[Miniflare 3] Re-enable concurrent dispatchFetch()s and batch proxy…

… heap frees (#713)

* Re-enable concurrent `dispatchFetch()`s and batch proxy heap frees

In order to fix tests when adding the magic proxy, we restricted
`dispatchFetch()` to one concurrent TCP connection. Unfortunately,
this prevented long-lived `dispatchFetch()` requests.

When proxies are garbage collected, we send a network request to
`workerd` to free the corresponding entry in the `ProxyServer` heap.
Garbage collection often happens in phases though, freeing lots of
objects at once. This caused many concurrent requests to `workerd`
(~60 in some tests), leading to many TCP connections being created.

This change re-enables using multiple TCP connections, and attempts
to address the root cause of the issues we were seeing, by batching
frees before sending the network request.

* Re-enable parallel tests

package.json

@@ -27,7 +27,7 @@
     "lint:fix": "npm run lint -- --fix",
     "prepublishOnly": "npm run lint && npm run clean && npm run build && npm run types:bundle && npm run test",
     "release": "./scripts/release.sh",
-    "test": "npm run build && ava --serial && rimraf ./.tmp",
+    "test": "npm run build && ava && rimraf ./.tmp",
     "types:build": "tsc && tsc -p packages/miniflare/src/workers/tsconfig.json",
     "types:bundle": "npm run types:build && node scripts/types.mjs"
   },

packages/miniflare/src/index.ts

@@ -22,7 +22,7 @@ import type {
 import exitHook from "exit-hook";
 import { $ as colors$ } from "kleur/colors";
 import stoppable from "stoppable";
-import { Client } from "undici";
+import { Dispatcher, Pool } from "undici";
 import SCRIPT_MINIFLARE_SHARED from "worker:shared/index";
 import SCRIPT_MINIFLARE_ZOD from "worker:shared/zod";
 import { WebSocketServer } from "ws";
@@ -557,7 +557,7 @@ export class Miniflare {
   readonly #removeRuntimeExitHook?: () => void;
   #runtimeEntryURL?: URL;
   #socketPorts?: SocketPorts;
-  #runtimeClient?: Client;
+  #runtimeDispatcher?: Dispatcher;
   #proxyClient?: ProxyClient;
 
   // Path to temporary directory for use as scratch space/"in-memory" Durable
@@ -1137,10 +1137,10 @@ export class Miniflare {
       `${secure ? "https" : "http"}://${accessibleHost}:${entryPort}`
     );
     if (previousEntryURL?.toString() !== this.#runtimeEntryURL.toString()) {
-      this.#runtimeClient = new Client(this.#runtimeEntryURL, {
+      this.#runtimeDispatcher = new Pool(this.#runtimeEntryURL, {
         connect: { rejectUnauthorized: false },
       });
-      registerAllowUnauthorizedDispatcher(this.#runtimeClient);
+      registerAllowUnauthorizedDispatcher(this.#runtimeDispatcher);
     }
     if (this.#proxyClient === undefined) {
       this.#proxyClient = new ProxyClient(
@@ -1291,7 +1291,7 @@ export class Miniflare {
     await this.ready;
 
     assert(this.#runtimeEntryURL !== undefined);
-    assert(this.#runtimeClient !== undefined);
+    assert(this.#runtimeDispatcher !== undefined);
 
     const forward = new Request(input, init);
     const url = new URL(forward.url);
@@ -1313,7 +1313,7 @@ export class Miniflare {
     }
 
     const forwardInit = forward as RequestInit;
-    forwardInit.dispatcher = this.#runtimeClient;
+    forwardInit.dispatcher = this.#runtimeDispatcher;
     const response = await fetch(url, forwardInit);
 
     // If the Worker threw an uncaught exception, propagate it to the caller

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

@@ -119,6 +119,14 @@ class ProxyClientBridge {
   // as the references will be invalid, and a new object with the same address
   // may be added to the "heap".
   readonly #finalizationRegistry: FinalizationRegistry<NativeTargetHeldValue>;
+  // Garbage collection passes will free lots of objects at once. Rather than
+  // sending a `DELETE` request for each address, we batch finalisations within
+  // 100ms of each other into one request. This ensures we don't create *lots*
+  // of TCP connections to `workerd` in `dispatchFetch()` for all the concurrent
+  // requests.
+  readonly #finalizeBatch: NativeTargetHeldValue[] = [];
+  #finalizeBatchTimeout?: NodeJS.Timeout;
+
   readonly sync = new SynchronousFetcher();
 
   constructor(public url: URL, readonly dispatchFetch: DispatchFetch) {
@@ -129,26 +137,37 @@ class ProxyClientBridge {
     return this.#version;
   }
 
-  #finalizeProxy = async (held: NativeTargetHeldValue) => {
-    // Sanity check: make sure the proxy hasn't been poisoned. We should
-    // unregister all proxies from the finalisation registry when poisoning,
-    // but it doesn't hurt to be careful.
-    if (held.version !== this.#version) return;
-
+  #finalizeProxy = (held: NativeTargetHeldValue) => {
     // Called when the `Proxy` with address `targetAddress` gets garbage
     // collected. This removes the target from the `ProxyServer` "heap".
+    this.#finalizeBatch.push(held);
+    clearTimeout(this.#finalizeBatchTimeout);
+    this.#finalizeBatchTimeout = setTimeout(this.#finalizeProxyBatch, 100);
+  };
+
+  #finalizeProxyBatch = async () => {
+    const addresses: number[] = [];
+    for (const held of this.#finalizeBatch.splice(0)) {
+      // Sanity check: make sure the proxy hasn't been poisoned. We should
+      // unregister all proxies from the finalisation registry when poisoning,
+      // but it doesn't hurt to be careful.
+      if (held.version === this.#version) addresses.push(held.address);
+    }
+    // If there are no addresses to free, we don't need to send a request
+    if (addresses.length === 0) return;
     try {
       await this.dispatchFetch(this.url, {
         method: "DELETE",
         headers: {
           [CoreHeaders.OP]: ProxyOps.FREE,
-          [CoreHeaders.OP_TARGET]: held.address.toString(),
+          [CoreHeaders.OP_TARGET]: addresses.join(","),
         },
       });
     } catch {
       // Ignore network errors when freeing. If this `dispatchFetch()` throws,
       // it's likely the runtime has shutdown, so the entire "heap" has been
-      // destroyed anyway.
+      // destroyed anyway. There's a small chance of a memory leak here if this
+      // threw for another reason.
     }
   };
 

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

@@ -140,11 +140,13 @@ export class ProxyServer implements DurableObject {
     // Get target to perform operations on
     if (targetHeader === null) return new Response(null, { status: 400 });
 
-    // If this is a FREE operation, remove the target from the heap
+    // If this is a FREE operation, remove the target(s) from the heap
     if (opHeader === ProxyOps.FREE) {
-      const targetAddress = parseInt(targetHeader);
-      assert(!Number.isNaN(targetAddress));
-      this.heap.delete(targetAddress);
+      for (const targetValue of targetHeader.split(",")) {
+        const targetAddress = parseInt(targetValue);
+        assert(!Number.isNaN(targetAddress));
+        this.heap.delete(targetAddress);
+      }
       return new Response(null, { status: 204 });
     }