Re: [w3c/ServiceWorker] ServiceWorkerAutoPreload (PR #1756)

@sisidovski commented on this pull request.



> +              1. Let |preloadResponseObject| be a new {{Response}} object associated with a new {{Headers}} object whose [=guard=] is "`immutable`".
+              1. [=header list/Append=] to |preloadRequestHeaders| a new [=header=] whose [=header/name=] is \`<code>Service-Worker-Navigation-Preload</code>\` and [=header/value=] is |registration|'s [=navigation preload header value=].
+              1. Set |preloadRequest|'s [=service-workers mode=] to "`none`".
+              1. Let |preloadFetchController| be null.
+              1. Run the following substeps [=in parallel=], but [=abort when=] |fetchController|'s [=fetch controller/state=] is "<code>terminated</code>" or "<code>aborted</code>":
+                  1. Set |preloadFetchController| to the result of [=Fetch|fetching=] |preloadRequest|.
+
+                      To [=fetch/processResponse=] for |navigationPreloadResponse|, run these substeps:
+
+                      1. If |navigationPreloadResponse|'s [=response/type=] is "`error`", reject |preloadResponse| with a `TypeError` and terminate these substeps.
+                      1. Associate |preloadResponseObject| with |navigationPreloadResponse|.
+                      1. Resolve |preloadResponse| with |preloadResponseObject|.
+              1. [=If aborted=], then:
+                  1. Let |deserializedError| be the result of [=deserialize a serialized abort reason=] given null and |workerRealm|.
+                  1. [=fetch controller/Abort=] |preloadFetchController| with |deserializedError|.
+          1. Else if |timingInfo|'s [=service worker timing info/worker matched router source=] is not "{{RouterSourceEnum/fetch-event}}", a user agent may run the following substeps:

We frame the autopreload is an optional optimization that the browser can apply at its choosing. So we want to use the static routing API as an opt-out mechanism. If we only invoke the autopreload only when the matched router source is `fetch-event`, that is more like an opt-in, not opt-out.

> +              1. Set |preloadRequest|'s [=service-workers mode=] to "`none`".
+              1. Let |preloadFetchController| be null.
+              1. Run the following substeps [=in parallel=], but [=abort when=] |fetchController|'s [=fetch controller/state=] is "<code>terminated</code>" or "<code>aborted</code>":
+                  1. Set |preloadFetchController| to the result of [=Fetch|fetching=] |preloadRequest|.
+
+                      To [=fetch/processResponse=] for |navigationPreloadResponse|, run these substeps:
+
+                      1. If |navigationPreloadResponse|'s [=response/type=] is "`error`", reject |preloadResponse| with a `TypeError` and terminate these substeps.
+                      1. Associate |preloadResponseObject| with |navigationPreloadResponse|.
+                      1. Resolve |preloadResponse| with |preloadResponseObject|.
+              1. [=If aborted=], then:
+                  1. Let |deserializedError| be the result of [=deserialize a serialized abort reason=] given null and |workerRealm|.
+                  1. [=fetch controller/Abort=] |preloadFetchController| with |deserializedError|.
+          1. Else if |timingInfo|'s [=service worker timing info/worker matched router source=] is not "{{RouterSourceEnum/fetch-event}}", a user agent may run the following substeps:
+
+              Note: A user agent may speculatively dispatch a network request in parallel with creating a fetch event in order to minimize the bootstrap cost.

In the explainer, we mention two possible scenarios that the browser may invoke the autopreload.
https://github.com/WICG/service-worker-auto-preload?tab=readme-ov-file#eligibility-criteria

I don't think we can fully specify when it's used since some conditions are not deterministic, but do you think we should say more here? 

> +
+                      1. If |navigationPreloadResponse|'s [=response/type=] is "`error`", reject |preloadResponse| with a `TypeError` and terminate these substeps.
+                      1. Associate |preloadResponseObject| with |navigationPreloadResponse|.
+                      1. Resolve |preloadResponse| with |preloadResponseObject|.
+              1. [=If aborted=], then:
+                  1. Let |deserializedError| be the result of [=deserialize a serialized abort reason=] given null and |workerRealm|.
+                  1. [=fetch controller/Abort=] |preloadFetchController| with |deserializedError|.
+          1. Else if |timingInfo|'s [=service worker timing info/worker matched router source=] is not "{{RouterSourceEnum/fetch-event}}", a user agent may run the following substeps:
+
+              Note: A user agent may speculatively dispatch a network request in parallel with creating a fetch event in order to minimize the bootstrap cost.
+
+              1. Let |autoPreloadFetchController| be null.
+              1. Set |responseForAutoPreload| to be a [=race response=] whose [=race response/value=] is "<code>pending</code>".
+              1. Run the following substeps [=in parallel=], but [=abort when=] |fetchController|'s [=fetch controller/state=] is "<code>terminated</code>" or "<code>aborted</code>":
+                  1. Set |autoPreloadFetchController| to the result of calling [=fetch=] given |request|, with [=fetch/processResponse=] set to the following steps given a [=/response=] |autoPreloadRequestResponse|:
+                  1. If |autoPreloadRequestResponse|'s [=response/status=] is [=ok status=], set |responseForAutoPreload|'s [=race response/value=] to |autoPreloadRequestResponse|.

Thanks! Updated to set the response to `responseForAutoPreload` regardless of the status.

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