Add additional traps to magic proxy stubs (#710)

This change adds `getOwnPropertyDescriptor`, `ownKeys` and
`getPrototypeOf` traps to magic proxy stubs. The first two allow
proxy stubs to be `JSON.stringify`ed. The last ensures proxies
aren't detected as plain objects.

Author: mrbbot

packages/miniflare/src/plugins/core/proxy/client.ts

@@ -185,7 +185,12 @@ class ProxyClientBridge {
 class ProxyStubHandler<T extends object> implements ProxyHandler<T> {
   readonly #version: number;
   readonly #stringifiedTarget: string;
-  readonly #known = new Map<string, unknown>();
+  readonly #knownValues = new Map<string, unknown>();
+  readonly #knownDescriptors = new Map<
+    string,
+    PropertyDescriptor | undefined
+  >();
+  #knownOwnKeys?: string[];
 
   revivers: ReducersRevivers = {
     ...revivers,
@@ -327,7 +332,7 @@ class ProxyStubHandler<T extends object> implements ProxyHandler<T> {
     if (typeof key === "symbol" || key === "then") return undefined;
 
     // See optimisation comments below for cases where this will be set
-    const maybeKnown = this.#known.get(key);
+    const maybeKnown = this.#knownValues.get(key);
     if (maybeKnown !== undefined) return maybeKnown;
 
     // Always perform a synchronous GET, if this returns a `Promise`, we'll
@@ -361,7 +366,7 @@ class ProxyStubHandler<T extends object> implements ProxyHandler<T> {
       // (e.g. accessing `R2ObjectBody#body` multiple times)
       result instanceof ReadableStream
     ) {
-      this.#known.set(key, result);
+      this.#knownValues.set(key, result);
     }
     return result;
   }
@@ -371,6 +376,54 @@ class ProxyStubHandler<T extends object> implements ProxyHandler<T> {
     return this.get(target, key, undefined) !== undefined;
   }
 
+  getOwnPropertyDescriptor(target: T, key: string | symbol) {
+    if (typeof key === "symbol") return undefined;
+
+    // Optimisation: assume constant prototypes of proxied objects, descriptors
+    // should never change after we've fetched them
+    const maybeKnown = this.#knownDescriptors.get(key);
+    if (maybeKnown !== undefined) return maybeKnown;
+
+    const syncRes = this.bridge.sync.fetch(this.bridge.url, {
+      method: "POST",
+      headers: {
+        [CoreHeaders.OP]: ProxyOps.GET_OWN_DESCRIPTOR,
+        [CoreHeaders.OP_KEY]: key,
+        [CoreHeaders.OP_TARGET]: this.#stringifiedTarget,
+      },
+    });
+    const result = this.#parseSyncResponse(
+      syncRes,
+      this.getOwnPropertyDescriptor
+    ) as PropertyDescriptor | undefined;
+
+    this.#knownDescriptors.set(key, result);
+    return result;
+  }
+
+  ownKeys(_target: T) {
+    // Optimisation: assume constant prototypes of proxied objects, own keys
+    // should never change after we've fetched them
+    if (this.#knownOwnKeys !== undefined) return this.#knownOwnKeys;
+
+    const syncRes = this.bridge.sync.fetch(this.bridge.url, {
+      method: "POST",
+      headers: {
+        [CoreHeaders.OP]: ProxyOps.GET_OWN_KEYS,
+        [CoreHeaders.OP_TARGET]: this.#stringifiedTarget,
+      },
+    });
+    const result = this.#parseSyncResponse(syncRes, this.ownKeys) as string[];
+
+    this.#knownOwnKeys = result;
+    return result;
+  }
+
+  getPrototypeOf(_target: T) {
+    // Return a `null` prototype, so users know this isn't a plain object
+    return null;
+  }
+
   #createFunction(key: string) {
     // Optimisation: if the function returns a `Promise`, we know it must be
     // async (assuming all async functions always return `Promise`s). When

packages/miniflare/src/workers/core/constants.ts

@@ -30,6 +30,10 @@ export const CoreBindings = {
 export const ProxyOps = {
   // Get the target or a property of the target
   GET: "GET",
+  // Get the descriptor for a property of the target
+  GET_OWN_DESCRIPTOR: "GET_OWN_DESCRIPTOR",
+  // Get the target's own property names
+  GET_OWN_KEYS: "GET_OWN_KEYS",
   // Call a method on the target
   CALL: "CALL",
   // Remove the strong reference to the target on the "heap", allowing it to be

packages/miniflare/src/workers/core/proxy.worker.ts

@@ -156,7 +156,7 @@ export class ProxyServer implements DurableObject {
     const targetName = target.constructor.name;
 
     let status = 200;
-    let result;
+    let result: unknown;
     let unbufferedRest: ReadableStream | undefined;
     if (opHeader === ProxyOps.GET) {
       // If no key header is specified, just return the target
@@ -168,6 +168,18 @@ export class ProxyServer implements DurableObject {
           headers: { [CoreHeaders.OP_RESULT_TYPE]: "Function" },
         });
       }
+    } else if (opHeader === ProxyOps.GET_OWN_DESCRIPTOR) {
+      if (keyHeader === null) return new Response(null, { status: 400 });
+      const descriptor = Object.getOwnPropertyDescriptor(target, keyHeader);
+      if (descriptor !== undefined) {
+        result = <PropertyDescriptor>{
+          configurable: descriptor.configurable,
+          enumerable: descriptor.enumerable,
+          writable: descriptor.writable,
+        };
+      }
+    } else if (opHeader === ProxyOps.GET_OWN_KEYS) {
+      result = Object.getOwnPropertyNames(target);
     } else if (opHeader === ProxyOps.CALL) {
       // We don't allow callable targets yet (could be useful to implement if
       // we ever need to proxy functions that return functions)

packages/miniflare/test/plugins/core/proxy/client.spec.ts

@@ -215,3 +215,26 @@ test("ProxyClient: returns empty ReadableStream synchronously", async (t) => {
   assert(objectBody != null);
   t.is(await text(objectBody.body), ""); // Synchronous empty stream access
 });
+test("ProxyClient: can `JSON.stringify()` proxies", async (t) => {
+  const mf = new Miniflare({ script: nullScript, r2Buckets: ["BUCKET"] });
+  t.teardown(() => mf.dispose());
+
+  const bucket = await mf.getR2Bucket("BUCKET");
+  const object = await bucket.put("key", "value");
+  assert(object !== null);
+  t.is(Object.getPrototypeOf(object), null);
+  const plainObject = JSON.parse(JSON.stringify(object));
+  t.deepEqual(plainObject, {
+    checksums: {
+      md5: "2063c1608d6e0baf80249c42e2be5804",
+    },
+    customMetadata: {},
+    etag: "2063c1608d6e0baf80249c42e2be5804",
+    httpEtag: '"2063c1608d6e0baf80249c42e2be5804"',
+    httpMetadata: {},
+    key: "key",
+    size: 5,
+    uploaded: object.uploaded.toISOString(),
+    version: object.version,
+  });
+});