Re: [w3c/ServiceWorker] Improve service worker script caching and update (#1283)

jungkees commented on this pull request.



> +            * |registration|'s [=service worker registration/update via cache mode=] is "`none`".
+            * The [=current global object=]'s [=force bypass cache for importscripts flag=] is set.
+            * |registration|'s [=last update check time=] is not null and the time difference in seconds calculated by the current time minus |registration|’s [=last update check time=] is greater than 86400.
+        1. Let |response| be the result of <a lt="fetch">fetching</a> |request|.
+        1. If |response|’s <a for="response" href="https://github.com/whatwg/fetch/issues/376">cache state</a> is not "<code>local</code>", set |registration|’s [=service worker registration/last update check time=] to the current time.
+        1. [=Extract a MIME type=] from the |response|'s [=unsafe response=]'s [=response/header list=]. If this MIME type (ignoring parameters) is not a [=JavaScript MIME type=], return a [=network error=].
+        1. If |response|'s <a>unsafe response</a>'s [=response/type=] is not "<code>error</code>", and |response|'s [=response/status=] is an <a>ok status</a>, then:
+            1. Let |newestWorker| be the result of running [=Get Newest Worker=] with |registration|.
+            1. Let |resource| be null.
+            1. If |newestWorker| is not null, set |resource| to |newestWorker|'s [=service worker/script resource map=][|request|'s [=request/url=]], or null if it does not [=map/exist=].
+            1. Set |serviceWorker|'s [=service worker/include updated resources flag=] if any of the following are true:
+                * |newestWorker| is null.
+                * |resource| is null.
+                * |resource|'s [=response/body=] is not byte-for-byte identical with |response|'s [=response/body=]. 
+            1. [=map/Set=] [=service worker/script resource map=][|request|'s [=request/url=]] to |response|.
+            1. Set |serviceWorker|'s [=classic scripts imported flag=].

The *classic scripts imported flag* is used to indicate the main script has some imported scripts so Update must not return early before actually checking the imported scripts even if the main scripts are byte identical. I named it *classic ..* because module scripts don't support `importScripts()` in the first place.

The job's *potentially include updated resources flag* is used to check if we can return early even without evaluating the script. That is, the script received from the network is byte identical and doesn't import any script. For module scripts, it checks all the responses gotten during fetching the module worker script graph.

The service worker's *include updated resources flag* is checked when Update couldn't determined from the earlier steps whether it could stop early.

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