fix(fetch-http-handler): move keepalive check to constructor (one-time) (#1310)

Author: kuhe

.changeset/eighty-impalas-watch.md

@@ -0,0 +1,5 @@
+---
+"@smithy/fetch-http-handler": patch
+---
+
+move keepAliveSupport check to FetchHttpHandler constructor

packages/fetch-http-handler/src/fetch-http-handler.ts

@@ -16,7 +16,7 @@ type FetchHttpHandlerConfig = FetchHttpHandlerOptions;
  * Detection of keepalive support. Can be overridden for testing.
  */
 export const keepAliveSupport = {
-  supported: Boolean(typeof Request !== "undefined" && "keepalive" in new Request("https://[::1]")),
+  supported: undefined as undefined | boolean,
 };
 
 /**
@@ -60,6 +60,11 @@ export class FetchHttpHandler implements HttpHandler<FetchHttpHandlerConfig> {
       this.config = options ?? {};
       this.configProvider = Promise.resolve(this.config);
     }
+    if (keepAliveSupport.supported === undefined) {
+      keepAliveSupport.supported = Boolean(
+        typeof Request !== "undefined" && "keepalive" in new Request("https://[::1]")
+      );
+    }
   }
 
   destroy(): void {