On 26/01/2023 04.25, Elijah Newren wrote:
On Wed, Jan 25, 2023 at 8:16 AM William Sprent <williams@xxxxxxxxxxx> wrote:
On 25/01/2023 06.11, Elijah Newren wrote:
It looks like Ævar and Victoria have both given really good reviews
already, but I think I spotted some additional things to comment on.
On Mon, Jan 23, 2023 at 3:46 AM William Sprent via GitGitGadget
<gitgitgadget@xxxxxxxxx> wrote:
From: William Sprent <williams@xxxxxxxxxxx>
There is currently no way to ask git the question "which files would be
part of a sparse checkout of commit X with sparse checkout patterns Y".
One use-case would be that tooling may want know whether sparse checkouts
of two commits contain the same content even if the full trees differ.
Could you say more about this usecase? Why does tooling need or want
to know this; won't a checkout of the new commit end up being quick
and simple? (I'm not saying your usecase is bad, just curious that it
had never occurred to me, and I'm afraid I'm still not sure what your
purpose might be.)
I'm thinking mainly about a monorepo context where there are a number of
distinct 'units' that can be described with sparse checkout patterns.
And perhaps there's some tooling that only wants to perform an action if
the content of a 'unit' changes.
So, you're basically wanting to do something like
git ls-tree --paths-matching-sparsity-file=<pattern-file> $COMMIT1
sparse-files
git ls-tree --paths-matching-sparsity-file=<pattern-file> $COMMIT2
sparse-files
sort sparse-files | uniq >relevant-files
git diff --name-only $COMMIT1 $COMMIT2 >changed-files
and then checking whether relevant-files and changed-files have a
non-empty intersection?
Well, the concrete use-case I'm exploring is something along the lines
of using the content hashes of sparse checkouts as cache keys for resource
heavy jobs (builds/tests/etc).
So, that would be something along the lines of,
git ls-tree -r --paths-matching-sparsity-file=<pattern-file> \
| sha1sum > cache-key
and then performing a lookup before performing an action (which would
then only be done in the context of the sparse checkout). My thinking
is that this only would require git and no additional tooling, which in
turn makes it very easy to reproduce the state where the job took place.
Would that potentially be better handled by
git diff --name-only $COMMIT1 $COMMIT2 | git check-ignore
--ignore-file=<pattern-file> --stdin
and seeing whether the output is non-empty? We'd have to add an
"--ignore-file" option to check-ignore to override reading of
.gitignore files and such, and it'd be slightly confusing because the
documentation talks about "ignored" files rather than "selected"
files, but that's a confusion point that has been with us ever since
the gitignore mechanism was repurposed for sparse checkouts. Or maybe
we could just add a check-sparsity helper, and then allow it to take
directories in-lieu of patterns.
I don't think it necessarily would be better handled by that. But it would
be workable. It would be a matter of collating the output of
git ls-tree -r <commit>
with
git ls-tree --name-only -r <commit> | git check-ignore ...
Which is less ergonomic. But it is also a less intrusive change.
Really, the main thing is to expose the sparse filtering logic somehow, and
allow for building tooling on top of it.
This seems nicer than opening a can of worms about letting every git
command specify a different set of sparsity rules.
I think you are the better judge of how much of a can of worms that would
be. I don't think it would be too out of line with how git acts in general
though, as we have things like the the 'GIT_INDEX_FILE' env-var.
Depending on the repo, it won't necessarily be quick to check out the
commit with the given patterns. However, it is more about it being
inconvenient to have to have a working directory, especially so if you
want use the tooling in some kind of service or query rapidly about
different revisions/patterns.
Another interesting use-case would be for tooling to use in conjunction
with 'git update-index --index-info'.
Sorry, I'm not following. Could you expound on this a bit?
I was imagining something along the lines of being able to generate new
tree objects based on what matches the given sparse checkout patterns.
Not that I have a specific use case for it right now.
I think what I'm trying to evoke with that paragraph is that this
enables integrations with git that seem interesting and weren't possible
before.
I'm not sure if it's interesting, frightening, or something else.
Hard to say without better descriptions of usecases, which we can't
have if we don't even have a usecase. I think I'd just strike this
paragraph.
[...]
Fair. Will do.
+ (*d)->pl.use_cone_patterns = core_sparse_checkout_cone;
Hmm, so the behavior still depends upon the current sparse-checkout
(or lack thereof), despite the documentation and rationale of your
feature as being there to check how a different sparse checkout would
behave?
I would hate to unconditionally turn cone_patterns off, since that
would come with a huge performance penalty for the biggest repos. But
turning it unconditionally on wouldn't be good for the non-cone users.
This probably suggests we need something like another flag, or perhaps
separate flags for each mode. Separate flags might provide the
benefit of allowing cone mode users to specify directories rather than
patterns, which would make it much easier for them to use.
I used 'core_sparse_checkout_cone' because I wanted to allow for the
cone mode optimisations, but I also figured that I should respect the
configuration. It doesn't change how the patterns are parsed in this case.
I agree that it is a bit awkward to have to "translate" the directories
into patterns when wanting to use cone mode. I can try adding
'--[no]-cone' flags and see how that feels. Together with Victoria's
suggestions that would result in having the following flags:
* --scope=(sparse|all)
* --sparse-patterns-file=<path>
* --[no]-cone: used together with --sparse-patterns-file to tell git
whether to interpret the patterns given as directories (cone) or
patterns (no-cone).
Which seems like a lot at first glance. But it allows for passing
directories instead of patterns for cone mode, and is similar to the
behaviour of 'sparse-checkout set'.
Does that seem like something that would make sense?
--sparse-patterns-file still implies patterns; I think that would need
some rewording.
Yeah. After sleeping on it, I also think that it becomes a difficult
interface to work with, and you'll get different results with the same
patterns whether you pass --cone or --no-cone, which seems error prone
to me.
For better or for worse, both cone and non-cone uses of sparse-checkouts
end up producing pattern files. And those pattern files do unambiguously
describe a filtering of the worktree whether it is in cone-mode or not.
Given that 'ls-tree' is more of a plumbing command, I think it might still
make sense to use the patterns. That would also make the interaction
a bit more logical to me -- e.g. if you want to override the patterns
you have to pass them in the same format as the ones that would be read
by default.
Then maybe it could eventually make sense to expose the translation of
cone-mode patterns as well, e.g.
git sparse-checkout set --cone --std-out dir1 dir2 dir3
or similar.
More importantly, though, based on your usecase description, I wonder
if you might be better served by either extending the check-ignore
subcommand or adding a similar helper ("check-sparsity"?), rather than
tweaking ls-tree.
