fix(federation): added the remove_introspection step in normalize_operation (#5580)

Co-authored-by: Renée <[email protected]>

Author: duckki

apollo-federation/src/operation/mod.rs

@@ -4901,6 +4901,7 @@ pub(crate) fn normalize_operation(
         schema,
         NormalizeSelectionOption::NormalizeRecursively,
     )?;
+    remove_introspection(&mut normalized_selection_set);
     normalized_selection_set.optimize_sibling_typenames(interface_types_with_interface_objects)?;
 
     let normalized_operation = Operation {
@@ -4915,6 +4916,21 @@ pub(crate) fn normalize_operation(
     Ok(normalized_operation)
 }
 
+// PORT_NOTE: This is a port of `withoutIntrospection` from JS version.
+fn remove_introspection(selection_set: &mut SelectionSet) {
+    // Note that, because we only apply this to the top-level selections, we skip all
+    // introspection, including __typename. In general, we don't want to ignore __typename during
+    // query plans, but at top-level, we can let the router execution deal with it rather than
+    // querying some service for that.
+
+    Arc::make_mut(&mut selection_set.selections).retain(|_, selection| {
+        !matches!(selection,
+            Selection::Field(field_selection) if
+                field_selection.field.field_position.is_introspection_typename_field()
+        )
+    });
+}
+
 fn runtime_types_intersect(
     type1: &CompositeTypeDefinitionPosition,
     type2: &CompositeTypeDefinitionPosition,

apollo-federation/src/query_plan/query_planner.rs

@@ -1313,4 +1313,51 @@ type User
         }
         "###);
     }
+
+    #[test]
+    fn drop_operation_root_level_typename() {
+        let subgraph1 = Subgraph::parse_and_expand(
+            "Subgraph1",
+            "https://Subgraph1",
+            r#"
+                type Query {
+                    t: T
+                }
+
+                type T @key(fields: "id") {
+                    id: ID!
+                    x: Int
+                }
+            "#,
+        )
+        .unwrap();
+        let subgraphs = vec![&subgraph1];
+        let supergraph = Supergraph::compose(subgraphs).unwrap();
+        let planner = QueryPlanner::new(&supergraph, Default::default()).unwrap();
+        let document = ExecutableDocument::parse_and_validate(
+            planner.api_schema().schema(),
+            r#"
+                query {
+                    __typename
+                    t {
+                        x
+                    }
+                }
+            "#,
+            "operation.graphql",
+        )
+        .unwrap();
+        let plan = planner.build_query_plan(&document, None).unwrap();
+        insta::assert_snapshot!(plan, @r###"
+        QueryPlan {
+          Fetch(service: "Subgraph1") {
+            {
+              t {
+                x
+              }
+            }
+          },
+        }
+        "###);
+    }
 }