util: ensure that cache entry lifetime is bigger than idle time

Motivation:
'document' the behavior, avoid miss configurations

Modification:
Add precondition into Cache constructor

Result:
Invalid cache objects can be constructed

Target: master
Acked-by: Albert Rossi
Acked-by: Lea Morschel

core/src/main/java/org/dcache/nfs/util/Cache.java

 /*
- * Copyright (c) 2009 - 2020 Deutsches Elektronen-Synchroton,
+ * Copyright (c) 2009 - 2022 Deutsches Elektronen-Synchroton,
  * Member of the Helmholtz Association, (DESY), HAMBURG, GERMANY
  *
  * This library is free software; you can redistribute it and/or modify
@@ -32,6 +32,8 @@ import java.util.concurrent.locks.ReentrantLock;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import static com.google.common.base.Preconditions.checkArgument;
+
 /**
  * A Dictionary where value associated with the key may become unavailable due
  * to validity timeout.
@@ -146,6 +148,9 @@ public class Cache<K, V> {
      */
     public Cache(final String name, int size, long entryLifeTime, long entryIdleTime,
             CacheEventListener<K, V> eventListener, Clock clock) {
+
+        checkArgument(entryLifeTime >= entryIdleTime, "Entry life time cant be smaller that idle time");
+
         _name = name;
         _size = size;
         _defaultEntryMaxLifeTime = entryLifeTime;

core/src/test/java/org/dcache/nfs/util/CacheTest.java

@@ -42,6 +42,13 @@ public class CacheTest {
                 new NopCacheEventListener(), _clock);
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void testCacheIdleBiggerThanMax() {
+        new Cache<>("test cache", 10,
+                5, 7,
+                new NopCacheEventListener(), _clock);
+    }
+
     @Test
     public void testPutGet() {