Allow resetting stringifyCanon used by canonicalStringify.

This reset will happen whenever cache.gc() is called, which makes sense
because it frees all memory associated with the replaced stringifyCanon,
at the cost of temporarily slowing down canonicalStringify (but without
any logical changes in the behavior/output of canonicalStringify).

src/cache/inmemory/inMemoryCache.ts

@@ -28,6 +28,7 @@ import {
   TypePolicies,
 } from './policies';
 import { hasOwn } from './helpers';
+import { resetStringifyCanon } from './object-canon';
 
 export interface InMemoryCacheConfig extends ApolloReducerConfig {
   resultCaching?: boolean;
@@ -262,6 +263,7 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
 
   // Request garbage collection of unreachable normalized entities.
   public gc() {
+    resetStringifyCanon();
     return this.optimisticData.gc();
   }
 
@@ -322,6 +324,7 @@ export class InMemoryCache extends ApolloCache<NormalizedCacheObject> {
   public reset(): Promise<void> {
     this.init();
     this.broadcastWatches();
+    resetStringifyCanon();
     return Promise.resolve();
   }
 

src/cache/inmemory/object-canon.ts

@@ -194,7 +194,10 @@ type SortedKeysInfo = {
   json: string;
 };
 
-const stringifyCanon = new ObjectCanon;
+let stringifyCanon = new ObjectCanon;
+export function resetStringifyCanon() {
+  stringifyCanon = new ObjectCanon;
+}
 const stringifyCache = new WeakMap<object, string>();
 
 // Since the keys of canonical objects are always created in lexicographically