Cache#computeIfAbsent should throw if loader returns null value

core/src/main/java/org/elasticsearch/common/cache/Cache.java

@@ -307,9 +307,10 @@ public class Cache<K, V> {
                     } catch (Exception e) {
                         throw new ExecutionException(e);
                     }
-                    if (value != null) {
-                        put(key, value, now);
+                    if (value == null) {
+                        throw new ExecutionException(new NullPointerException("loader returned a null value"));
                     }
+                    put(key, value, now);
                 }
             }
         }

core/src/test/java/org/elasticsearch/common/cache/CacheTests.java

@@ -29,6 +29,8 @@ import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.concurrent.atomic.AtomicReferenceArray;
 
+import static org.hamcrest.CoreMatchers.instanceOf;
+
 public class CacheTests extends ESTestCase {
     private int numberOfEntries;
 
@@ -491,6 +493,16 @@ public class CacheTests extends ESTestCase {
         }
     }
 
+    public void testComputeIfAbsentThrowsExceptionIfLoaderReturnsANullValue() {
+        final Cache<Integer, String> cache = CacheBuilder.<Integer, String>builder().build();
+        try {
+            cache.computeIfAbsent(1, k -> null);
+            fail("expected ExecutionException");
+        } catch (ExecutionException e) {
+            assertThat(e.getCause(), instanceOf(NullPointerException.class));
+        }
+    }
+
     // test that the cache is not corrupted under lots of concurrent modifications, even hitting the same key
     // here be dragons: this test did catch one subtle bug during development; do not remove lightly
     public void testTorture() throws InterruptedException {