fix: implement ALS runner fallback for cloudflare workers support

Author: logaretm

packages/nextjs/src/server/prepareSafeIdGeneratorContext.ts

@@ -1,5 +1,5 @@
 import {
-  type _INTERNAL_RandomSafeContextRunner as _INTERNAL_RandomSafeContextRunner,
+  type _INTERNAL_RandomSafeContextRunner as RandomSafeContextRunner,
   debug,
   GLOBAL_OBJ,
 } from '@sentry/core';
@@ -15,17 +15,53 @@ type OriginalAsyncLocalStorage = typeof AsyncLocalStorage;
  */
 export function prepareSafeIdGeneratorContext(): void {
   const sym = Symbol.for('__SENTRY_SAFE_RANDOM_ID_WRAPPER__');
-  const globalWithSymbol: typeof GLOBAL_OBJ & { [sym]?: _INTERNAL_RandomSafeContextRunner } = GLOBAL_OBJ;
-  const als = getAsyncLocalStorage();
-  if (!als || typeof als.snapshot !== 'function') {
-    DEBUG_BUILD &&
-      debug.warn(
-        '[@sentry/nextjs] No AsyncLocalStorage found in the runtime or AsyncLocalStorage.snapshot() is not available, skipping safe random ID generator context preparation, you may see some errors with cache components.',
-      );
+  const globalWithSymbol: typeof GLOBAL_OBJ & { [sym]?: RandomSafeContextRunner } = GLOBAL_OBJ;
+
+  // Get initial snapshot - if unavailable, don't set up the wrapper at all
+  const initialSnapshot = getAsyncLocalStorageSnapshot();
+  if (!initialSnapshot) {
     return;
   }
 
-  globalWithSymbol[sym] = als.snapshot();
+  // We store a wrapper function instead of the raw snapshot because in serverless
+  // environments (e.g., Cloudflare Workers), the snapshot is bound to the request
+  // context it was created in. Once that request ends, the snapshot becomes invalid.
+  // The wrapper catches this and creates a fresh snapshot for the current request context.
+  let cachedSnapshot: RandomSafeContextRunner = initialSnapshot;
+
+  globalWithSymbol[sym] = <T>(callback: () => T): T => {
+    try {
+      return cachedSnapshot(callback);
+    } catch (error) {
+      // Only handle AsyncLocalStorage-related errors, rethrow others
+      if (!isAsyncLocalStorageError(error)) {
+        throw error;
+      }
+
+      // Snapshot likely stale, try to get a fresh one and retry
+      const freshSnapshot = getAsyncLocalStorageSnapshot();
+      // No snapshot available, fall back to direct execution
+      if (!freshSnapshot) {
+        return callback();
+      }
+
+      // Update the cached snapshot
+      cachedSnapshot = freshSnapshot;
+
+      // Retry the callback with the fresh snapshot
+      try {
+        return cachedSnapshot(callback);
+      } catch (retryError) {
+        // Only fall back for AsyncLocalStorage errors, rethrow others
+        if (!isAsyncLocalStorageError(retryError)) {
+          throw retryError;
+        }
+        // If fresh snapshot also fails with ALS error, fall back to direct execution
+        return callback();
+      }
+    }
+  };
+
   DEBUG_BUILD && debug.log('[@sentry/nextjs] Prepared safe random ID generator context');
 }
 
@@ -47,3 +83,21 @@ function getAsyncLocalStorage(): OriginalAsyncLocalStorage | undefined {
 
   return undefined;
 }
+
+function getAsyncLocalStorageSnapshot(): RandomSafeContextRunner | undefined {
+  const als = getAsyncLocalStorage();
+
+  if (!als || typeof als.snapshot !== 'function') {
+    DEBUG_BUILD &&
+      debug.warn(
+        '[@sentry/nextjs] No AsyncLocalStorage found in the runtime or AsyncLocalStorage.snapshot() is not available, skipping safe random ID generator context preparation, you may see some errors with cache components.',
+      );
+    return undefined;
+  }
+
+  return als.snapshot();
+}
+
+function isAsyncLocalStorageError(error: unknown): boolean {
+  return error instanceof Error && error.message.includes('AsyncLocalStorage');
+}