fix: Merge data to init document (#293)

Init document is the aggregated data documen that the user has
specified using multiple `add_data` calls. Each query evaluation
starts of by initializing the current data to the init document.

Previously `add_data` was incorrectly added to the current document,
causing the added data to be lost if the addition happened after query
evaluation.

With this fix, scenarios where data addition may be interspersed with
query evaluation calls are supported.

Also provide a get_data method to obtain the (init) data document.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/tests/interpreter/mod.rs

@@ -422,3 +422,32 @@ fn one_yaml() -> Result<()> {
 fn run(path: &str) {
     yaml_test(path).unwrap()
 }
+
+#[test]
+fn test_get_data() -> Result<()> {
+    let mut engine = Engine::new();
+
+    // Merge { "x" : 1, "y" : {} }
+    engine.add_data(Value::from_json_str(r#"{ "x" : 1, "y" : {}}"#)?)?;
+
+    // Merge { "z" : 2 }
+    engine.add_data(Value::from_json_str(r#"{ "z" : 2 }"#)?)?;
+
+    // Add a policy
+    engine.add_policy("policy.rego".to_string(), "package a".to_string())?;
+
+    // Evaluate virtual data document. The virtual document includes all rules as well.
+    let v_data = engine.eval_query("data".to_string(), false)?.result[0].expressions[0]
+        .value
+        .clone();
+    // There must be an empty package.
+    assert_eq!(v_data["a"], Value::new_object());
+
+    // Get the data document.
+    let data = engine.get_data();
+
+    // There must NOT be any value of `a`.
+    assert_eq!(data["a"], Value::Undefined);
+
+    Ok(())
+}