spec says cache should return responses with immutable headers, but no browser does that

[wanderview]

While working on a glitch for a chrome-specific issue, I noticed a weird behavior in all browsers. The spec says that the responses returned from the Cache API should be immutable. See step 5.4.2.1 here:

https://w3c.github.io/ServiceWorker/#cache-matchall

This glitch overwrites a header in a cache-returned response, though:

https://code-cache-mime-type.glitch.me/

And it appears to work in chrome, firefox, and safari. I hit some issue in edge which prevented the service worker from installing for some reason, but I couldn't figure out why.

Given that all browsers are shipping this behavior, should we change the spec? There is some risk of implementation now that sites could break if we start making these immutable now.

[jakearchibald]

Given that all browsers are shipping this behavior, should we change the spec?

Seems reasonable.

[annevk]

Well, is this for all responses that can be stored in the cache? How does it work for CORS responses? There's some detail lacking here.

[jakearchibald]

Whatever we do here, we should try to do the same for responses returned from fetch().

If we can't allow headers to be mutable, we should run a test to see if we can make the cache ones immutable. As in, get some usage stats.