Re: [w3c/ServiceWorker] Handle asynchronous modules (top-level await). (#1444)

littledan commented on this pull request.

This patch has the semantics I was imagining. It's probably worth including a note about the unfortunate compatibility situation leading to the synchronous check of the promise (which is generally a design anti-pattern).

Or, to the folks here more familiar with ServiceWorker usage: would it be OK to skip the synchronous error check, and just rely on the error being reported a bit later, and letting the rest of the initialization algorithm complete as if the module loaded successfully?

> @@ -2801,8 +2803,21 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
           1. Set |workerGlobalScope|'s [=ServiceWorkerGlobalScope/force bypass cache for import scripts flag=] if |forceBypassCache| is true.
           1. Create a new {{WorkerLocation}} object and associate it with |workerGlobalScope|.
           1. If |serviceWorker| is an <a>active worker</a>, and there are any <a>tasks</a> queued in |serviceWorker|'s <a>containing service worker registration</a>'s [=service worker registration/task queues=], <a lt="queue a task">queue</a> them to |serviceWorker|'s <a>event loop</a>'s [=/task queues=] in the same order using their original <a>task sources</a>.
-          1. Let |evaluationStatus| be the result of <a lt="run a classic script">running the classic script</a> |script| if |script| is a <a>classic script</a>, otherwise, the result of <a lt="run a module script">running the module script</a> |script| if |script| is a [=module script=].
-          1. If |evaluationStatus|.\[[Value]] is empty, this means the script was not evaluated. Set |startFailed| to true and abort these steps.
+          1. Let |evaluationStatus| be null.
+          1. If |script| is a <a>classic script</a>, then:
+              1. Set |evaluationStatus| to the result of <a lt="run a classic script">running the classic script</a> |script|.
+              1. If |evaluationStatus|.\[[Value]] is empty, this means the script was not evaluated. Set |startFailed| to true and abort these steps.
+          1. Otherwise, if |script| is a [=module script=], then:
+              1. Let |evaluationPromise| be the result of <a lt="run a module script">running the module script</a> |script|, with report errors set to false.
+              1. If |evaluationPromise|.\[[PromiseState]] is "fulfilled":
+                  1. Set |evaluationStatus| to [$NormalCompletion$](|evaluationPromise|.\[[PromiseResult]]).

I'm not sure why we need this synchronous check for "fulfilled"; isn't this path handled just as well by line 2816?

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