Fix __typename inside router response

.changesets/fix_fix_typename.md

@@ -0,0 +1,22 @@
+### Fix various edge cases for `__typename` field ([PR #6009](https://github.com/apollographql/router/pull/6009))
+
+The router now correctly handles the `__typename` field used on operation root types, even when the subgraph's root type has a name that differs from the supergraph's root type.
+
+For example, in query like this:
+```graphql
+{
+  ...RootFragment
+}
+
+fragment RootFragment on Query {
+  __typename
+  me {
+    name
+  }
+}
+```
+Even if the subgraph's root type returns a `__typename` that differs from `Query`, the router will still use `Query` as the value of the `__typename` field.
+
+This change also includes fixes for other edge cases related to the handling of `__typename` fields. For a detailed technical description of the edge cases that were fixed, please see [this description](https://github.com/apollographql/router/pull/6009#issue-2529717207).
+
+By [@IvanGoncharov](https://github.com/IvanGoncharov) in https://github.com/apollographql/router/pull/6009

apollo-router/src/query_planner/execution.rs

@@ -311,6 +311,8 @@ impl PlanNode {
                             let _ = primary_sender.send((value.clone(), errors.clone()));
                         } else {
                             let _ = primary_sender.send((value.clone(), errors.clone()));
+                            // primary response should be an empty object
+                            value.deep_merge(Value::Object(Default::default()));
                         }
                     }
                     .instrument(tracing::info_span!(

apollo-router/src/services/supergraph/tests.rs

@@ -1103,12 +1103,12 @@ async fn subscription_without_header() {
 async fn root_typename_with_defer_and_empty_first_response() {
     let subgraphs = MockedSubgraphs([
         ("user", MockSubgraph::builder().with_json(
-                serde_json::json!{{"query":"{currentUser{activeOrganization{__typename id}}}"}},
+                serde_json::json!{{"query":"{... on Query{currentUser{activeOrganization{__typename id}}}}"}},
                 serde_json::json!{{"data": {"currentUser": { "activeOrganization": { "__typename": "Organization", "id": "0" } }}}}
             ).build()),
         ("orga", MockSubgraph::builder().with_json(
             serde_json::json!{{
-                "query":"query($representations:[_Any!]!){_entities(representations:$representations){...on Organization{suborga{__typename id}}}}",
+                "query":"query($representations:[_Any!]!){_entities(representations:$representations){...on Organization{suborga{id name}}}}",
                 "variables": {
                     "representations":[{"__typename": "Organization", "id":"0"}]
                 }
@@ -1117,33 +1117,11 @@ async fn root_typename_with_defer_and_empty_first_response() {
                 "data": {
                     "_entities": [{ "suborga": [
                     { "__typename": "Organization", "id": "1"},
-                    { "__typename": "Organization", "id": "2"},
+                    { "__typename": "Organization", "id": "2", "name": "A"},
                     { "__typename": "Organization", "id": "3"},
                     ] }]
                 },
-                }}
-        )
-        .with_json(
-            serde_json::json!{{
-                "query":"query($representations:[_Any!]!){_entities(representations:$representations){...on Organization{name}}}",
-                "variables": {
-                    "representations":[
-                        {"__typename": "Organization", "id":"1"},
-                        {"__typename": "Organization", "id":"2"},
-                        {"__typename": "Organization", "id":"3"}
-
-                        ]
-                }
-            }},
-            serde_json::json!{{
-                "data": {
-                    "_entities": [
-                    { "__typename": "Organization", "id": "1"},
-                    { "__typename": "Organization", "id": "2", "name": "A"},
-                    { "__typename": "Organization", "id": "3"},
-                    ]
-                }
-                }}
+            }}
         ).build())
     ].into_iter().collect());
 
@@ -1156,23 +1134,77 @@ async fn root_typename_with_defer_and_empty_first_response() {
         .await
         .unwrap();
 
+    let query = r#"
+        query {
+            ...OnlyTypename
+            ... @defer {
+                currentUser {
+                    activeOrganization {
+                        id
+                        suborga {
+                            id
+                            name
+                        }
+                    }
+                }
+            }
+        }
+
+        fragment OnlyTypename on Query {
+          __typename
+        }
+    "#;
     let request = supergraph::Request::fake_builder()
-            .context(defer_context())
-            .query(
-                "query { __typename ... @defer { currentUser { activeOrganization { id  suborga { id name } } } } }",
-            )
-            .build()
-            .unwrap();
+        .context(defer_context())
+        .query(query)
+        .build()
+        .unwrap();
 
     let mut stream = service.oneshot(request).await.unwrap();
     let res = stream.next_response().await.unwrap();
-    assert_eq!(
-        res.data.as_ref().unwrap().get("__typename"),
-        Some(&serde_json_bytes::Value::String("Query".into()))
-    );
+
+    insta::assert_json_snapshot!(res, @r###"
+    {
+      "data": {
+        "__typename": "Query"
+      },
+      "hasNext": true
+    }
+    "###);
 
     // Must have 2 chunks
-    let _ = stream.next_response().await.unwrap();
+    let res = stream.next_response().await.unwrap();
+    insta::assert_json_snapshot!(res, @r###"
+    {
+      "hasNext": false,
+      "incremental": [
+        {
+          "data": {
+            "currentUser": {
+              "activeOrganization": {
+                "id": "0",
+                "suborga": [
+                  {
+                    "id": "1",
+                    "name": null
+                  },
+                  {
+                    "id": "2",
+                    "name": "A"
+                  },
+                  {
+                    "id": "3",
+                    "name": null
+                  }
+                ]
+              }
+            }
+          },
+          "path": []
+        }
+      ]
+    }
+    "###);
 }
 
 #[tokio::test]