[PATCH v4 4/4] sparse-index: update do_read_index to ensure correct sparsity

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

From: Victoria Dye <vdye@github.com>

If `command_requires_full_index` is false, ensure correct in-core index
sparsity on read by calling `ensure_correct_sparsity`. This change is meant
to update the how the index is read in a command after sparse index-related
repository settings are modified. Previously, for example, if `index.sparse`
were changed from `true` to `false`, the in-core index on the next command
would be sparse. The index would only be expanded to full when it was next
written to disk.

By adding a call to `ensure_correct_sparsity`, the in-core index now matches
the sparsity dictated by the relevant repository settings as soon as it is
read into memory, rather than when it is later written to disk.

Helped-by: Junio C Hamano <gitster@pobox.com>
Co-authored-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Victoria Dye <vdye@github.com>
---
 read-cache.c                             |  8 ++++++
 t/t1092-sparse-checkout-compatibility.sh | 31 ++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/read-cache.c b/read-cache.c
index a78b88a41bf..b3772ba70a1 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2337,9 +2337,17 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 
 	if (!istate->repo)
 		istate->repo = the_repository;
+
+	/*
+	 * If the command explicitly requires a full index, force it
+	 * to be full. Otherwise, correct the sparsity based on repository
+	 * settings and other properties of the index (if necessary).
+	 */
 	prepare_repo_settings(istate->repo);
 	if (istate->repo->settings.command_requires_full_index)
 		ensure_full_index(istate);
+	else
+		ensure_correct_sparsity(istate);
 
 	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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