incorrect query plan generated when spreading interface fragment into multiple implementations

[benjaminjkraft]

Consider the following query, against the federation demo service:

fragment Price on Product {
  price
}

query {
  topProducts {
    __typename
    ... on Book {
      ...Price
    }
    ... on Furniture {
      ...Price
    }
  }
}

Apollo gateway generates an incorrect query plan: it should simply pass on an equivalent query to the products service, but in fact it entirely omits asking for price on Furniture, and only asks for it on Book.

Test case to reproduce this on master

diff --git packages/apollo-gateway/src/__tests__/buildQueryPlan.test.ts packages/apollo-gateway/src/__tests__/buildQueryPlan.test.ts
index 156f7f93..4f210413 100644
--- packages/apollo-gateway/src/__tests__/buildQueryPlan.test.ts
+++ packages/apollo-gateway/src/__tests__/buildQueryPlan.test.ts
@@ -768,6 +768,46 @@ describe('buildQueryPlan', () => {
         }
       `);
     });
+
+    it(`should not get confused by a fragment spread multiple times`, () => {
+      const query = gql`
+        fragment Price on Product {
+          price
+        }
+
+        query {
+          topProducts {
+            __typename
+            ... on Book {
+              ...Price
+            }
+            ... on Furniture {
+              ...Price
+            }
+          }
+        }
+      `;
+
+      const queryPlan = buildQueryPlan(buildOperationContext(schema, query));
+
+      expect(queryPlan).toMatchInlineSnapshot(`
+                QueryPlan {
+                  Fetch(service: "product") {
+                    {
+                      topProducts {
+                        __typename
+                        ... on Book {
+                          price
+                        }
+                        ... on Furniture {
+                          price
+                        }
+                      }
+                    }
+                  },
+                }
+            `);
+    });
   });
 
   // GraphQLError: Cannot query field "isbn" on type "Book"

Test output excerpt:

  ● buildQueryPlan › for abstract types › should not get confused by a fragment spread multiple times

    expect(received).toMatchInlineSnapshot(snapshot)

    Snapshot name: `buildQueryPlan for abstract types should not get confused by a fragment spread multiple times 1`

    - Snapshot
    + Received

    @@ -4,12 +4,9 @@
            topProducts {
              __typename
              ... on Book {
                price
              }
    -         ... on Furniture {
    -           price
    -         }
            }
          }
        },
      }

[benjaminjkraft]

It's possible this is related to #384, but there's not quite enough information there for me to know.

[abernix]

@benjaminjkraft Do you know if this is still happening on the latest version of @apollo/gateway? Do you mind opening a PR with this test to verify it is fixed (or not!)?

[benjaminjkraft]

You got it: apollographql/apollo-server#4281. From a quick look, it looks like it is still happening.

(Edit: removed what I previously wrote here -- I had misread the diff.)

[abernix]

The fix for this was released in @apollo/gateway@0.27.1! 🎉