CNDB-14182: Add some null checks in InMemoryTrie

Currently returning a null from an Upserter may corrupt
the trie state which may end up in a serious problem.
It is better to crash instead.

Author: pkolaczk

src/java/org/apache/cassandra/db/tries/InMemoryTrie.java

@@ -22,7 +22,10 @@
 import java.util.concurrent.atomic.AtomicReferenceArray;
 import java.util.function.Predicate;
 
+import javax.annotation.Nonnull;
+
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
 import com.google.common.base.Predicates;
 
 import org.agrona.concurrent.UnsafeBuffer;
@@ -296,8 +299,9 @@ private int allocateNewObject()
      * @return A content id that can be used to reference the content, encoded as ~index where index is the
      *         position of the value in the content array.
      */
-    private int addContent(T value) throws TrieSpaceExhaustedException
+    private int addContent(@Nonnull T value) throws TrieSpaceExhaustedException
     {
+        Preconditions.checkNotNull(value, "Content value cannot be null");
         int index = objectAllocator.allocate();
         int leadBit = getBufferIdx(index, CONTENTS_START_SHIFT, CONTENTS_START_SIZE);
         int ofs = inBufferOffset(index, leadBit, CONTENTS_START_SIZE);
@@ -1329,9 +1333,9 @@ public interface UpsertTransformerWithKeyProducer<T, U>
          * @param existing Existing content for this key, or null if there isn't any.
          * @param update   The update, always non-null.
          * @param keyState An interface that can be used to retrieve the path of the value being updated.
-         * @return The combined value to use.
+         * @return The combined value to use. Cannot be null.
          */
-        T apply(T existing, U update, KeyProducer<T> keyState);
+        @Nonnull T apply(T existing, @Nonnull U update, @Nonnull KeyProducer<T> keyState);
     }
 
     /**
@@ -1353,7 +1357,7 @@ public interface UpsertTransformer<T, U> extends UpsertTransformerWithKeyProduce
          * @param update   The update, always non-null.
          * @return The combined value to use. Cannot be null.
          */
-        T apply(T existing, U update);
+        @Nonnull T apply(T existing, @Nonnull U update);
 
         /**
          * Version of the above that also provides the path of a value being updated.
@@ -1363,7 +1367,7 @@ public interface UpsertTransformer<T, U> extends UpsertTransformerWithKeyProduce
          * @param keyState An interface that can be used to retrieve the path of the value being updated.
          * @return The combined value to use. Cannot be null.
          */
-        default T apply(T existing, U update, KeyProducer<T> keyState)
+        default @Nonnull T apply(T existing, @Nonnull U update, @Nonnull KeyProducer<T> keyState)
         {
             return apply(existing, update);
         }
@@ -1441,7 +1445,10 @@ void applyContent() throws TrieSpaceExhaustedException
             {
                 T existingContent = state.getContent();
                 T combinedContent = transformer.apply(existingContent, content, state);
-                state.setContent(combinedContent, // can be null
+                if (combinedContent == null)
+                    throw new AssertionError("Transformer " + transformer + " returned null content for "
+                                             + existingContent + ", " + content);
+                state.setContent(combinedContent,
                                  state.currentDepth >= forcedCopyDepth); // this is called at the start of processing
             }
         }

test/unit/org/apache/cassandra/db/tries/CellReuseTest.java

@@ -304,9 +304,9 @@ static void addThrowingEntry(ByteComparable b,
                    {
                        if (upd instanceof Boolean)
                        {
-                           if (upd != null && !((Boolean) upd))
+                           if (!((Boolean) upd))
                                throw new TestException();
-                           return null;
+                           return true;
                        }
                        else
                            return upd;