Introduce Service-Worker-Max-Age header

[annevk]

@ehsan, please correct me if this didn't match what we discussed.

Since service workers use byte-for-byte comparison rather than HTTP cache semantics, there's a risk that when using a service worker library, it becomes harder to update, since you need to change the library code.

So it would be good that in addition to the byte-for-byte comparison, there's also a header that indicates when the browser should start ignoring the service worker being byte-for-byte identical.

[ehsan]

That sounds like an accurate summary!

[annevk]

The max-age is counted since installation.

[slightlyoff]

There's pretty strong consensus from the F2F that this is worth doing in M2 (IIRC). Marking as such.

[delapuente]

The are two perspectives here. One option is to extend byte-to-byte comparison to the whole imported scripts (importScripts) graph. If one node has change, update. The other is related with #839. The header Service-Worker-Force-Update proposed by @jakearchibald can be enough for this but it's hard to automatically calculate in the server cause it involves to calculate the import graph there.

[collimarco]

One option is to extend byte-to-byte comparison to the whole imported scripts (importScripts) graph.

+1 for this

Personally I don't like the idea of having to return a custom header to force the refresh of the imported scripts.

This is because at Pushpad (which is a BaaS) we have a script that is imported in our customers' service workers. When we update our script we want the update to be propagated. However we cannot ask our customers to return custom headers for their service worker (some of them for example have simple websites or blogs powered by Wordpress).

I have described more extensively about our situation and why we need automatic updates of the imported scripts in this blog post.

[jakearchibald]

We're going to check imported scripts by default #839 (comment)