[PATCH 2/2] sparse-index: update index read to consider index.sparse config

["Victoria Dye via GitGitGadget" <gitgitgadget@gmail.com>]

From: Victoria Dye <vdye@github.com>

Use the index.sparse config setting to expand or collapse the index when
read. Previously, index.sparse would determine how the index would be
written to disk, but would not enforce whether the index is read into memory
as full or sparse. Now, the index is expanded when a sparse index is read
with `index.sparse=false` and is collapsed to sparse when a full index is
read with `index.sparse=true` (and the command does not require a full
index).

This makes the behavior of `index.sparse` more intuitive, as it now clearly
enables/disables usage of a sparse index. It also provides users with a way
to disable the sparse index per-command (e.g., for troubleshooting purposes)
without needing to re-initialize the sparse-checkout.

Co-authored-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Victoria Dye <vdye@github.com>
---
 read-cache.c                             |  5 +++-
 t/t1092-sparse-checkout-compatibility.sh | 31 ++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/read-cache.c b/read-cache.c
index a78b88a41bf..c3f31718b19 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2338,8 +2338,11 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	if (!istate->repo)
 		istate->repo = the_repository;
 	prepare_repo_settings(istate->repo);
-	if (istate->repo->settings.command_requires_full_index)
+	if (!istate->repo->settings.sparse_index ||
+	    istate->repo->settings.command_requires_full_index)
 		ensure_full_index(istate);
+	else if (!istate->sparse_index)
+		convert_to_sparse(istate, 0);
 
 	return istate->cache_nr;
 
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index ca91c6a67f8..59accde1fa3 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -694,6 +694,37 @@ test_expect_success 'sparse-index is expanded and converted back' '
 	test_region index ensure_full_index trace2.txt
 '
 
+test_expect_success 'index.sparse disabled inline uses full index' '
+	init_repos &&
+
+	# When index.sparse is disabled inline with `git status`, the
+	# index is expanded at the beginning of the execution then never
+	# converted back to sparse. It is then written to disk as a full index.
+	rm -f trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+		git -C sparse-index -c index.sparse=false status &&
+	! test_region index convert_to_sparse trace2.txt &&
+	test_region index ensure_full_index trace2.txt &&
+
+	# Since index.sparse is set to true at a repo level, the index
+	# is converted from full to sparse when read, then never expanded
+	# over the course of `git status`. It is written to disk as a sparse
+	# index.
+	rm -f trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+		git -C sparse-index status &&
+	test_region index convert_to_sparse trace2.txt &&
+	! test_region index ensure_full_index trace2.txt &&
+
+	# Now that the index has been written to disk as sparse, it is not
+	# converted to sparse (or expanded to full) when read by `git status`.
+	rm -f trace2.txt &&
+	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+		git -C sparse-index status &&
+	! test_region index convert_to_sparse trace2.txt &&
+	! test_region index ensure_full_index trace2.txt
+'
+
 ensure_not_expanded () {
 	rm -f trace2.txt &&
 	echo >>sparse-index/untracked.txt &&
-- 
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