[PATCH v5 2/4] sparse-index: avoid unnecessary cache tree clearing

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

From: Victoria Dye <vdye@github.com>

When converting a full index to sparse, clear and recreate the cache tree
only if the cache tree is not fully valid. The convert_to_sparse operation
should exit silently if a cache tree update cannot be successfully completed
(e.g., due to a conflicted entry state). However, because this failure
scenario only occurs when at least a portion of the cache tree is invalid,
we can save ourselves the cost of clearing and recreating the cache tree by
skipping the check when the cache tree is fully valid.

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Victoria Dye <vdye@github.com>
---
 sparse-index.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/sparse-index.c b/sparse-index.c
index 7b7ff79e044..85613cd8a3a 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -175,17 +175,20 @@ int convert_to_sparse(struct index_state *istate, int flags)
 	if (index_has_unmerged_entries(istate))
 		return 0;
 
-	/* Clear and recompute the cache-tree */
-	cache_tree_free(&istate->cache_tree);
-	/*
-	 * Silently return if there is a problem with the cache tree update,
-	 * which might just be due to a conflict state in some entry.
-	 *
-	 * This might create new tree objects, so be sure to use
-	 * WRITE_TREE_MISSING_OK.
-	 */
-	if (cache_tree_update(istate, WRITE_TREE_MISSING_OK))
-		return 0;
+	if (!cache_tree_fully_valid(istate->cache_tree)) {
+		/* Clear and recompute the cache-tree */
+		cache_tree_free(&istate->cache_tree);
+
+		/*
+		 * Silently return if there is a problem with the cache tree update,
+		 * which might just be due to a conflict state in some entry.
+		 *
+		 * This might create new tree objects, so be sure to use
+		 * WRITE_TREE_MISSING_OK.
+		 */
+		if (cache_tree_update(istate, WRITE_TREE_MISSING_OK))
+			return 0;
+	}
 
 	remove_fsmonitor(istate);
 
-- 
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