fix(gateway): maintain immutability when merging selection sets (apol…

…lographql#4239)

packages/apollo-gateway/CHANGELOG.md

@@ -5,6 +5,7 @@
 > The changes noted within this `vNEXT` section have not been released yet.  New PRs and commits which introduce changes should include an entry in this `vNEXT` section as part of their development.  When a release is being prepared, a new header will be (manually) created below and the appropriate changes within that release will be moved into the new section.
 
 - The default branch of the repository has been changed to `main`.  As this changed a number of references in the repository's `package.json` and `README.md` files (e.g., for badges, links, etc.), this necessitates a release to publish those changes to npm. [PR #4302](https://github.com/apollographql/apollo-server/pull/4302)
+- __FIX__: The `mergeFieldNodeSelectionSets` method no longer mutates original FieldNode objects. Before, it was updating the selection set of the original object, corrupting the data accross requests.
 
 ## 0.16.9
 

packages/apollo-gateway/src/FieldSet.ts

@@ -151,13 +151,16 @@ function mergeFieldNodeSelectionSets(
       nonAliasedFieldNodes,
     ).values(),
   ).map((nodesWithSameName) => {
-    const node = nodesWithSameName[0];
+    const node = { ...nodesWithSameName[0] };
     if (node.selectionSet) {
-      node.selectionSet.selections = mergeFieldNodeSelectionSets(
-        nodesWithSameName.flatMap(
-          (node) => node.selectionSet?.selections || [],
+      node.selectionSet = {
+        ...node.selectionSet,
+        selections: mergeFieldNodeSelectionSets(
+          nodesWithSameName.flatMap(
+            (node) => node.selectionSet?.selections || [],
+          ),
         ),
-      );
+      };
     }
     return node;
   });

packages/apollo-gateway/src/__tests__/gateway/queryPlanCache.test.ts

@@ -104,3 +104,118 @@ it('supports multiple operations and operationName', async () => {
     ],
   });
 });
+
+it('does not corrupt cached queryplan data across requests', async () => {
+  const serviceA = {
+    name: 'a',
+    typeDefs: gql`
+      type Query {
+        user: User
+      }
+
+      type User @key(fields: "id") {
+        id: ID!
+        preferences: Preferences
+      }
+
+      type Preferences {
+        favorites: Things
+      }
+
+      type Things {
+        color: String
+        animal: String
+      }
+    `,
+    resolvers: {
+      Query: {
+        user() {
+          return {
+            id: '1',
+            preferences: {
+              favorites: { color: 'limegreen', animal: 'platypus' },
+            },
+          };
+        },
+      },
+    },
+  };
+
+  const serviceB = {
+    name: 'b',
+    typeDefs: gql`
+      extend type User @key(fields: "id") {
+        id: ID! @external
+        preferences: Preferences @external
+        favoriteColor: String
+          @requires(fields: "preferences { favorites { color } }")
+        favoriteAnimal: String
+          @requires(fields: "preferences { favorites { animal } }")
+      }
+
+      extend type Preferences {
+        favorites: Things @external
+      }
+
+      extend type Things {
+        color: String @external
+        animal: String @external
+      }
+    `,
+    resolvers: {
+      User: {
+        favoriteColor(user: any) {
+          return user.preferences.favorites.color;
+        },
+        favoriteAnimal(user: any) {
+          return user.preferences.favorites.animal;
+        },
+      },
+    },
+  };
+
+  const gateway = new ApolloGateway({
+    localServiceList: [serviceA, serviceB],
+    buildService: service => {
+      return new LocalGraphQLDataSource(buildFederatedSchema([service]));
+    },
+  });
+
+  const { schema, executor } = await gateway.load();
+
+  const server = new ApolloServer({ schema, executor });
+
+  const call = createTestClient(server);
+
+  const query1 = `#graphql
+    query UserFavoriteColor {
+      user {
+        favoriteColor
+      }
+    }
+  `;
+
+  const query2 = `#graphql
+    query UserFavorites {
+      user {
+        favoriteColor
+        favoriteAnimal
+      }
+    }
+  `;
+
+  const result1 = await call.query({
+    query: query1,
+  });
+  const result2 = await call.query({
+    query: query2,
+  });
+  const result3 = await call.query({
+    query: query1,
+  });
+
+  expect(result1.errors).toEqual(undefined);
+  expect(result2.errors).toEqual(undefined);
+  expect(result3.errors).toEqual(undefined);
+  expect(result1).toEqual(result3);
+});

packages/apollo-gateway/src/__tests__/integration/requires.test.ts

@@ -235,7 +235,6 @@ it('collapses nested requires with user-defined fragments', async () => {
                 preferences {
                   favorites {
                     animal
-                    color
                   }
                 }
               }