Re: [w3c/ServiceWorker] Skip executing fetch handlers if it is no-op. (PR #1672)

@annevk commented on this pull request.



> +    <h3 id="empty-fetch-handler-identification-algorithm"><dfn>Empty Fetch Handler Identification</dfn></h3>
+
+      : Input
+      :: |workerGlobalScope|, a [=service worker/global object=].
+      : Output
+      :: a boolean
+
+      1. If |workerGlobalScope|'s [=set of event types to handle=] does not [=set/contain=] <code>fetch</code>, then returns false.
+      1. Let |eventListenerList| be an empty [=list=].
+      1. [=list/For each=] |eventListener| of |workerGlobalScope|'s [=set of event types to handle=]:
+          1. If |eventListener|'s <a spec="dom">type</a> is <code>fetch</code>, then [=list/append=] |eventListener| to |eventListenerList|.
+      1. [=list/For each=] |eventListener| of |eventListenerList|:
+          1. If |eventListener|'s [=callback=] is not null:
+              1. Set |callback| to the result of [=converting|convert to an ECMAScript value=] |eventListener|'s [=callback=] to an ECMAScript value.
+              1. If [$IsCallable$](|callback|) is false:
+                  1. Let |getResult| be [=Completion=]([$Get$](|callback|), <code>handleEvent</code>).
+                  1. If |getResult| is [=abrupt completion=], then return false.
+                  1. Set |callback| to |getResult|.
+              1. If [$IsCallable$](|callback|) is false, then return false.
+              1. If |callback|'s [=function body=] is not empty (i.e. either a [=statement=] or [=declaration=] exist), then return false.
+
+              Note: it detects something like `onfetch = () => {}`. Some sites have a fetch event listener with empty body to make them recognized as progressive web application (PWA).

@domenic I think this still doesn't work for `onfetch` as HTML wraps that function in its own function when creating the callback, right? See https://github.com/WebKit/standards-positions/issues/129#issuecomment-1431008116.

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