HADOOP-15056. Fix TestUnbuffer#testUnbufferException failure. Contributed by Jack Bearden.

(cherry picked from commit 19e089420999dd9d97d981dcd0abd64b6166152d)
(cherry picked from commit 1d1401ad3898834a740f4b9bfcea83d28635a3b6)
(cherry picked from commit 31436a12d5cd7c8d96414fcf37f9b8aebe497cd3)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/StreamCapabilitiesPolicy.java

@@ -22,6 +22,8 @@ import java.io.InputStream;
 
 import org.apache.hadoop.classification.InterfaceAudience;
 import org.apache.hadoop.classification.InterfaceStability;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Static methods to implement policies for {@link StreamCapabilities}.
@@ -29,6 +31,10 @@ import org.apache.hadoop.classification.InterfaceStability;
 @InterfaceAudience.Public
 @InterfaceStability.Evolving
 public class StreamCapabilitiesPolicy {
+  public static final String CAN_UNBUFFER_NOT_IMPLEMENTED_MESSAGE =
+          "claims unbuffer capabilty but does not implement CanUnbuffer";
+  static final Logger LOG = LoggerFactory.getLogger(
+          StreamCapabilitiesPolicy.class);
   /**
    * Implement the policy for {@link CanUnbuffer#unbuffer()}.
    *
@@ -40,11 +46,14 @@ public class StreamCapabilitiesPolicy {
           && ((StreamCapabilities) in).hasCapability(
           StreamCapabilities.UNBUFFER)) {
         ((CanUnbuffer) in).unbuffer();
+      } else {
+        LOG.debug(in.getClass().getName() + ":"
+                + " does not implement StreamCapabilities"
+                + " and the unbuffer capability");
       }
     } catch (ClassCastException e) {
-      throw new UnsupportedOperationException("this stream " +
-          in.getClass().getName() +
-          " claims to unbuffer but forgets to implement CanUnbuffer");
+      throw new UnsupportedOperationException(in.getClass().getName() + ": "
+              + CAN_UNBUFFER_NOT_IMPLEMENTED_MESSAGE);
     }
   }
 }

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/fs/TestUnbuffer.java

@@ -131,17 +131,27 @@ public class TestUnbuffer {
   }
 
   /**
-   * Test unbuffer method which throws an Exception with class name included.
+   * Test that a InputStream should throw an exception when not implementing
+   * CanUnbuffer
+   *
+   * This should throw an exception when the stream claims to have the
+   * unbuffer capability, but actually does not implement CanUnbuffer.
    */
   @Test
   public void testUnbufferException() {
-    FSInputStream in = Mockito.mock(FSInputStream.class);
-    FSDataInputStream fs = new FSDataInputStream(in);
+    abstract class BuggyStream
+            extends FSInputStream
+            implements StreamCapabilities {
+    }
+
+    BuggyStream bs = Mockito.mock(BuggyStream.class);
+    Mockito.when(bs.hasCapability(Mockito.anyString())).thenReturn(true);
 
     exception.expect(UnsupportedOperationException.class);
-    exception.expectMessage("this stream " + in.getClass().getName()
-        + " does not support unbuffering");
+    exception.expectMessage(
+            StreamCapabilitiesPolicy.CAN_UNBUFFER_NOT_IMPLEMENTED_MESSAGE);
 
+    FSDataInputStream fs = new FSDataInputStream(bs);
     fs.unbuffer();
   }
 }