Conditionally create an instance of CancellationException in Channel.…

[qwwdfsad]

…cancel()

Avoid creating costly exception when the channel is cancelled to save a few cycles when it's not necessary. Cancellation is heavy-enough when the channel is open, so the single check won't worsen it.

[elizarov]

👍 Good stuff

[paulferaud]

This changed the behavior for completed callbackFlow that are still blocked. Example:

  @Test
  fun p2384() = runBlocking {
    val myFlow = callbackFlow<Any> {
      close()

      // Should throw?
      delay(Duration.INFINITE)
    }

    // Should complete?
    myFlow.collect()
  }

Here, delay used to throw a cancellation exception, and the flow would complete.
Now, it blocks on delay, and collect() never completes.

[qwwdfsad]

Thanks for the report, that's an unexpected side-effect, filed #2506