CancelledKeyException during channel registration

[Gillespie, Oli]

Hi,

I noticed this surprising (to me) behaviour, and wonder whether it's 
expected or could be considered a bug. I'm not an expert in this area so 
apologies if this is trivial.

When registering a SocketChannel with a Selector for the first time, 
it's possible to get a CancelledKeyException even though this is the 
first register call.

Exception in thread "main" java.nio.channels.CancelledKeyException
     at 
java.base/sun.nio.ch.SelectionKeyImpl.ensureValid(SelectionKeyImpl.java:75)
     at 
java.base/sun.nio.ch.SelectionKeyImpl.interestOps(SelectionKeyImpl.java:104)
     at java.base/sun.nio.ch.SelectorImpl.register(SelectorImpl.java:222)
     at 
java.base/java.nio.channels.spi.AbstractSelectableChannel.register(AbstractSelectableChannel.java:236)
     at 
java.base/java.nio.channels.SelectableChannel.register(SelectableChannel.java:260)
     at KeyCancelled.main(KeyCancelled.java:20)

The javadoc states:

@throws  CancelledKeyException
     If this channel is currently registered with the given selector
       but the corresponding key has already been cancelled

However in this case the channel is _not_ registered, as shown by 
SocketChannel.isRegistered() returning false.

This following sequence triggers this issue:

1. Thread 1 starts SelectableChannel.register
2. A new SelectionKey becomes visible via Selector.keys()
3. Thread 2 iterates Selector.keys() and calls SelectorKey.cancel()
4. Thread 1 (still in the register() invocation) finds that the key is 
cancelled and throws CancelledKeyException

Below is a small reproducer which usually exhibits this issue:

import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;

public class KeyCancelled {
     public static void main(String[] args) throws Exception {
         Selector s = Selector.open();

         new Thread(() -> {
             for (int i = 0; i < 100_000; i++) {
                 s.keys().forEach(SelectionKey::cancel);
             }
         }).start();

         for (int i = 0; i < 10_000; i++) {
             SocketChannel c = s.provider().openSocketChannel();
             c.configureBlocking(false);
             // Sometimes this throws CancelledKeyException, because the 
key is cancelled by
             // the other thread part-way through the register call.
             c.register(s, SelectionKey.OP_READ);
             // c.isRegistered() is false here after the exceptional case
         }
     }
}

So, is this expected, a bug, or a gap in documentation?

Many thanks,

Oli



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