Throwing exception if cursor state is null

src/workerd/api/sql-test.js

@@ -536,7 +536,9 @@ async function test(storage) {
     assert.deepEqual(rawResults[3], [4,5,6,1,2,3])
 
     // Once an iterator is consumed, it can no longer access the columnNames.
-    assert.deepEqual(iterator.columnNames, [])
+    requireException(() => {
+      iterator.columnNames
+    }, "Error: Cannot call .getColumnNames after Cursor iterator has been consumed.")
 
     // Also works with cursors returned from .exec
     const execIterator = sql.exec(`SELECT * FROM abc, cde`)

src/workerd/api/sql.c++

@@ -118,14 +118,17 @@ jsg::Ref<SqlStorage::Cursor::RawIterator> SqlStorage::Cursor::raw(jsg::Lock&) {
   return jsg::alloc<RawIterator>(JSG_THIS);
 }
 
+// Returns the set of column names for the current Cursor. An exception will be thrown if the
+// iterator has already been fully consumed. The resulting columns may contain duplicate entries,
+// for instance a `SELECT *` across a join of two tables that share a column name.
 kj::Array<jsg::V8Ref<v8::String>> SqlStorage::Cursor::getColumnNames(jsg::Lock& js) {
   KJ_IF_MAYBE(s, state) {
     cachedColumnNames.ensureInitialized(js, (*s)->query);
     return KJ_MAP(name, this->cachedColumnNames.get()) {
       return name.addRef(js);
     };
   } else {
-    return kj::Array<jsg::V8Ref<v8::String>>();
+    kj::throwFatalException(JSG_KJ_EXCEPTION(FAILED, Error, "Cannot call .getColumnNames after Cursor iterator has been consumed."));
   }
 }