[PATCH v4 3/4] sparse-index: add ensure_correct_sparsity function

["Victoria Dye via GitGitGadget" <gitgitgadget@xxxxxxxxx>]

From: Victoria Dye <vdye@xxxxxxxxxx>

The `ensure_correct_sparsity` function is intended 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 first checks
whether a sparse index is allowed (per repository & sparse checkout pattern
settings). If the sparse index may be used, the index is converted to
sparse; otherwise, it is explicitly expanded with `ensure_full_index`.

Helped-by: Junio C Hamano <gitster@xxxxxxxxx>
Signed-off-by: Victoria Dye <vdye@xxxxxxxxxx>
---
 sparse-index.c | 33 +++++++++++++++++++++++++++++----
 sparse-index.h |  1 +
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/sparse-index.c b/sparse-index.c
index 85613cd8a3a..a1d505d50e9 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -122,17 +122,17 @@ static int index_has_unmerged_entries(struct index_state *istate)
 	return 0;
 }
 
-int convert_to_sparse(struct index_state *istate, int flags)
+static int is_sparse_index_allowed(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)
 		istate->repo = the_repository;
 
 	if (!(flags & SPARSE_INDEX_MEMORY_ONLY)) {
+		int test_env;
+
 		/*
 		 * The sparse index is not (yet) integrated with a split index.
 		 */
@@ -168,6 +168,19 @@ int convert_to_sparse(struct index_state *istate, int flags)
 	if (!istate->sparse_checkout_patterns->use_cone_patterns)
 		return 0;
 
+	return 1;
+}
+
+int convert_to_sparse(struct index_state *istate, int flags)
+{
+	/*
+	 * If the index is already sparse, empty, or otherwise
+	 * cannot be converted to sparse, do not convert.
+	 */
+	if (istate->sparse_index || !istate->cache_nr ||
+	    !is_sparse_index_allowed(istate, flags))
+		return 0;
+
 	/*
 	 * NEEDSWORK: If we have unmerged entries, then stay full.
 	 * Unmerged entries prevent the cache-tree extension from working.
@@ -316,6 +329,18 @@ 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)
+{
+	/*
+	 * If the index can be sparse, make it sparse. Otherwise,
+	 * ensure the index is full.
+	 */
+	if (is_sparse_index_allowed(istate, 0))
+		convert_to_sparse(istate, 0);
+	else
+		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..656bd835b25 100644
--- a/sparse-index.h
+++ b/sparse-index.h
@@ -4,6 +4,7 @@
 struct index_state;
 #define SPARSE_INDEX_MEMORY_ONLY (1 << 0)
 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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