Fix Overlay bug with nested field deletion (#10016)

* Fix Overlay bug with nested field deletion

* Add more tests.

Author: wu-hui

Firestore/core/src/model/mutation.cc

@@ -167,7 +167,7 @@ TransformMap Mutation::Rep::LocalTransformResults(
 
 absl::optional<Mutation> Mutation::CalculateOverlayMutation(
     const MutableDocument& doc, const absl::optional<FieldMask>& mask) {
-  if ((!doc.has_local_mutations()) || (mask.has_value() && mask->empty())) {
+  if ((!doc.has_local_mutations())) {
     return absl::nullopt;
   }
 
@@ -199,9 +199,13 @@ absl::optional<Mutation> Mutation::CalculateOverlayMutation(
           path = path.PopLast();
           value = doc_value.Get(path);
         }
-        HARD_ASSERT(value.has_value());
-        patch_value.Set(
-            path, Message<google_firestore_v1_Value>(DeepClone(value.value())));
+        if (value.has_value()) {
+          patch_value.Set(path, Message<google_firestore_v1_Value>(
+                                    DeepClone(value.value())));
+        } else {
+          patch_value.Delete(path);
+        }
+
         mask_set.insert(path);
       }
     }

Firestore/core/test/unit/model/mutation_test.cc

@@ -73,7 +73,7 @@ void VerifyOverlayRoundTrips(const MutableDocument& doc,
   MutableDocument doc_for_mutations = doc.Clone();
   MutableDocument doc_for_overlay = doc.Clone();
 
-  absl::optional<FieldMask> mask;
+  absl::optional<FieldMask> mask = FieldMask();
   for (const Mutation& mutation : mutations) {
     mask = mutation.ApplyToLocalView(doc_for_mutations, mask, now);
   }
@@ -712,6 +712,43 @@ TEST(MutationTest, OverlayWithFieldDeletionOfNestedField) {
   VerifyOverlayRoundTrips(doc, {patch1, patch2, patch3});
 }
 
+// See: https://github.com/firebase/firebase-ios-sdk/issues/9985
+TEST(MutationTest, OverlayWithFieldDeletionOfNestedFieldAndParentField) {
+  MutableDocument doc = Doc("collection/key", 1, Map("foo", 1));
+  Mutation patch1 =
+      PatchMutation("collection/key", Map("foo", "foo-patched-value"),
+                    {{"bar.baz", Increment(1)}});
+  Mutation patch2 =
+      PatchMutation("collection/key", Map("foo", "foo-patched-value"),
+                    {{"bar.baz", ServerTimestamp()}, {"a.b.c", Increment(1)}});
+  Mutation patch3 =
+      PatchMutation("collection/key", Map("foo", "foo-patched-value"),
+                    {Field("bar.baz"), Field("a.b.c")}, {});
+
+  Mutation patch4 =
+      PatchMutation("collection/key", Map("foo", "foo-patched-value"),
+                    {Field("bar"), Field("a.b")}, {});
+
+  VerifyOverlayRoundTrips(doc, {patch1, patch2, patch3, patch4});
+}
+
+// See: https://github.com/firebase/firebase-ios-sdk/issues/10018
+// Same root cause as OverlayWithFieldDeletionOfNestedFieldAndParentField,
+// different way to trigger.
+TEST(MutationTest, OverlayWorksWithDeletingSameField) {
+  MutableDocument doc = Doc("collection/key", 1, Map("foo", 1));
+  Mutation patch1 =
+      PatchMutation("collection/key", Map("foo", "foo-patched-value"),
+                    {{"bar", ServerTimestamp()}});
+  Mutation patch2 = PatchMutation(
+      "collection/key", Map("foo", "foo-patched-value"), {Field("bar")}, {});
+
+  Mutation patch3 = PatchMutation(
+      "collection/key", Map("foo", "foo-patched-value"), {Field("bar")}, {});
+
+  VerifyOverlayRoundTrips(doc, {patch1, patch2, patch3});
+}
+
 TEST(MutationTest, OverlayCreatedFromSetToEmptyWithMerge) {
   MutableDocument doc = DeletedDoc("collection/key", 1);
   Mutation merge = MergeMutation("collection/key", Map(), {});