Allow multiple Speculation Rules

[midzer]

Is your feature request related to a problem? Please describe.
According to https://developer.chrome.com/blog/prerender-pages/#multiple-speculation-rules the Speculation API allows multiple Speculation Rules. Right now prerender: true can only prerender a single URL in viewport.

Describe the solution you'd like
Remove

quicklink/src/index.mjs

Line 247 in e6565ed

if (isSpecRulesExists()) {

Additional context
Would be happy to open a PR for this.

Have a nice weekend
midzer

[philwolstenholme]

I noticed this today too, did earlier versions of the API only support one script element, perhaps?

test/test-prerender-only.html can be used to reproduce the issue. Note how in the screenshot below, only 1.html will be prerendered:

[tunetheweb]

did earlier versions of the API only support one script element, perhaps?

Yes that's correct. It's a relatively recent change (Chrome 107 I think) that allowed multiple prerenders.

[philwolstenholme]

I've had a look around the repo and made a few changes locally. I think there are a few things we will need to do:

• Remove isSpecRulesExists() function definition and usage
• Remove prerenderLimit const definition
• Remove if (toPrerender.size < prerenderLimit) { check
• Add toPrerender.clear(); to the 'reset' function returned from listen() (toPrefetch.clear(); is there already)
• Call toPrerender.clear(); after adding the Speculation Rules script elements to prevent a build up of the same URLs being included as new links are found on scroll (see example below), maybe add a mechanism to avoid adding a URL twice if a script element is created for it once, then again when the user scrolls back to that part of the page?
• Remove or update comments prerendering conditions comments now that '2)' has been removed:

  quicklink/src/index.mjs

  Lines 239 to 258 in e6565ed

  	// prerendering preconditions:
  	// 1) whether UA supports spec rules.. If not, fallback to prefetch
  	if (!hasSpecRulesSupport()) {
  	prefetch (urls);
  	return Promise.reject(new Error('This browser does not support the speculation rules API. Falling back to prefetch.'));
  	}
  	// 2) whether spec rules is already defined (and with this we also covered when we have created spec rules before)
  	if (isSpecRulesExists()) {
  	return Promise.reject(new Error('Speculation Rules is already defined and cannot be altered.'));
  	}
  	// 3) whether it's a same origin url,
  	for (const url of [].concat(urls)) {
  	if (!isSameOrigin(url)) {
  	return Promise.reject(new Error('Only same origin URLs are allowed: ' + url));
  	}
  	toPrerender.add(url);
  	}

Example of not clearing out the toPrerender set:

<script type="speculationrules">
  { "prerender": [{ "source": "list", "urls": ["http://localhost:8080/work"] }] }
</script>
<script type="speculationrules">
  { "prerender": [{ "source": "list", "urls": ["http://localhost:8080/work", "http://localhost:8080/contact"] }] }
</script>
<script type="speculationrules">
  { "prerender": [{ "source": "list", "urls": ["http://localhost:8080/work", "http://localhost:8080/contact", "http://localhost:8080/github-stars"] }] }
</script>
<script type="speculationrules">
  { "prerender": [{ "source": "list", "urls": ["http://localhost:8080/work", "http://localhost:8080/contact", "http://localhost:8080/github-stars", "http://localhost:8080/github-stars?no-js"] }] }
</script>

[philwolstenholme]

All that said, I did quite quickly start seeing Max number of prerendering exceeded/MaxNumOfRunningPrerendersExceeded messages in the experimental Prerender2 dev tools panel, so I'm thinking that Quicklink's approach of prerendering anything in the viewport might be a bit too resource intensive compared to only prerendering links that are being hovered over or touchstart'ed on.

[tunetheweb]

Yeah there's current a limit of 10 prerenders. IMHO that is more than enough, and prerendering even that many (which I could easily see happening if prerendering all in-viewport links) seems like a lot to me.

[JiangWeixian]

quicklink export prerender API, currently, if call prerender multiple times, still throw this error. If user use prerender API manual, I think we should not limit prerender urls size.

Maybe should mv isSpecRulesExists check function into listen. 🤔