fix: normalize field set selection (#3162)

`FieldSet` scalar represents a selection set without outer braces. This
means that users could potentially specify some selections that could be
normalized (i.e. eliminate duplicate field selections, hoist/collapse
unnecessary inline fragments, etc). Previously we were using `@requires`
field set selection AS-IS for edge conditions. With this change we will
now normalize the `FieldSet` selections before using them as fetch node
conditions.

<!-- FED-390 -->

.changeset/slow-dots-explode.md

@@ -0,0 +1,8 @@
+---
+"@apollo/query-planner": patch
+"@apollo/federation-internals": patch
+---
+
+fix: normalize field set selection sets
+
+`FieldSet` scalar represents a selection set without outer braces. This means that users could potentially specify some selections that could be normalized (i.e. eliminate duplicate field selections, hoist/collapse unnecessary inline fragments, etc). Previously we were using `@requires` field set selection AS-IS for edge conditions. With this change we will now normalize the `FieldSet` selections before using them as fetch node conditions.

internals-js/src/federation.ts

@@ -2302,7 +2302,7 @@ export function parseFieldSetArgument({
         }
       });
     }
-    return selectionSet;
+    return selectionSet.normalize({ parentType, recursive: true });
   } catch (e) {
     if (!(e instanceof GraphQLError) || !decorateValidationErrors) {
       throw e;

query-planner-js/src/__tests__/buildPlan.test.ts

@@ -2986,6 +2986,94 @@ describe('@requires', () => {
       }
     `);
   });
+
+  it(`normalizes requires field set selection`, () => {
+    const subgraph1 = {
+      name: 'Subgraph1',
+      typeDefs: gql`
+        type Query {
+          t: T
+        }
+
+        type T @key(fields: "id") {
+          id: ID!
+          a: A
+        }
+
+        type A {
+          a1: Int
+          a2: Int
+        }
+      `,
+    };
+
+    const subgraph2 = {
+      name: 'Subgraph2',
+      typeDefs: gql`
+        type T @key(fields: "id") {
+          id: ID!
+          a: A @external
+          b: Int @requires(fields: "a { ... on A { a1 a2 } a1 }")
+        }
+
+        type A @external {
+          a1: Int
+          a2: Int
+        }
+      `,
+    };
+
+    const [api, queryPlanner] = composeAndCreatePlanner(subgraph1, subgraph2);
+    const operation = operationFromDocument(
+      api,
+      gql`
+        {
+          t {
+            b
+          }
+        }
+      `,
+    );
+
+    const plan = queryPlanner.buildQueryPlan(operation);
+    expect(plan).toMatchInlineSnapshot(`
+      QueryPlan {
+        Sequence {
+          Fetch(service: "Subgraph1") {
+            {
+              t {
+                __typename
+                id
+                a {
+                  a1
+                  a2
+                }
+              }
+            }
+          },
+          Flatten(path: "t") {
+            Fetch(service: "Subgraph2") {
+              {
+                ... on T {
+                  __typename
+                  id
+                  a {
+                    a1
+                    a2
+                  }
+                }
+              } =>
+              {
+                ... on T {
+                  b
+                }
+              }
+            },
+          },
+        },
+      }
+    `);
+  });
 });
 
 describe('fetch operation names', () => {