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The official Couchbase JVM Core IO Library
/*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.couchbase.client.core.deps.io.netty.util.internal.shaded.com.couchbase.client.core.deps.org.jctools.queues;
import com.couchbase.client.core.deps.io.netty.util.internal.shaded.com.couchbase.client.core.deps.org.jctools.util.InternalAPI;
/**
* A note to maintainers on index assumptions: in a single threaded world it would seem intuitive to assume:
*
* producerIndex >= consumerIndex
*
* As an invariant, but in a concurrent, long running settings all of the following need to be considered:
*
* -
consumerIndex > producerIndex
: due to counter overflow (unlikey with longs, but easy to reason)
* -
consumerIndex > producerIndex
: due to consumer FastFlow like implementation discovering the
* element before the counter is updated.
* -
producerIndex - consumerIndex < 0
: due to above.
* -
producerIndex - consumerIndex > Integer.MAX_VALUE
: as linked buffers allow constructing queues
* with more than Integer.MAX_VALUE
elements.
*
*
*/
@InternalAPI
public final class IndexedQueueSizeUtil
{
public static int size(IndexedQueue iq)
{
/*
* It is possible for a thread to be interrupted or reschedule between the read of the producer and
* consumer indices, therefore protection is required to ensure size is within valid range. In the
* event of concurrent polls/offers to this method the size is OVER estimated as we read consumer
* index BEFORE the producer index.
*/
long after = iq.lvConsumerIndex();
long size;
while (true)
{
final long before = after;
final long currentProducerIndex = iq.lvProducerIndex();
after = iq.lvConsumerIndex();
if (before == after)
{
size = (currentProducerIndex - after);
break;
}
}
// Long overflow is impossible (), so size is always positive. Integer overflow is possible for the unbounded
// indexed queues.
if (size > Integer.MAX_VALUE)
{
return Integer.MAX_VALUE;
}
// concurrent updates to cIndex and pIndex may lag behind other progress enablers (e.g. FastFlow), so we need
// to check bounds
else if (size < 0)
{
return 0;
}
else if (iq.capacity() != MessagePassingQueue.UNBOUNDED_CAPACITY && size > iq.capacity())
{
return iq.capacity();
}
else
{
return (int) size;
}
}
public static boolean isEmpty(IndexedQueue iq)
{
// Order matters!
// Loading consumer before producer allows for producer increments after consumer index is read.
// This ensures this method is conservative in it's estimate. Note that as this is an MPMC there is
// nothing we can do to make this an exact method.
return (iq.lvConsumerIndex() >= iq.lvProducerIndex());
}
@InternalAPI
public interface IndexedQueue
{
long lvConsumerIndex();
long lvProducerIndex();
int capacity();
}
}