How to understand coroutine cancellation is cooperative

Question

In Kotlin, coroutine cancellation is cooperative. How should I understand it?

Link to Kotlin documentation.

Answer 1

If you have a Java background, you may be familiar with the thread interruption mechanism. Any thread can call thread.interrupt() and the receiving thread will get a signal in the form of a Boolean isInterrupted flag becoming true. The receiving thread may check the flag at any time with currentThread.isInterrupted() — or it may ignore it completely. That's why this mechanism is said to be cooperative.

Kotlin's coroutine cancellation mechanism is an exact replica of this: you have a coroutineContext.isActive flag that you (or a function you call) may check.

In both cases some well-known functions, for example Thread.sleep() in Java and delay() in Kotlin, check this flag and throw an InterruptedException and CancellationException, respectively. These methods/functions are said to be "interruptible" / "cancellable".