Fix NPE in nodetool compactionhistory

patch by Pierre N. and yukim; reviewed by yukim for CASSANDRA-9758

CHANGES.txt

@@ -1,4 +1,5 @@
 2.1.10
+ * Fix NPE in nodetool compactionhistory (CASSANDRA-9758)
  * (Pig) support BulkOutputFormat as a URL parameter (CASSANDRA-7410)
  * BATCH statement is broken in cqlsh (CASSANDRA-10272)
  * Added configurable warning threshold for GC duration (CASSANDRA-8907)

src/java/org/apache/cassandra/utils/FBUtilities.java

@@ -28,7 +28,6 @@ import java.net.UnknownHostException;
 import java.nio.ByteBuffer;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
-import java.text.NumberFormat;
 import java.util.*;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
@@ -36,6 +35,9 @@ import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.zip.Checksum;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 import com.google.common.base.Joiner;
 import com.google.common.collect.AbstractIterator;
 import org.apache.cassandra.io.util.FileUtils;
@@ -472,8 +474,18 @@ public class FBUtilities
         return s;
     }
 
-    public static String toString(Map<?,?> map)
+    /**
+     * Make straing out of the given {@code Map}.
+     *
+     * @param map Map to make string.
+     * @return String representation of all entries in the map,
+     *         where key and value pair is concatenated with ':'.
+     */
+    @Nonnull
+    public static String toString(@Nullable Map<?, ?> map)
     {
+        if (map == null)
+            return "";
         Joiner.MapJoiner joiner = Joiner.on(", ").withKeyValueSeparator(":");
         return joiner.join(map);
     }

test/unit/org/apache/cassandra/utils/FBUtilitiesTest.java

@@ -27,6 +27,11 @@ import java.nio.charset.StandardCharsets;
 import com.google.common.primitives.Ints;
 import org.junit.Test;
 
+import java.util.Map;
+import java.util.TreeMap;
+
+import static org.junit.Assert.assertEquals;
+
 public class FBUtilitiesTest
 {
     @Test
@@ -70,6 +75,20 @@ public class FBUtilitiesTest
         }
     }
 
+    @Test
+    public void testToString()
+    {
+        // null turns to empty string
+        assertEquals("", FBUtilities.toString(null));
+        Map<String, String> map = new TreeMap<>();
+        // empty map turns to empty string
+        assertEquals("", FBUtilities.toString(map));
+        map.put("aaa", "bbb");
+        assertEquals("aaa:bbb", FBUtilities.toString(map));
+        map.put("ccc", "ddd");
+        assertEquals("aaa:bbb, ccc:ddd", FBUtilities.toString(map));
+    }
+
     @Test(expected=CharacterCodingException.class)
     public void testDecode() throws IOException
     {