Re: [whatwg/fetch] New Proposal: Fetch retry (Issue #1838)

rakina left a comment (whatwg/fetch#1838)

The fetch retry [Origin Trial](https://developer.chrome.com/origintrials/#/register_trial/2119506574631239681) has been running on Chrome for a while. Some takeaways based on partner feedback:
-  Overall the current shape of the API seems ok. We needed to add a function (`Request.getRetryOptions()`) so that partners can do feature-detection and do slicing correctly in their A/B experiments.
- Some of the requests are using `POST` so `retryNonIdempotent` is useful.
- `Retry-Attempts` is needed by some partners to identify whether a request they're receiving is a retry or not, to evaluate the effectiveness of the fetch retry itself (and potentially tweak their parameters/retry strategy). From [past comments](https://github.com/whatwg/fetch/issues/1838#issuecomment-3068061166) this header might be OK if we don't send it for `no-cors` mode, and only send for CORS with pre-flight. Is that correct?
- Currently if the fetch encountered redirects, then failed, we always retry from the beginning. There's some interest to do the retry from the last successful hop instead (so if we go A -> B -> fail to get to C, retry from B instead). I wonder if there are downsides / problems of doing that (potentially as an option so users can pick what works best for their case)?
- There are some servers that can't deduplicate at all (e.g. going to third party servers), so there's some interest in a `retryOnlyIfServerUnreached` option. Not sure if it's entirely possible to spec, but at least in Chrome impl, we can know that e.g. a connection wasn't established at all, or if there are DNS problems, where we're pretty sure that the fetch request never reached the server.

I'm wondering if people here have thoughts on the potential additions from these feedback. I've also requested to add fetch retry to the WHATWG TPAC agenda, where we can hopefully finalize the API shape, and figure out technical details etc. 

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