Re: [PATCH v2 0/5] Sparse Index: Integrate with 'git add'

[Derrick Stolee <stolee@gmail.com>]

On 7/28/2021 7:13 PM, Elijah Newren wrote:
> On Mon, Jul 26, 2021 at 9:18 AM Derrick Stolee via GitGitGadget
> <gitgitgadget@gmail.com> wrote:
...
>>  * a full proposal for what to do with "git (add|mv|rm)" and paths outside
>>    the cone is delayed to another series (with an RFC round) because the
>>    behavior of the sparse-index matches a full index with sparse-checkout.
> 
> I think this makes sense.
> 
> I've read through the patches, and I like this version...with one
> exception.  Can we mark the test added in patch 1 under
> 
>      # 3. Rename the file to another sparse filename and
>      #    accept conflict markers as resolved content.
> 
> as NEEDSWORK or even MAYNEEDWORK?

I have no objection to adding a blurb such as:

	# NEEDSWORK: allowing adds outside the sparse cone can be
	# confusingto users, as the file can disappear from the
	# worktree without warning in later Git commands.

And perhaps I'm misunderstanding the situation a bit, but that
seems to apply not just to this third case, but all of them. I
don't see why the untracked case is special compared to the
tracked case. More investigation may be required on my part.

>  I'm still quite unconvinced that it
> is testing for correct behavior, and don't want to paint ourselves
> into a corner.  In particular, we don't allow folks to "git add
> $IGNORED_FILE" without a --force override because it's likely to be a
> mistake. 

I agree about ignored files, and that is true whether or not they
are in the sparse cone.

> I think the same logic holds for adding untracked files
> outside the sparsity cone.  But it's actually even worse than that
> case because there's a secondary level of surprise too: adding files
> outside the sparsity cone will result in delayed user surprises when
> the next git command that happens to call unpack_trees() (which are
> found all over the codebase) removes the file from the working tree.
> I've had some such reports already.

I believe this is testing a realistic scenario that users are
hitting in the wild today. I would believe that users succeed with
these commands more often than they are confused by the file
disappearing from the worktree in a later Git command, so having
this sequence of events be documented as a potential use case has
some value.

I simultaneously don't think it is behavior we want to commit to
as a contract for all future Git versions, but there is value in
showing how this situation changes with any future meddling. In
particular: will users be able to self-discover the "new" way of
doing things?

The proposal part of changing how add/mv/rm behave in these cases
would need to adjust this test with something that would also help
direct users to a helpful resolution. For example, the first run
of

	git add sparse/dir/file

could error out with an error message saying "The pathspec is
outside of your sparse cone, so staging the file might lead to
a staged change that is removed from your working directory."
But we should _also_ include two strategies for getting out of
this state:

1. Adjust your sparse-checkout definition so this file is in scope.

-or- (and this is the part that would be new)

2. If you understand the risks of staging a file outside the sparse
   cone, then run 'git add --sparse sparse/dir/file'.

(Insert whatever option would be appropriate for --sparse here.)

Such a warning message would allow users who follow the steps listed
in the test to know how to adjust their usage to then get into a
good state.

> If that test is marked as NEEDSWORK or even as the correct behavior
> still being under dispute, then you can happily apply my:

I would classify this as "The test documents current behavior, but
isn't a contract for future behavior." With a concept such as my
suggestion above, the test could be modified to check for the
warning and then run the second command with the extra option and
complete the test's expectations. Having the existing behavior
documented in a test helps demonstrate how behavior is changing.

We we've discussed, we want to give such a behavior change the
right venue for feedback and suggestions for alternate approaches,
and this series is not the right place for that. Hopefully you
can tell that it is on my mind and that I want to recommend a
change in the near future.

Thanks,
-Stolee