Fix NPE when writetime() or ttl() are nested inside a fn call

Patch by Tyler Hobbs; reviewed by Benjamin Lerer for CASSANDRA-8451

CHANGES.txt

@@ -1,4 +1,6 @@
 2.0.12:
+ * Fix NPE when writetime() or ttl() calls are wrapped by
+   another function call (CASSANDRA-8451)
  * Fix NPE after dropping a keyspace (CASSANDRA-8332)
  * Fix error message on read repair timeouts (CASSANDRA-7947)
  * Default DTCS base_time_seconds changed to 60 (CASSANDRA-8417)

src/java/org/apache/cassandra/cql3/statements/Selection.java

@@ -186,11 +186,8 @@ public abstract class Selection
             {
                 Selector selector = makeSelector(cfDef, rawSelector, names, metadata);
                 selectors.add(selector);
-                if (selector instanceof WritetimeOrTTLSelector)
-                {
-                    collectTimestamps |= ((WritetimeOrTTLSelector)selector).isWritetime;
-                    collectTTLs |= !((WritetimeOrTTLSelector)selector).isWritetime;
-                }
+                collectTimestamps |= selector.usesTimestamps();
+                collectTTLs |= selector.usesTTLs();
             }
             return new SelectionWithProcessing(names, metadata, selectors, collectTimestamps, collectTTLs);
         }
@@ -374,6 +371,12 @@ public abstract class Selection
     private interface Selector extends AssignementTestable
     {
         public ByteBuffer compute(ResultSetBuilder rs) throws InvalidRequestException;
+
+        /** Returns true if the selector acts on a column's timestamp, false otherwise. */
+        public boolean usesTimestamps();
+
+        /** Returns true if the selector acts on a column's TTL, false otherwise. */
+        public boolean usesTTLs();
     }
 
     private static class SimpleSelector implements Selector
@@ -399,6 +402,16 @@ public abstract class Selection
             return receiver.type.isValueCompatibleWith(type);
         }
 
+        public boolean usesTimestamps()
+        {
+            return false;
+        }
+
+        public boolean usesTTLs()
+        {
+            return false;
+        }
+
         @Override
         public String toString()
         {
@@ -431,6 +444,22 @@ public abstract class Selection
             return receiver.type.isValueCompatibleWith(fun.returnType());
         }
 
+        public boolean usesTimestamps()
+        {
+            for (Selector s : argSelectors)
+                if (s.usesTimestamps())
+                    return true;
+            return false;
+        }
+
+        public boolean usesTTLs()
+        {
+            for (Selector s : argSelectors)
+                if (s.usesTTLs())
+                    return true;
+            return false;
+        }
+
         @Override
         public String toString()
         {
@@ -476,6 +505,17 @@ public abstract class Selection
             return receiver.type.isValueCompatibleWith(isWritetime ? LongType.instance : Int32Type.instance);
         }
 
+
+        public boolean usesTimestamps()
+        {
+            return isWritetime;
+        }
+
+        public boolean usesTTLs()
+        {
+            return !isWritetime;
+        }
+
         @Override
         public String toString()
         {