Improve dnslink resolution

[Kubuxu]

This would prevent possible delay of loading the page when DNS is slow (on bad connections DNS resolution can take few seconds).

So start loading page via normal HTTP and start resolving dnslink, if dnslink is available terminate normal HTTP connection and redirect to local gateway.

[lidel]

Very good idea. Thank you!

Current SDK provides two HTTP events (same for the future one)
that we can use to hook into request pipeline before request is made:

• http-on-modify-request Called as a http request is made. The channel is available to allow you to modify headers and such.
• http-on-opening-request Similar to http-on-modify-request, but called earlier (synchronously during the channel's asyncOpen() call), and some channel atttributes (proxyInfo) may be missing. Use only if your observer must be called before asyncOpen returns.

– https://developer.mozilla.org/en-US/docs/Observer_Notifications#HTTP_request

Currently, all the redirect logic of this addon is in http-on-modify-request handler.

I guess we could hook into earlier http-on-opening-request and only check if host is in cache.
If not, do an asynchronous dnslookup with a callback that puts the result in cache.
Then in http-on-modify-request we would just check the cache.
Result: no slowdown caused by synchronous lookups or slow DNS/network.

There is a risk of dnslink lookup taking longer than the delay between those two events, but it would be only for an initial cache miss (and we could save cache between browser restarts).
Not sure if we can do anything more with these APIs.

Should I give it a try?

[the8472]

At the very least we could do an async XHR and suspend the nsIRequest instead of blocking the main thread waiting for a response and resume the request once we got a response. This way at least the UI stays responsive.

[lidel]

Thank you @the8472, it was a very good insight.

• dnslink lookups are now executed asynchronously and only if DNS support is enabled, record was not found in cache and API is up
• request is suspended until dnslink lookup callback is fired

Submitted v1.5.4 to AMO.

[lidel]

Reopening: I feel it may be a good idea to revisit this in WebExtension rewrite and see if something similar can be introduced.