check for an implementation of AsyncLocalStorage (#866)

Add a runtime check for `wrangler dev` local mode to avoid erroring in environments with no `AsyncLocalStorage` class

Certain runtime APIs are only available to workers during the "request context",
which is any code that returns after receiving a request and before returning
a response.

Miniflare emulates this behavior by using an [`AsyncLocalStorage`](https://nodejs.org/api/async_context.html#class-asynclocalstorage) and
[checking at runtime](https://github.com/cloudflare/miniflare/blob/master/packages/shared/src/context.ts#L21-L36)
to see if you're using those APIs during the request context.

In certain environments `AsyncLocalStorage` is unavailable, such as in a
[webcontainer](https://github.com/stackblitz/webcontainer-core).
This function figures out if we're able to run those "request context" checks
and returns [a set of options](https://miniflare.dev/core/standards#global-functionality-limits)
that indicate to miniflare whether to run the checks or not.

.changeset/many-numbers-cheer.md

@@ -0,0 +1,19 @@
+---
+"wrangler": patch
+---
+
+Add a runtime check for `wrangler dev` local mode to avoid erroring in environments with no `AsyncLocalStorage` class
+
+Certain runtime APIs are only available to workers during the "request context",
+which is any code that returns after receiving a request and before returning
+a response.
+
+Miniflare emulates this behavior by using an [`AsyncLocalStorage`](https://nodejs.org/api/async_context.html#class-asynclocalstorage) and
+[checking at runtime](https://github.com/cloudflare/miniflare/blob/master/packages/shared/src/context.ts#L21-L36)
+to see if you're using those APIs during the request context.
+
+In certain environments `AsyncLocalStorage` is unavailable, such as in a
+[webcontainer](https://github.com/stackblitz/webcontainer-core).
+This function figures out if we're able to run those "request context" checks
+and returns [a set of options](https://miniflare.dev/core/standards#global-functionality-limits)
+that indicate to miniflare whether to run the checks or not.

packages/wrangler/src/miniflare-cli/index.ts

@@ -2,6 +2,7 @@ import { Log, LogLevel, Miniflare } from "miniflare";
 import yargs from "yargs";
 import { hideBin } from "yargs/helpers";
 import { enumKeys } from "./enum-keys";
+import { getRequestContextCheckOptions } from "./request-context";
 
 async function main() {
   const args = await yargs(hideBin(process.argv))
@@ -12,8 +13,10 @@ async function main() {
     }).argv;
 
   const logLevel = LogLevel[args.log ?? "INFO"];
+  const requestContextCheckOptions = await getRequestContextCheckOptions();
   const config = {
     ...JSON.parse((args._[0] as string) ?? "{}"),
+    ...requestContextCheckOptions,
     log: new Log(logLevel),
   };
 

packages/wrangler/src/miniflare-cli/request-context.ts

@@ -0,0 +1,40 @@
+import type { MiniflareOptions } from "miniflare";
+
+/**
+ * Certain runtime APIs are only available to workers during the "request context",
+ * which is any code that returns after receiving a request and before returning
+ * a response.
+ *
+ * Miniflare emulates this behavior by using an [`AsyncLocalStorage`](https://nodejs.org/api/async_context.html#class-asynclocalstorage) and
+ * [checking at runtime](https://github.com/cloudflare/miniflare/blob/master/packages/shared/src/context.ts#L21-L36)
+ * to see if you're using those APIs during the request context.
+ *
+ * In certain environments `AsyncLocalStorage` is unavailable, such as in a
+ * [webcontainer](https://github.com/stackblitz/webcontainer-core).
+ * This function figures out if we're able to run those "request context" checks
+ * and returns [a set of options](https://miniflare.dev/core/standards#global-functionality-limits)
+ * that indicate to miniflare whether to run the checks or not.
+ */
+export const getRequestContextCheckOptions = async (): Promise<
+  Pick<MiniflareOptions, "globalAsyncIO" | "globalTimers" | "globalRandom">
+> => {
+  // check that there's an implementation of AsyncLocalStorage
+  let hasAsyncLocalStorage = true;
+  try {
+    // ripped from  the example here https://nodejs.org/api/async_context.html#class-asynclocalstorage
+    const { AsyncLocalStorage } = await import("node:async_hooks");
+    const storage = new AsyncLocalStorage<string>();
+
+    hasAsyncLocalStorage = storage.run("some-value", () => {
+      return storage.getStore() === "some-value";
+    });
+  } catch (e) {
+    hasAsyncLocalStorage = false;
+  }
+
+  return {
+    globalAsyncIO: hasAsyncLocalStorage,
+    globalRandom: hasAsyncLocalStorage,
+    globalTimers: hasAsyncLocalStorage,
+  };
+};

packages/wrangler/src/pages.tsx

@@ -25,6 +25,7 @@ import { fetchResult } from "./cfetch";
 import { readConfig } from "./config";
 import { FatalError } from "./errors";
 import { logger } from "./logger";
+import { getRequestContextCheckOptions } from "./miniflare-cli/request-context";
 import openInBrowser from "./open-in-browser";
 import { toUrlPath } from "./paths";
 import { requireAuth } from "./user";
@@ -1287,6 +1288,9 @@ export const pages: BuilderCallback<unknown, unknown> = (yargs) => {
             ? await generateAssetsFetch(directory)
             : invalidAssetsFetch;
 
+        const requestContextCheckOptions =
+          await getRequestContextCheckOptions();
+
         const miniflare = new Miniflare({
           port,
           watch: true,
@@ -1351,6 +1355,7 @@ export const pages: BuilderCallback<unknown, unknown> = (yargs) => {
           cachePersist: true,
           liveReload,
 
+          ...requestContextCheckOptions,
           ...miniflareArgs,
         });