Deduplicate variable definitions for subqueries

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

@@ -248,6 +248,83 @@ describe('executeQueryPlan', () => {
             `);
   });
 
+  it('should not duplicate variable definitions', async () => {
+    const query = gql`
+      query Test($first: Int!) {
+        first: topReviews(first: $first) {
+          body
+          author {
+            name
+          }
+        }
+        second: topReviews(first: $first) {
+          body
+          author {
+            name
+          }
+        }
+      }
+    `;
+
+    const operationContext = buildOperationContext(schema, query);
+    const queryPlan = buildQueryPlan(operationContext);
+
+    const requestContext = buildRequestContext();
+    requestContext.request.variables = { first: 3 };
+
+    const response = await executeQueryPlan(
+      queryPlan,
+      serviceMap,
+      requestContext,
+      operationContext,
+    );
+
+    expect(response.data).toMatchInlineSnapshot(`
+      Object {
+        "first": Array [
+          Object {
+            "author": Object {
+              "name": "Ada Lovelace",
+            },
+            "body": "Love it!",
+          },
+          Object {
+            "author": Object {
+              "name": "Ada Lovelace",
+            },
+            "body": "Too expensive.",
+          },
+          Object {
+            "author": Object {
+              "name": "Alan Turing",
+            },
+            "body": "Could be better.",
+          },
+        ],
+        "second": Array [
+          Object {
+            "author": Object {
+              "name": "Ada Lovelace",
+            },
+            "body": "Love it!",
+          },
+          Object {
+            "author": Object {
+              "name": "Ada Lovelace",
+            },
+            "body": "Too expensive.",
+          },
+          Object {
+            "author": Object {
+              "name": "Alan Turing",
+            },
+            "body": "Could be better.",
+          },
+        ],
+      }
+    `);
+  });
+
   it('can execute an introspection query', async () => {
     const operationContext = buildOperationContext(
       schema,

packages/apollo-gateway/src/executeQueryPlan.ts

@@ -339,14 +339,27 @@ function mapFetchNodeToVariableDefinitions(
   node: FetchNode,
 ): VariableDefinitionNode[] {
   const variableUsage = node.variableUsages;
-  return variableUsage
-    ? variableUsage.map(({ node, type }) => ({
+  if (!variableUsage) {
+    return [];
+  }
+
+  const variableMap = variableUsage.reduce((map, { node, type }) => {
+    const key = `${node.name.value}_${type.toString()}`;
+
+    if (!map.has(key)) {
+      map.set(key, {
         kind: Kind.VARIABLE_DEFINITION,
         variable: node,
         type: astFromType(type),
-      }))
-    : [];
+      });
+    }
+
+    return map;
+  }, new Map<string, VariableDefinitionNode>());
+
+  return Array.from(variableMap.values());
 }
+
 function operationForRootFetch(
   fetch: FetchNode,
   operation: OperationTypeNode = 'query',