pointing that out.

>
> Even when failed_parse==true, i.e. we found that the tree object's
> data cannot be understood, if we have skip-tree bit set, that means
> we do not care---we won't be descending into its children anyway.
>
Yes.

> > +# Make sure tree:0 does not iterate through any trees.
> > +
> > +test_expect_success 'filter a GIANT tree through tree:0' '
> > +     GIT_TRACE=1 git -C r3 rev-list \
> > +             --objects --filter=tree:0 HEAD 2>filter_trace &&
> > +     grep "Skipping contents of tree [.][.][.]" filter_trace >actual &&
>
> Here you are not jus tmaking sure SKIP_TREE bit is set for some
> tree, but it is set when base->buf is an empty string (i.e. the top
> level tree)?  Which makes sense, and the next text makes sure that
> between the two commits, the number of total "top level" trees is 2,
> but I wonder if it is more direct to also make sure that the code is
> not even seeing or skipping any tree inside these top level trees
> (i.e. the same message but for ""!=base->buf should never appear in
> the trace).
Makes sense. I added another check in this test for other "Skipping" messages.

Thank you for reviewing.