Github user geomacy commented on a diff in the pull request:
https://github.com/apache/brooklyn-server/pull/520#discussion_r97600990
--- Diff:
core/src/main/java/org/apache/brooklyn/core/sensor/ReleaseableLatch.java ---
@@ -0,0 +1,95 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you 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 org.apache.brooklyn.core.sensor;
+
+import java.util.concurrent.Semaphore;
+
+import org.apache.brooklyn.api.entity.Entity;
+import org.apache.brooklyn.util.core.task.DeferredSupplier;
+import org.apache.brooklyn.util.core.task.ImmediateSupplier;
+import org.apache.brooklyn.util.exceptions.Exceptions;
+import org.apache.brooklyn.util.guava.Maybe;
+
+// DeferredSupplier used as a marker interface to prevent coercion. When
resolved it must evaluate to {@code Boolean.TRUE}.
+public interface ReleaseableLatch extends DeferredSupplier<Boolean>,
ImmediateSupplier<Boolean> {
+ /**
+ * Increment usage count for the {@code caller} entity
+ */
+ void acquire(Entity caller);
+
+ /**
+ * Decrement usage count for the {@code caller} entity
+ */
+ void release(Entity caller);
+
+ static abstract class AbstractReleaseableLatch implements
ReleaseableLatch {
+ // Instances coerce to TRUE as they are non-null.
+ @Override public Boolean get() {return Boolean.TRUE;}
+ @Override public Maybe<Boolean> getImmediately() {return
Maybe.of(Boolean.TRUE);}
+ }
+
+ ReleaseableLatch NOP = new Factory.NopLatch();
+
+ static class Factory {
+ private static class NopLatch extends AbstractReleaseableLatch {
+ @Override public void acquire(Entity caller) {}
+ @Override public void release(Entity caller) {}
+ }
+
+ private static class MaxConcurrencyLatch extends
AbstractReleaseableLatch {
+ private int permits;
+ private transient final Semaphore sem;
+
+ public MaxConcurrencyLatch(int permits) {
+ this.permits = permits;
+ this.sem = new Semaphore(permits);
+ }
+
+ @Override
+ public void acquire(Entity caller) {
--- End diff --
Why is no use made of `caller` here and in `release` - by the description
of the contract above "Increment usage count for the {@code caller} entity"
surely it should? Reading that, I wouldn't expect that if I have two entities
and do `latch.acquire(entity1)`, I would then be able to do
`latch.release(entity2)`, or indeed `latch.release(null)`, and still release a
permit on the semaphore, but that's what will happen. It may be worth updating
the comment to indicate that implementations are free to ignore the entity if
that suits them.
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