ios – Why doesn’t calling serialQueue.sync() end in a impasse?

Whereas learning DispatchQueue, I got here throughout one thing I don’t fairly perceive.

I created a customized serial queue and executed a synchronous block on it. Once I printed the thread that was working the block, it turned out to be the primary thread. This confused me, as a result of the queue wasn’t the primary queue, but the block nonetheless executed on the primary thread.

let serialQueue = DispatchQueue(label: "serial.queue")

serialQueue.sync {
    print("present thread = (Thread.present)") // present thread = _NSMainThread
}

Within the documentation for sync(), I discovered the next assertion:

As a efficiency optimization, this operate executes blocks on the
present thread at any time when potential, with one exception: Blocks submitted
to the primary dispatch queue at all times run on the primary thread.

Primarily based on this description, I assumed that if the block was certainly executed on the present thread (the primary thread on this case), then a impasse ought to have occurred.

Right here’s the reasoning I had:

1. The primary thread calls serialQueue.sync(). As a result of it’s synchronous, the primary thread turns into blocked till the block completes.
2. The block is submitted to the serial queue, which then makes an attempt to execute it on the primary thread (the present thread).
3. Nonetheless, for the reason that primary thread is already blocked, it wouldn’t have the ability to course of the duty, which ought to trigger a impasse.

However opposite to my expectation, no impasse occurred and the code executed simply tremendous. I can’t determine why.

Query.

1. Why is a block executed on the primary thread regardless that it was submitted to a customized serial queue?
2. If, because the documentation suggests, sync() executes on the present thread as an optimization, why doesn’t this result in a impasse on this case?
3. Can the primary thread nonetheless execute different duties whereas it’s blocked? I want to perceive how precisely the primary thread operates.