sparse-index: update do_read_index to ensure correct sparsity

Unless `command_requires_full_index` forces index expansion, ensure in-core
index sparsity matches config settings on read by calling
`ensure_correct_sparsity`. This makes the behavior of the in-core index more
consistent between different methods of updating sparsity: manually changing
the `index.sparse` config setting vs. executing
`git sparse-checkout --[no-]sparse-index init`

Although index sparsity is normally updated with `git sparse-checkout init`,
ensuring correct sparsity after a manual `index.sparse` change has some
practical benefits:

1. It allows for command-by-command sparsity toggling with
   `-c index.sparse=<true|false>`, e.g. when troubleshooting issues with the
   sparse index.
2. It prevents users from experiencing abnormal slowness after setting
   `index.sparse` to `true` due to use of a full index in all commands until
   the on-disk index is updated.

Helped-by: Junio C Hamano <gitster@pobox.com>
Co-authored-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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;
 

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 &&