Fix encoding of cell names for super columns (CASSANDRA-12235 follow-up)

patch by Sylvain Lebresne; reviewed by Aleksey Yeschenko for
CASSANDRA-12335

src/java/org/apache/cassandra/db/LegacyLayout.java

@@ -264,6 +264,8 @@ public static ByteBuffer encodeCellName(CFMetaData metadata, ClusteringPrefix cl
         // We use comparator.size() rather than clustering.size() because of static clusterings
         int clusteringSize = metadata.comparator.size();
         int size = clusteringSize + (metadata.isDense() ? 0 : 1) + (collectionElement == null ? 0 : 1);
+        if (metadata.isSuper())
+            size = clusteringSize + 1;
         ByteBuffer[] values = new ByteBuffer[size];
         for (int i = 0; i < clusteringSize; i++)
         {
@@ -282,10 +284,23 @@ public static ByteBuffer encodeCellName(CFMetaData metadata, ClusteringPrefix cl
             values[i] = v;
         }
 
-        if (!metadata.isDense())
-            values[clusteringSize] = columnName;
-        if (collectionElement != null)
-            values[clusteringSize + 1] = collectionElement;
+        if (metadata.isSuper())
+        {
+            // We need to set the "column" (in thrift terms) name, i.e. the value corresponding to the subcomparator.
+            // What it is depends if this a cell for a declared "static" column or a "dynamic" column part of the
+            // super-column internal map.
+            assert columnName != null; // This should never be null for supercolumns, see decodeForSuperColumn() above
+            values[clusteringSize] = columnName.equals(CompactTables.SUPER_COLUMN_MAP_COLUMN)
+                                   ? collectionElement
+                                   : columnName;
+        }
+        else
+        {
+            if (!metadata.isDense())
+                values[clusteringSize] = columnName;
+            if (collectionElement != null)
+                values[clusteringSize + 1] = collectionElement;
+        }
 
         return CompositeType.build(isStatic, values);
     }