perf: Reduce unnecessary validation on responses (#3350)

Previously we were validating that responses were JSON, but we already
do that elsewhere and are guaranteed for the responses to be valid by
the time we get to `isValidResponse`. Additionally, we don't need an
entirely byte accurate length for the response, if the Snap goes beyond
the limit they simply won't receive a response from the Snap.

Author: FrederikBolding

packages/snaps-execution-environments/coverage.json

@@ -1,6 +1,6 @@
 {
   "branches": 90.78,
   "functions": 94.96,
-  "lines": 90.85,
-  "statements": 90.27
+  "lines": 90.84,
+  "statements": 90.26
 }

packages/snaps-execution-environments/src/common/BaseSnapExecutor.ts

@@ -346,7 +346,7 @@ export class BaseSnapExecutor {
     });
   }
 
-  async #respond(id: JsonRpcId, response: Record<string, unknown>) {
+  async #respond(id: JsonRpcId, response: Record<string, Json>) {
     if (!isValidResponse(response)) {
       // Instead of throwing, we directly respond with an error.
       // This prevents an issue where we wouldn't respond when errors were non-serializable

packages/snaps-execution-environments/src/common/utils.test.ts

@@ -63,10 +63,6 @@ describe('isValidResponse', () => {
     expect(isValidResponse('foo')).toBe(false);
   });
 
-  it('returns false if the value is not JSON serializable', () => {
-    expect(isValidResponse({ foo: BigInt(0) })).toBe(false);
-  });
-
   it('returns false if the value is too large', () => {
     expect(isValidResponse({ foo: '1'.repeat(100_000_000) })).toBe(false);
   });

packages/snaps-execution-environments/src/common/utils.ts

@@ -1,6 +1,8 @@
 import type { RequestArguments } from '@metamask/providers';
 import { rpcErrors } from '@metamask/rpc-errors';
-import { assert, getJsonSize, getSafeJson, isObject } from '@metamask/utils';
+import { getJsonSizeUnsafe } from '@metamask/snaps-utils';
+import type { Json } from '@metamask/utils';
+import { assert, getSafeJson, isObject } from '@metamask/utils';
 
 import { log } from '../logging';
 
@@ -128,16 +130,13 @@ export function sanitizeRequestArguments(value: unknown): RequestArguments {
  * @param response - The response.
  * @returns True if the response is valid, otherwise false.
  */
-export function isValidResponse(response: Record<string, unknown>) {
+export function isValidResponse(response: Record<string, Json>) {
   if (!isObject(response)) {
     return false;
   }
 
-  try {
-    // If the JSON is invalid this will throw and we should return false.
-    const size = getJsonSize(response);
-    return size < MAX_RESPONSE_JSON_SIZE;
-  } catch {
-    return false;
-  }
+  // We know that the response is valid JSON already and we don't
+  // need the size to be exact.
+  const size = getJsonSizeUnsafe(response);
+  return size < MAX_RESPONSE_JSON_SIZE;
 }

packages/snaps-utils/src/index.executionenv.ts

@@ -3,5 +3,6 @@ export * from './errors';
 export * from './handlers/exports';
 export * from './handlers/types';
 export * from './iframe';
+export * from './json';
 export * from './logging';
 export * from './types';