From: Trieu Huynh <vikingtc4@gmail.com>
Date: Wed, 01 Apr 2026 19:31:34 GMT
Subject: Re: [GSoC PATCH] backfill: auto-detect sparse-checkout from config
Message-ID: <buisigjsw3zrcy6bqaic2zefypq37kimju32eufquppsvkgkvx@cqd3cwj6an6t>
In-Reply-To: <xmqqo6k40wbl.fsf@gitster.g>

On Tue, Mar 31, 2026 at 09:59:10AM -0700, Junio C Hamano wrote:
> Trieu Huynh <vikingtc4@gmail.com> writes:
> 
> > git backfill currently initializes the `sparse` field in
> > backfill_context to 0. This causes the command to always perform a
> > full backfill by default, even when the repository has sparse-checkout
> > enabled in its configuration (core.sparseCheckout).
> >
> > Because 'sparse' is explicitly set to 0 at initialization, any later
> > logic intended to auto-detect the setting from the repository
> > configuration becomes dead code, as it only triggers if the value
> > is negative (sentinel).
> >
> > Change the initial value of .sparse to -1. This allows the command
> > to correctly fallback to the repository's sparse-checkout settings
> > when the '--sparse' or '--no-sparse' options are not provided on the
> > command line.
> 
> The author of bff45557 (backfill: add --sparse option, 2025-02-03),
> where this .sparse member originates, CC'ed for more intelligent
> input than my review can offer ;-)
> 
> 
> > Add a test case in t5620-backfill.sh to verify that 'git backfill'
> > automatically respects the sparse-checkout configuration without
> > explicit flags.
> >
> > Signed-off-by: Trieu Huynh <vikingtc4@gmail.com>
> > ---
> >  builtin/backfill.c  |  2 +-
> >  t/t5620-backfill.sh | 15 +++++++++++++++
> >  2 files changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/builtin/backfill.c b/builtin/backfill.c
> > index 4b2db94173..0f31844ce7 100644
> > --- a/builtin/backfill.c
> > +++ b/builtin/backfill.c
> > @@ -124,7 +124,7 @@ int cmd_backfill(int argc, const char **argv, const char *prefix, struct reposit
> >  		.repo = repo,
> >  		.current_batch = OID_ARRAY_INIT,
> >  		.min_batch_size = 50000,
> > -		.sparse = 0,
> > +		.sparse = -1,
> >  		.show_progress = -1,
> >  	};
> >  	struct option options[] = {
> 
> I am a bit confused by this change.  What's the difference between
> using -1 (which you picked) and 1 as the initial value for this
> member?  From the proposed log message, I would have expected a new
> code that says "ah, we notice, from this member being -1, that the
> user did not specify --no-sparse or --sparse, so let's figure out if
> our working tree is sparsely checked out ourselves and set it either
> to 0 or to 1", but there is nothing like that in the code.  It seems
> that the updated code relies on the fact that this part of
> do_backfill() only cares if .sparse is zero or not, and ...
> 
> 	if (ctx->sparse) {
> 		CALLOC_ARRAY(info.pl, 1);
> 		if (get_sparse_checkout_patterns(info.pl)) {
> 			path_walk_info_clear(&info);
> 			return error(_("problem loading sparse-checkout"));
> 		}
> 	}
> 
> ... relies on get_sparse_checkout_patterns() not to do any harm when
> the working tree is not sparsely checked out.
> 
> I am not sure if we want to call it "auto-detction".  It looks more
> like "default to --sparse, relying that --sparse is a no-op in a
> non-sparse working tree" at least to me.  Not that it is necessarily
> wrong, and when people do "backfill" knowing that the working tree
> is sparse, I am sympathetic if they prefer to keep the sparseness,
> so such a change of default may be beneficial.
> 
actually, the logic IIUC here is:
- first, ctx.sprase originally is set to 0.
- then, it check user's options. Assume, there is no option passed,
still 0.
- then, it check repo's config (core.sparseCheckout (default to 0 in
enviroment.c) but it doesn't since the guard:
	if (ctx.sparse < 0)
		ctx.sparse = cfg->apply_sparse_checkout;
- evenly. ctx.sparse still 0 eventhough in case the
core.sparseCheckout = 1 (git config core.sparseCheckout true)

IMHO, this change set the default value to -1, then it can fallback to
repo's config value if user has no-op passing (default to 0 (full
backfill if user doesnt intent to config previously either).
> Derrick, what do you think?
> 
> > diff --git a/t/t5620-backfill.sh b/t/t5620-backfill.sh
> > index 91b5115732..a1a8d736db 100755
> > --- a/t/t5620-backfill.sh
> > +++ b/t/t5620-backfill.sh
> > @@ -149,6 +149,21 @@ test_expect_success 'backfill --sparse' '
> >  	test_line_count = 0 missing
> >  '
> >  
> > +test_expect_success 'backfill auto-detects sparse-checkout from config' '
> > +	git clone --sparse --filter=blob:none \
> > +		--single-branch --branch=main \
> > +		"file://$(pwd)/srv.bare" backfill-auto-sparse &&
> > +
> > +	git -C backfill-auto-sparse rev-list --quiet --objects --missing=print HEAD >missing &&
> > +	test_line_count = 44 missing &&
> > +
> > +	GIT_TRACE2_EVENT="$(pwd)/auto-sparse-trace" git \
> > +		-C backfill-auto-sparse backfill &&
> > +
> > +	test_trace2_data promisor fetch_count 4 <auto-sparse-trace &&
> > +	test_trace2_data path-walk paths 5 <auto-sparse-trace
> > +'
> > +
> >  test_expect_success 'backfill --sparse without cone mode (positive)' '
> >  	git clone --no-checkout --filter=blob:none		\
> >  		--single-branch --branch=main 		\