pool: Allow mover queues with zero capacity

PoolV4#mover_set_max_active allows the queue capacity to be set
to zero, yet the AdjustableSemaphore rejects this value (even
though the default capacity for a newly created semaphore is
zero).

Target: trunk
Require-book: no
Require-notes: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5425/

modules/dcache/src/main/java/org/dcache/util/AdjustableSemaphore.java

@@ -1,8 +1,12 @@
 package org.dcache.util;
 
+import com.google.common.base.Preconditions;
+
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.TimeUnit;
 
+import static com.google.common.base.Preconditions.*;
+
 /**
  * A simple implementation of an adjustable semaphore.
  *
@@ -44,10 +48,7 @@ public AdjustableSemaphore() {
      * @param newMax
      */
     public synchronized void setMaxPermits(int newMax) {
-        if (newMax < 1) {
-            throw new IllegalArgumentException("Semaphore size must be at least 1,"
-                    + " was " + newMax);
-        }
+        checkArgument(newMax >= 0);
 
         int delta = newMax - this.maxPermits;