Re: [PATCH] unpack-trees: do not set SKIP_WORKTREE on submodules

[Matheus Tavares Bernardino <matheus.bernardino@usp.br>]

On Tue, Jun 16, 2020 at 10:21 PM Elijah Newren via GitGitGadget
<gitgitgadget@gmail.com> wrote:
>
> From: Elijah Newren <newren@gmail.com>
>
> As noted in commit e7d7c73249 ("git-sparse-checkout: clarify
> interactions with submodules", 2020-06-10), sparse-checkout cannot
> remove submodules even if they don't match the sparsity patterns,
> because doing so would risk data loss -- unpushed, uncommitted, or
> untracked changes could all be lost.  That commit also updated the
> documentation to point out that submodule initialization state was a
> parallel, orthogonal reason that entries in the index might not be
> present in the working tree.
>
> However, sparsity and submodule initialization weren't actually fully
> orthogonal yet.  The SKIP_WORKTREE handling in unpack_trees would
> attempt to set the SKIP_WORKTREE bit on submodules when the submodule
> did not match the sparsity patterns.  This resulted in innocuous but
> potentially alarming warning messages:
>
>     warning: unable to rmdir 'sha1collisiondetection': Directory not empty
>
> It could also make things confusing since the entry would be marked as
> SKIP_WORKTREE in the index but actually still be present in the working
> tree:
>
>     $ git ls-files -t | grep sha1collisiondetection
>     S sha1collisiondetection
>     $ ls -al sha1collisiondetection/ | wc -l
>     13
>
> Submodules have always been their own form of "partial checkout"
> behavior, with their own "present or not" state determined by running
> "git submodule [init|deinit|update]".  Enforce that separation by having
> the SKIP_WORKTREE logic not touch submodules and allow submodules to
> continue using their own initialization state for determining if the
> submodule is present.

Makes sense to me.

I'm just thinking about the possible implications in grep (with
mt/grep-sparse-checkout). As you mentioned in [1], users might think
of "git grep $rev $pat" as an optimized version of "git checkout $rev
&& git grep $pat". And, in this sense, they probably expect the output
of these two operations to be equal. But if we don't set the
SKIP_WORKTREE bit for submodules they might diverge.

As an example, if we have a repository like:
.
|-- A
|   `-- sub
`-- B

And the [cone-mode] sparsity rules:
/*
!/*/
/B/

Then, "git grep --recurse-submodules $rev $pat" would search only in B
(as A doesn't match the sparsity patterns and thus, is not recursed
into). But "git checkout $rev && git grep --recurse-submodules $pat"
would search in both B and A/sub (as the latter would not have the
SKIP_WORKTREE bit set).

This might be a problem for git-grep, not git-sparse-checkout. But I'm
not sure how we could solve it efficiently, as the submodule might be
deep down in a path whose first dir was already ignored for not
matching the sparsity patterns. Is this a problem we should consider,
or is it OK if the outputs of these two operations diverge?

[1]: https://lore.kernel.org/git/CABPp-BFKHivKffBPO0M_s-JtpiLyEMLZr+sX9_yZk9ZX7ULtbw@mail.gmail.com/