Re: [PATCH 06/27] checkout-index: ensure full index

From: Elijah Newren <newren@gmail.com>
To: Derrick Stolee <stolee@gmail.com>
Cc: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>,
	Git Mailing List <git@vger.kernel.org>,
	Junio C Hamano <gitster@pobox.com>,
	Derrick Stolee <derrickstolee@github.com>,
	Derrick Stolee <dstolee@microsoft.com>
Subject: Re: [PATCH 06/27] checkout-index: ensure full index
Date: Wed, 17 Mar 2021 15:36:46 -0700	[thread overview]
Message-ID: <CABPp-BFaNOY8xPCTBzU+5p7FtWuC0kazh6DpQ1oKYVJNKw+UXg@mail.gmail.com> (raw)
In-Reply-To: <5c886fd7-710d-ac4a-c63a-c1d000c29126@gmail.com>

On Wed, Mar 17, 2021 at 2:33 PM Derrick Stolee <stolee@gmail.com> wrote:
>
> On 3/17/2021 5:10 PM, Elijah Newren wrote:
> > On Wed, Mar 17, 2021 at 1:05 PM Derrick Stolee <stolee@gmail.com> wrote:
> >>
> >> On 3/17/2021 1:50 PM, Elijah Newren wrote:
> >>> On Tue, Mar 16, 2021 at 2:17 PM Derrick Stolee via GitGitGadget
> >>> <gitgitgadget@gmail.com> wrote:
> >>> With the caveat in the commit message, this change looks okay, but
> >>> checkout-index may be buggy regardless of the presence of
> >>> ensure_full_index().  If ensure_full_index() really is needed here
> >>> because it needs to operate on all SKIP_WORKTREE paths and not just
> >>> leading directories, that's because it's writing all those
> >>> SKIP_WORKTREE entries to the working tree.  When it writes them to the
> >>> working tree, is it clearing the SKIP_WORKTREE bit?  If not, we're in
> >>> a bit of a pickle...
> >>
> >> Perhaps I'm unclear in my intentions with this series: _every_
> >> insertion of ensure_full_index() is intended to be audited with
> >> tests in the future. Some might need behavior change, and others
> >> will not. In this series, I'm just putting in the protections so
> >> we don't accidentally trigger unexpected behavior.
> >
> > I think this may be part of my qualms -- what do you mean by not
> > accidentally triggering unexpected behavior?  In particular, does your
> > statement imply that whatever behavior you get after putting in
> > ensure_full_index() is "expected"?  I think I'm reading that
> > implication into it, and objecting that the behavior with the
> > ensure_full_index() still isn't expected.  You've only removed a
> > certain class of unexpected behavior, namely code that wasn't written
> > to expect tree entries that suddenly gets them.  You haven't handled
> > the class of "user wants to work with a subset of files, why are all
> > these unrelated files being munged/updated/computed/shown/etc."
> > unexpected behavior.
>
> My intention is to ensure that (at this moment) choosing to use
> the on-disk sparse-index format does not alter Git's end-to-end
> behavior.
>
> I want to avoid as much as possible a state where enabling the
> sparse-index can start changing how Git commands behave, perhaps
> in destructive ways.
>
> By adding these checks, we ensure the in-memory data structure
> matches whatever a full index would have created, and then the
> behavior matches what Git would do there. It might not be the
> "correct" behavior, but it is _consistent_.

That sounds good.  Could this be included in
Documentation/technical/sparse-index.txt?  There's only an oblique
reference to it when talking about grep and rm, which incidentally
were already updated by Matheus.  Perhaps also a reference to the
warning in Documentation/git-sparse-checkout.txt would be worthwhile.

> > I'm worrying that expectations are being set up such that working with
> > just a small section of the code will be unusably hard.  There may be
> > several commands/flags where it could make sense to operate on either
> > (a) all files in the repo or (b) just on files within your sparse
> > paths.  If, though, folks interpret operate-on-all-files as the
> > "normal" mode (and history suggests they will), then people start
> > adding all kinds of --no-do-this-sparsely flags to each command, and
> > then users who want sparse operation have to remember to type such a
> > flag with each and every command they ever run -- despite having taken
> > at least three steps already to get a sparse-index.
> >
> > I believe the extended discussions (for _months_!) on just grep & rm,
> > plus watching a --sparse patch being floated just in the last day for
> > ls-files suggest to me that this is a _very_ likely outcome and I'm
> > worried about it.
>
> It's these behavior changes that I would like to delay as much as
> possible and focus on the format and making commands fast that don't
> need a change in behavior.

Delaying sounds great, just so long as that delay doesn't also cement
the behavior and confuse consistency for correctness.

I still think we don't have correct behavior for sparse-checkouts in
many cases (I mean, when "git reset --hard" throws errors about not
removing files and then removes them, we've obviously got some
problems), but we had a decade long cementing of sorts with
SKIP_WORKTREE and now a year-or-two long cementing since
sparse-checkout was introduced and we never went through and cleaned
up the commands. We should at some point, especially since we put the
huge scary warning in Documenation/git-sparse-checkout.txt expressly
for this purpose.

(I would have started this sooner, but trying to feed merge-ort and
keep up with patch review already keeps me at less time on non-git
projects than I think is expected for me.  Once merge-ort is done...)

> (Yes, there will be exceptions, like when "git add" specifically
> adds a file that is in a directory that should be out of the cone,
> but the user added it anyway. Atypical behavior like that can be
> slow for now.)

I agree that there will be exceptions where we can't make the behavior
be fast, but I disagree with that specific example.  "git add" should
just give a warning message and not add any file outside the cone, for
both sparse-index and sparse-checkout.  That can be done quickly.

I know you want to delay the discussion of behavior fixes for specific
commands somewhat, but this particular discussion has already been
ongoing for about 4-12 months now (started at [1] and [2]) and it's
reached a point where I've put my Reviewed-by on it; see [3] for the
git-add piece specifically.

[1] https://lore.kernel.org/git/9f2135f90ffea7f4ccb226f506bf554deab324cc.1605205427.git.matheus.bernardino@usp.br/
[2] https://lore.kernel.org/git/0b9b4c4b414a571877163667694afa3053bf8890.1585027716.git.matheus.bernardino@usp.br/
[3] https://lore.kernel.org/git/66d5c71182274c78e1fcfe84e77deb17e4f0d7e6.1615588109.git.matheus.bernardino@usp.br/

> >> Since tests take time to write and review, I was hoping that these
> >> insertions were minimal enough to get us to a safe place where we
> >> can remove the guards carefully.
> >>
> >> So with that in mind...
> >>
> >>> Might be nice to add a
> >>> /* TODO: audit if this is needed; if it is, we may have other bugs... */
> >>> or something like that.  But then again, perhaps you're considering
> >>> all uses of ensure_full_index() to be need-to-be-reaudited codepaths?
> >>> If so, and we determine we really do need one and want to keep it
> >>> indefinitely, will we mark those with a comment about why it's
> >>> considered correct?
> >>>
> >>> I just want a way to know what still needs to be audited and what
> >>> doesn't without doing a lot of history spelunking...
> >>
> >> ...every insertion "needs to be audited" in the future. That's a
> >> big part of the next "phases" in the implementation plan.
> >>
> >> As you suggest, it might be a good idea to add a comment to every
> >> insertion, to mark it as un-audited, such as:
> >>
> >>         /* TODO: test if ensure_full_index() is necessary */
> >>
> >> We can come back later to delete the comment if it truly is
> >> necessary (and add tests to guarantee correct behavior). We can
> >> also remove the comment _and_ the call by modifying the loop
> >> behavior to do the right thing in some cases.
> >
> > If it's "needs to be audited for both performance reasons (can we
> > operate on fewer entries as an invisible doesn't-change-results
> > optimization) and correctness reasons (should we operate on fewer
> > entries and given a modified result within a sparse-index because
> > users would expect that, but maybe provide a special flag for the
> > users who want to operate on all files in the repo)" and there's also
> > an agreement that either audited or unaudited ones will be marked (or
> > both), then great, I'm happy.  If not, can we discuss which part of my
> > performance/correctness/marking we aren't in agreement on?
>
> I will mark all of the ones I'm inserting. My hope is to eventually
> remove it entirely except for when disabling the sparse-index. That
> is likely too far out to really hope for, but it is the direction I
> am trying to go.
>
> As I indicate that we should carefully test each of these instances
> where ensure_full_index() _might_ be necessary before removing them,
> it is even more important to test the scenarios where the behavior
> changes from a full index with sparse-checkout. Preferably, we just
> change the behavior under sparse-checkout and then the sparse-index
> can match that (see "test_sparse_match" in t1092).

Makes sense.  I agree that it'd be nice to have the two generally
match, though I think we should be open to there being special cases
that differ.  The only one I can think of right now is `git ls-files`
(list the entries in the index) -- since there are tree entries in a
sparse-index, ls-files would naturally show the tree entries.
However, we can discuss that and any other cases -- if there are any
-- later.