`no-cache` option prevents preflight requests from being cached

[pkuzco]

In the current implementation of the matrix-js-sdk, setting the cache property of fetch() to no-cache (-> Cache-Control: max-age=0) prevents browsers from caching preflight (OPTIONS) requests. This behavior leads to unnecessary network overhead, as preflight requests are sent before every cross-origin request, even when the CORS policy remains unchanged.

https://github.com/matrix-org/matrix-js-sdk/blob/develop/src/http-api/fetch.ts#L294

The older implementation from 3 years ago doesn't seem to specify any cache-control directive on requests:
https://github.com/matrix-org/matrix-js-sdk/blob/e37aab29672256218d30876e96dd17fedc0506f7/src/http-api.ts

I'm not saying the new behavior is incorrect, but it's worth raising the issue and perhaps discuss it, as it has performance implications.

[pkuzco]

PR #5020 fixes this issue.

[t3chguy]

I can't see documentation which states that no-cache would mean preflight Access-Control-Max-Age is ignored - heck https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CORS even uses no-cache in its example explaining Access-Control-Max-Age. Got any links?

[pkuzco]

This might be browser-specific, but at least in Chrome:

• Per fetch spec, no-cache maps to Cache-Control: max-age=0
• !RetrieveCacheFlags(request.load_flags) fails in https://github.com/chromium/chromium/blob/main/services/network/cors/preflight_controller.cc#L733
• Because LOAD_VALIDATE_CACHE flag is set
• And that flag is set when Cache-Control: max-age=0 header is present: https://github.com/chromium/chromium/blob/main/net/http/http_cache_util.cc#L46, https://github.com/chromium/chromium/blob/main/net/http/http_cache_util.cc#L99

btw, I may have overstated the performance gain in issue description, since preflight caching remains bound to the full URL.