[PATCH 7/7] read-tree: make three-way merge sparse-aware

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

From: Victoria Dye <vdye@github.com>

Enable use of 'merged_sparse_dir' in 'threeway_merge'. As with two-way
merge, the contents of each conflicted sparse directory are merged without
referencing the index, avoiding sparse index expansion.

Signed-off-by: Victoria Dye <vdye@github.com>
---
 builtin/read-tree.c                      |  5 -----
 t/t1092-sparse-checkout-compatibility.sh |  3 ++-
 unpack-trees.c                           | 16 ++++++++++++----
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index 5a421de2629..dc2283fce53 100644
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -230,11 +230,6 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 			break;
 		case 3:
 		default:
-			/*
-			 * TODO: update threeway_merge to handle edit/edit conflicts in
-			 * sparse directories.
-			 */
-			ensure_full_index(&the_index);
 			opts.fn = threeway_merge;
 			break;
 		}
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index d6f19682d65..5447d314632 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -1412,7 +1412,8 @@ test_expect_success 'sparse index is not expanded: read-tree' '
 
 	ensure_not_expanded checkout -b test-branch update-folder1 &&
 	for MERGE_TREES in "update-folder2" \
-			   "base update-folder2"
+			   "base update-folder2" \
+			   "base HEAD update-folder2"
 	do
 		ensure_not_expanded read-tree -mu $MERGE_TREES &&
 		ensure_not_expanded reset --hard HEAD || return 1
diff --git a/unpack-trees.c b/unpack-trees.c
index a4ace53904e..9e79cb13bd4 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -2635,16 +2635,24 @@ int threeway_merge(const struct cache_entry * const *stages,
 	 */
 	/* #14, #14ALT, #2ALT */
 	if (remote && !df_conflict_head && head_match && !remote_match) {
-		if (index && !same(index, remote) && !same(index, head))
-			return reject_merge(index, o);
+		if (index && !same(index, remote) && !same(index, head)) {
+			if (S_ISSPARSEDIR(index->ce_mode))
+				return merged_sparse_dir(stages, 4, o);
+			else
+				return reject_merge(index, o);
+		}
 		return merged_entry(remote, index, o);
 	}
 	/*
 	 * If we have an entry in the index cache, then we want to
 	 * make sure that it matches head.
 	 */
-	if (index && !same(index, head))
-		return reject_merge(index, o);
+	if (index && !same(index, head)) {
+		if (S_ISSPARSEDIR(index->ce_mode))
+			return merged_sparse_dir(stages, 4, o);
+		else
+			return reject_merge(index, o);
+	}
 
 	if (head) {
 		/* #5ALT, #15 */
-- 
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