[PATCH v2 2/3] sparse-index: add ensure_correct_sparsity function

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

From: Victoria Dye <vdye@github.com>

The purpose of the `ensure_correct_sparsity` function is to provide a means
of aligning the in-core index with the sparsity required by the repository
settings and other properties of the index. The function will first attempt
to convert the index to sparse, now with a "SPARSE_INDEX_VERIFY_ALLOWED"
flag that forces the function to return a nonzero value if repository
settings do not allow use of a sparse index. If a nonzero value is returned,
the index is expanded to full with `ensure_full_index`.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Victoria Dye <vdye@github.com>
---
 sparse-index.c | 42 +++++++++++++++++++++++++++++++++++++++---
 sparse-index.h |  2 ++
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/sparse-index.c b/sparse-index.c
index 7b7ff79e044..4273453e078 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -122,11 +122,10 @@ static int index_has_unmerged_entries(struct index_state *istate)
 	return 0;
 }
 
-int convert_to_sparse(struct index_state *istate, int flags)
+static int can_convert_to_sparse(struct index_state *istate, int flags)
 {
 	int test_env;
-	if (istate->sparse_index || !istate->cache_nr ||
-	    !core_apply_sparse_checkout || !core_sparse_checkout_cone)
+	if (!core_apply_sparse_checkout || !core_sparse_checkout_cone)
 		return 0;
 
 	if (!istate->repo)
@@ -187,6 +186,30 @@ int convert_to_sparse(struct index_state *istate, int flags)
 	if (cache_tree_update(istate, WRITE_TREE_MISSING_OK))
 		return 0;
 
+	return 1;
+}
+
+int convert_to_sparse(struct index_state *istate, int flags)
+{
+	int verify = flags & SPARSE_INDEX_VERIFY_ALLOWED;
+
+	/*
+	 * If validating with strict checks against whether the sparse index is
+	 * allowed, we want to check `can_convert_to_sparse` *before* exiting
+	 * early due to an already sparse or empty index.
+	 *
+	 * If not performing strict validation, the order is reversed to avoid
+	 * the more expensive checks in `can_convert_to_sparse` whenver possible.
+	 */
+	if (verify) {
+		if (!can_convert_to_sparse(istate, flags))
+			return -1;
+		else if (istate->sparse_index || !istate->cache_nr)
+			return 0;
+	} else if (istate->sparse_index || !istate->cache_nr ||
+		   !can_convert_to_sparse(istate, flags))
+		return 0;
+
 	remove_fsmonitor(istate);
 
 	trace2_region_enter("index", "convert_to_sparse", istate->repo);
@@ -313,6 +336,19 @@ void ensure_full_index(struct index_state *istate)
 	trace2_region_leave("index", "ensure_full_index", istate->repo);
 }
 
+void ensure_correct_sparsity(struct index_state *istate)
+{
+	/*
+	 * First check whether the index can be converted to sparse by attempting
+	 * to convert it with the SPARSE_INDEX_VERIFY_ALLOWED flag. If the
+	 * SPARSE_INDEX_VERIFY_ALLOWED checks indicate that the index cannot
+	 * be converted because repository settings and/or index properties
+	 * do not allow it, expand the index to full.
+	 */
+	if (convert_to_sparse(istate, SPARSE_INDEX_VERIFY_ALLOWED))
+		ensure_full_index(istate);
+}
+
 /*
  * This static global helps avoid infinite recursion between
  * expand_to_path() and index_file_exists().
diff --git a/sparse-index.h b/sparse-index.h
index 9f3d7bc7faf..b61754f1f76 100644
--- a/sparse-index.h
+++ b/sparse-index.h
@@ -3,7 +3,9 @@
 
 struct index_state;
 #define SPARSE_INDEX_MEMORY_ONLY (1 << 0)
+#define SPARSE_INDEX_VERIFY_ALLOWED (1 << 1)
 int convert_to_sparse(struct index_state *istate, int flags);
+void ensure_correct_sparsity(struct index_state *istate);
 
 /*
  * Some places in the codebase expect to search for a specific path.
-- 
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