common: prevent mergeSelectionsets from mutating its input

packages/indexer-common/package.json

@@ -25,6 +25,7 @@
     "@graphprotocol/common-ts": "2.0.3",
     "@graphprotocol/cost-model": "0.1.16",
     "@thi.ng/heaps": "1.2.38",
+    "@types/lodash.clonedeep": "^4.5.7",
     "@types/lodash.intersection": "^4.4.7",
     "@types/lodash.xor": "^4.5.7",
     "@urql/core": "2.4.4",
@@ -39,6 +40,7 @@
     "graphql": "16.3.0",
     "graphql-tag": "2.12.6",
     "jayson": "3.6.6",
+    "lodash.clonedeep": "^4.5.0",
     "lodash.groupby": "^4.6.0",
     "lodash.intersection": "^4.4.0",
     "lodash.isequal": "^4.5.0",

packages/indexer-common/src/__tests__/subgraph.test.ts

@@ -1,4 +1,4 @@
-import { DocumentNode } from 'graphql'
+import { DocumentNode, print } from 'graphql'
 import {
   SubgraphFreshnessChecker,
   LoggerInterface,
@@ -7,6 +7,8 @@ import {
 } from '../subgraphs'
 import { QueryResult } from '../network-subgraph'
 import gql from 'graphql-tag'
+import { mergeSelectionSets } from '../utils'
+import { merge } from 'evt/lib/Evt.merge'
 
 /* eslint-disable @typescript-eslint/no-explicit-any */
 export const mockProvider: ProviderInterface & any = {
@@ -48,6 +50,39 @@ function mockQueryResult(blockNumber: number): QueryResult<any> & {
   }
 }
 /* eslint-enable @typescript-eslint/no-explicit-any */
+const blockNumberQuery = gql`
+  {
+    _meta {
+      block {
+        number
+      }
+    }
+  }
+`
+
+const expectedMergedQuery = gql`
+  query TestQuery {
+    foo {
+      id
+    }
+    _meta {
+      block {
+        number
+      }
+    }
+  }
+`
+
+describe('mergeSelectionSets function', () => {
+  it('can append the block number to an existing query', () => {
+    let result: DocumentNode
+    // Repetition required to test `mergeSelectionSets` doesn't mutate its inputq
+    for (let i = 0; i < 3; i++) {
+      result = mergeSelectionSets(testSubgraphQuery, blockNumberQuery)
+    }
+    expect(print(result!)).toEqual(print(expectedMergedQuery))
+  })
+})
 
 describe('SubgraphFreshnessChecker', () => {
   beforeEach(jest.resetAllMocks)

packages/indexer-common/src/utils.ts

@@ -7,6 +7,7 @@ import {
 import { Logger, Metrics, timer } from '@graphprotocol/common-ts'
 import { indexerError, IndexerErrorCode } from './errors'
 import { DocumentNode, SelectionSetNode, Kind } from 'graphql'
+import cloneDeep from 'lodash.clonedeep'
 
 export const parseBoolean = (
   val: string | boolean | number | undefined | null,
@@ -68,13 +69,15 @@ export function mergeSelectionSets(
   first: DocumentNode,
   second: DocumentNode,
 ): DocumentNode {
-  const firstSelectionSet = extractSelectionSet(first)
+  // Work on a copy to avoid mutating inupt
+  const copy = cloneDeep(first)
+  const firstSelectionSet = extractSelectionSet(copy)
   const secondSelectionSet = extractSelectionSet(second)
   firstSelectionSet.selections = [
     ...firstSelectionSet.selections,
     ...secondSelectionSet.selections,
   ]
-  return first
+  return copy
 }
 
 function extractSelectionSet(document: DocumentNode): SelectionSetNode {