alternator: correct handling of missing item in GetItem

According to the documentation, trying to GetItem a non-existant item
should result in an empty response - NOT a response with an empty "Item"
map as we do before this patch.

This patch fixes this case, and adds a test case for it. As usual,
we verify that the test case also works on Amazon DynamoDB, to verify
DynamoDB really behaves the way we thik it does.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

alternator-test/test_item.py

@@ -166,3 +166,11 @@ def test_item_operations_nonexistent_table(dynamodb):
     with pytest.raises(ClientError, match='ResourceNotFoundException'):
         dynamodb.meta.client.put_item(TableName='non_existent_table',
             Item={'a':{'S':'b'}})
+
+# Fetching a non-existant item. According to the DynamoDB doc, "If there is no
+# matching item, GetItem does not return any data and there will be no Item
+# element in the response."
+def test_get_item_missing_item(test_table):
+    p = random_string()
+    c = random_string()
+    assert not "Item" in test_table.get_item(Key={'p': p, 'c': c}, ConsistentRead=True)

alternator/executor.cc

@@ -478,6 +478,13 @@ static Json::Value describe_item(schema_ptr schema, const query::partition_slice
     query::result_view::consume(*query_result, slice, cql3::selection::result_set_builder::visitor(builder, *schema, selection));
 
     auto result_set = builder.build();
+    if (result_set->empty()) {
+        // If there is no matching item, we're supposed to return an empty
+        // object without an Item member - not one with an empty Item member
+        return Json::objectValue;
+    }
+    // FIXME: I think this can't really be a loop, there should be exactly
+    // one result after above we handled the 0 result case
     for (auto& result_row : result_set->rows()) {
         const auto& columns = selection.get_columns();
         auto column_it = columns.begin();