Implement a URLSession backend for viaduct on iOS

[rfk]

This was mentioned in slack last week, wanted to file it for future reference.

Our networking library currently uses hyper on iOS, which bottoms out at the system networking and TLS stack, but brings with it a bunch of intermediate dependencies. We should consider doing a thin viaduct backend that directly uses the native URLSession infrastructure provided by the platform, which would reduce dependency footprint and simplify reasoning about the system.

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[rfk]

/cc @garvankeeley @thomcc; no particular urgency here IIUC, but worth thinking about.

[thomcc]

This doesn't sound hard. The same backend as for the android code could hopefully be used, just with swift powering it rather than Kotlin. The one caveat is that the API exposed by viaduct backends must be synchronous, whereas URLSession is async.

That said, swift has some locks and stuff even if it's much more limited than rust there, so we can hopefully make that work.

[garvankeeley]

Happy to help.
Quick tip for URLSession, instantiate it with the arg delegateQueue: .main, so that the response calls back to main (default behaviour is off-main response calls).
More relevant, IIUC we want a sync urlsession wrapper implementation which should be trivial on the swift side (pseudocode):

// got called from rust to load an url
semaphore.setup()
urlSession.dataTask(url: url) { data, error in
    // got data or error
    semaphore.notify()
}
semaphore.wait()
// go back to rust with data and error state info

[rfk]

Would it make sense to allow the caller to pass in a URLSession instance in a similar way to how a-c can pass in a specific Fetch implementation?