Fix returning thrown errors from QP (#457)

The Rust side is expecting a single string in `exception.stacktrace`,
but JS was providing an array of strings. It doesn't really make sense
to split `error.toString()` as normally the first line will be the error
message, and only the later lines will be part of the stack trace, so
I think it's best if we use a single string.

Ref:

https://github.com/apollographql/federation-rs/blob/35052d8064e7ca50d09226febdbbe0d55cc5d18b/federation-2/router-bridge/src/planner.rs#L121-L136

Fixes apollographql/router#4783

federation-2/router-bridge/js-src/plan_worker.ts

@@ -305,7 +305,7 @@ async function run() {
           extensions: {
             code: "QUERY_PLANNING_FAILED",
             exception: {
-              stacktrace: e.toString().split(/\n/),
+              stacktrace: e.toString(),
             },
           },
         };
@@ -330,7 +330,7 @@ async function run() {
         extensions: {
           code: "QUERY_PLANNING_FAILED",
           exception: {
-            stacktrace: e.toString().split(/\n/),
+            stacktrace: e.toString(),
           },
         },
       };

federation-2/router-bridge/src/planner.rs

@@ -2216,4 +2216,89 @@ feature https://specs.apollo.dev/unsupported-feature/v0.1 is for: SECURITY but i
         .data
         .unwrap()).unwrap());
     }
+
+    #[tokio::test]
+    async fn propagate_internal_qp_errors() {
+        let schema = r#"
+        schema
+          @link(url: "https://specs.apollo.dev/link/v1.0")
+          @link(url: "https://specs.apollo.dev/join/v0.2", for: EXECUTION)
+        {
+          query: Query
+          subscription: Subscription
+        }
+        
+        directive @join__field(graph: join__Graph!, requires: join__FieldSet, provides: join__FieldSet, type: String, external: Boolean, override: String, usedOverridden: Boolean) repeatable on FIELD_DEFINITION | INPUT_FIELD_DEFINITION
+        directive @join__graph(name: String!, url: String!) on ENUM_VALUE
+        directive @join__implements(graph: join__Graph!, interface: String!) repeatable on OBJECT | INTERFACE
+        directive @join__type(graph: join__Graph!, key: join__FieldSet, extension: Boolean! = false, resolvable: Boolean! = true) repeatable on OBJECT | INTERFACE | UNION | ENUM | INPUT_OBJECT | SCALAR
+        directive @link(url: String, as: String, for: link__Purpose, import: [link__Import]) repeatable on SCHEMA
+                
+        scalar link__Import
+        enum link__Purpose {
+          SECURITY
+          EXECUTION
+        }
+
+        type Computer
+          @join__type(graph: COMPUTERS)
+        {
+          id: ID!
+          errorField: String
+          nonNullErrorField: String!
+          gpus: [GPU]
+        }
+
+        type GPU @join__type(graph: COMPUTERS) @join__type(graph: GPUS) {
+          id: ID!
+          wattage: Int! @join__field(graph: GPUS)
+        }
+        
+        scalar join__FieldSet
+        
+        enum join__Graph {
+          COMPUTERS @join__graph(name: "computers", url: "http://localhost:4001/")
+          GPUS @join__graph(name: "gpus", url: "http://localhost:4002/")
+        }
+
+
+        type Query
+          @join__type(graph: COMPUTERS)
+        {
+          computer(id: ID!): Computer
+        }
+        type Subscription @join__type(graph: COMPUTERS) {
+          computer(id: ID!): Computer
+        }"#;
+
+        let planner = Planner::<serde_json::Value>::new(
+            schema.to_string(),
+            QueryPlannerConfig {
+                incremental_delivery: Some(IncrementalDeliverySupport {
+                    enable_defer: Some(true),
+                }),
+                graphql_validation: true,
+                reuse_query_fragments: None,
+                generate_query_fragments: None,
+                debug: Default::default(),
+            },
+        )
+        .await
+        .unwrap();
+
+        insta::assert_snapshot!(serde_json::to_string_pretty(
+            &planner
+                .plan(
+                    "subscription { computer(id: 1) { ... @defer { gpus { wattage } } } }"
+                        .to_string(),
+                    None,
+                    PlanOptions::default(),
+                )
+                .await
+                .unwrap()
+                .errors
+                .unwrap()
+        )
+        .unwrap());
+    }
 }

federation-2/router-bridge/src/snapshots/router_bridge__planner__error_display__propagate_internal_qp_errors.snap

@@ -0,0 +1,15 @@
+---
+source: router-bridge/src/planner.rs
+expression: "serde_json::to_string_pretty(&planner.plan(\"subscription { computer(id: 1) { ... @defer { gpus { wattage } } } }\".to_string(),\n                                None,\n                                PlanOptions::default()).await.unwrap().errors.unwrap()).unwrap()"
+---
+[
+  {
+    "message": "@defer is not supported on subscriptions",
+    "extensions": {
+      "code": "QUERY_PLANNING_FAILED",
+      "exception": {
+        "stacktrace": "Error: @defer is not supported on subscriptions"
+      }
+    }
+  }
+]