Use a shared Fetcher instance for ServiceWorkerGlobalScope requests

We were creating a new Fetcher instance for every request
despite the fact that the Fetcher itself was stateless and
not storing any request-specific data. This changes it so
that all requests to a given ServiceWorkerGlobalScope instance
share a single default Fetcher instance.

Author: jasnell

src/workerd/api/global-scope.c++

@@ -218,8 +218,18 @@ kj::Promise<DeferredProxy<void>> ServiceWorkerGlobalScope::request(kj::HttpMetho
     }
   }
 
+  if (defaultFetcher == kj::none) {
+    // The default fetcher can be shared across requests since it does not persist any
+    // request-specific state. We can create one lazily here for the first request
+    // handled by this global scope and avoid the extra allocations on subsequence requests.
+    // We could also consider creating it lazily when first accessed, but Fetcher is very
+    // lightweight so this seems sufficient for now.
+    defaultFetcher =
+        js.alloc<Fetcher>(IoContext::NEXT_CLIENT_CHANNEL, Fetcher::RequiresHostAndProtocol::YES);
+  }
+
   auto jsRequest = js.alloc<Request>(js, method, url, Request::Redirect::MANUAL, kj::mv(jsHeaders),
-      js.alloc<Fetcher>(IoContext::NEXT_CLIENT_CHANNEL, Fetcher::RequiresHostAndProtocol::YES),
+      KJ_ASSERT_NONNULL(defaultFetcher).addRef(),
       /* signal */ kj::mv(abortSignal), kj::mv(cf), kj::mv(body),
       /* thisSignal */ kj::none, Request::CacheMode::NONE);
 

src/workerd/api/global-scope.h

@@ -930,6 +930,7 @@ class ServiceWorkerGlobalScope: public WorkerGlobalScope {
   jsg::UnhandledRejectionHandler unhandledRejections;
   kj::Maybe<jsg::JsRef<jsg::JsValue>> processValue;
   kj::Maybe<jsg::JsRef<jsg::JsValue>> bufferValue;
+  kj::Maybe<jsg::Ref<Fetcher>> defaultFetcher;
 
   // Global properties such as scheduler, crypto, caches, self, and origin should
   // be monkeypatchable / mutable at the global scope.