Moving to the nio-dev list.
Registration is not an atomic operation. The channel is registered (this
creates the selection key) and then the key's interest set is changed.
If I read your test case correctly, it is cancelling the key before the
key's interest set is changed and this leads to the CancelledKeyException.
Yes, I agree this may be surprising. Can you submit a bug for this?
-Alan
On 31/05/2022 11:38, Gillespie, Oli wrote:
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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