merge-ort: expand only for out-of-cone conflicts

Merge conflicts happen often enough to want to avoid expanding a sparse
index when they happen, as long as those conflicts are within the
sparse-checkout cone. If a conflict exists outside of the
sparse-checkout cone, then we still need to expand before iterating over
the index entries. This is critical to do in advance because of how the
original_cache_nr is tracked to allow inserting and replacing cache
entries.

Iterate over the conflicted files and check if any paths are outside of
the sparse-checkout cone. If so, then expand the full index.

Add a test that demonstrates that we do not expand the index, even when
we hit a conflict within the sparse-checkout cone.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

merge-ort.c

@@ -4060,11 +4060,18 @@ static int record_conflicted_index_entries(struct merge_options *opt)
 
 	/*
 	 * We are in a conflicted state. These conflicts might be inside
-	 * sparse-directory entries, so expand the index preemtively.
-	 * Also, we set original_cache_nr below, but that might change if
+	 * sparse-directory entries, so check if any entries are outside
+	 * of the sparse-checkout cone preemptively.
+	 *
+	 * We set original_cache_nr below, but that might change if
 	 * index_name_pos() calls ask for paths within sparse directories.
 	 */
-	ensure_full_index(index);
+	strmap_for_each_entry(&opt->priv->conflicted, &iter, e) {
+		if (!path_in_sparse_checkout(e->key, index)) {
+			ensure_full_index(index);
+			break;
+		}
+	}
 
 	/* If any entries have skip_worktree set, we'll have to check 'em out */
 	state.force = 1;

t/t1092-sparse-checkout-compatibility.sh

@@ -617,8 +617,17 @@ test_expect_success 'sparse-index is expanded and converted back' '
 ensure_not_expanded () {
 	rm -f trace2.txt &&
 	echo >>sparse-index/untracked.txt &&
-	GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
-		git -C sparse-index "$@" &&
+
+	if test "$1" = "!"
+	then
+		shift &&
+		test_must_fail env \
+			GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+			git -C sparse-index "$@" || return 1
+	else
+		GIT_TRACE2_EVENT="$(pwd)/trace2.txt" GIT_TRACE2_EVENT_NESTING=10 \
+			git -C sparse-index "$@" || return 1
+	fi &&
 	test_region ! index ensure_full_index trace2.txt
 }
 
@@ -658,6 +667,23 @@ test_expect_success 'sparse-index is not expanded' '
 	)
 '
 
+test_expect_success 'sparse-index is not expanded: merge conflict in cone' '
+	init_repos &&
+
+	for side in right left
+	do
+		git -C sparse-index checkout -b expand-$side base &&
+		echo $side >sparse-index/deep/a &&
+		git -C sparse-index commit -a -m "$side" || return 1
+	done &&
+
+	(
+		sane_unset GIT_TEST_MERGE_ALGORITHM &&
+		git -C sparse-index config pull.twohead ort &&
+		ensure_not_expanded ! merge -m merged expand-right
+	)
+'
+
 # NEEDSWORK: a sparse-checkout behaves differently from a full checkout
 # in this scenario, but it shouldn't.
 test_expect_success 'reset mixed and checkout orphan' '