Don't update backlinks in Mixed self-assignment (#7384)

Setting a Mixed field to ObjLink equal to the current value removed the
existing backlink and then exited before adding the new one, leaving things in
an invalid state.

CHANGELOG.md

@@ -7,6 +7,7 @@
 ### Fixed
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * List KVO information was being populated for non-list collections ([PR #7378](https://github.com/realm/realm-core/pull/7378), since v14.0.0)
+* Setting a Mixed property to an ObjLink equal to its existing value would remove the existing backlinks and then exit before re-adding them, resulting in later assertion failures due to the backlink state being invalid ([PR #7384](https://github.com/realm/realm-core/pull/7384), since v14.0.0).
 * Opening file with file format 23 in read-only mode will crash ([#7388](https://github.com/realm/realm-core/issues/7388), since v14.0.0)
 
 ### Breaking changes

src/realm/obj.cpp

@@ -1168,10 +1168,11 @@ Obj& Obj::set<Mixed>(ColKey col_key, Mixed value, bool is_default)
     }
 
     Mixed old_value = get_unfiltered_mixed(col_ndx);
-    ObjLink old_link{};
-    ObjLink new_link{};
     if (old_value.is_type(type_TypedLink)) {
-        old_link = old_value.get<ObjLink>();
+        if (old_value == value) {
+            return *this;
+        }
+        auto old_link = old_value.get<ObjLink>();
         recurse = remove_backlink(col_key, old_link, state);
     }
     else if (old_value.is_type(type_Dictionary)) {
@@ -1187,10 +1188,8 @@ Obj& Obj::set<Mixed>(ColKey col_key, Mixed value, bool is_default)
         if (m_table->is_asymmetric()) {
             throw IllegalOperation("Links not allowed in asymmetric tables");
         }
-        new_link = value.template get<ObjLink>();
+        auto new_link = value.get<ObjLink>();
         m_table->get_parent_group()->validate(new_link);
-        if (new_link == old_link)
-            return *this;
         set_backlink(col_key, new_link);
     }
 

test/test_mixed_null_assertions.cpp

@@ -443,3 +443,26 @@ TEST(Mixed_set_non_link_assertion)
     dest_obj.remove(); // triggers an assertion failure if the backlink was not removed
     source_obj.remove();
 }
+
+TEST(Mixed_LinkSelfAssignment)
+{
+    Group g;
+    auto source = g.add_table("source");
+    auto dest = g.add_table("dest");
+    ColKey mixed_col = source->add_column(type_Mixed, "mixed");
+    auto source_obj = source->create_object();
+    auto dest_obj = dest->create_object();
+
+    CHECK_EQUAL(dest_obj.get_backlink_count(), 0);
+
+    source_obj.set(mixed_col, Mixed{ObjLink{dest->get_key(), dest_obj.get_key()}});
+    CHECK_EQUAL(dest_obj.get_backlink_count(), 1);
+
+    // Re-assign the same link, which should not update backlinks
+    source_obj.set(mixed_col, Mixed{ObjLink{dest->get_key(), dest_obj.get_key()}});
+    CHECK_EQUAL(dest_obj.get_backlink_count(), 1);
+
+    dest_obj.remove();
+    CHECK_EQUAL(source_obj.get<Mixed>(mixed_col), Mixed());
+    source_obj.remove();
+}