rpc: multicall requires methodName as the struct's first member

Faults clearly instead of the confusing positional parse error it replaces.

src/rpc/xmlrpc_tinyxml2.cc

@@ -326,15 +326,20 @@ process_document(const tinyxml2::XMLDocument* doc, tinyxml2::XMLPrinter* printer
     auto& result_list     = result.as_list();
     auto  parent_elements = element_access(doc->RootElement(), {"params", "param", "value", "array", "data"});
     for (auto child = parent_elements->FirstChildElement("value"); child; child = child->NextSiblingElement("value")) {
-      auto sub_method_name = element_access(child, {"struct", "member", "value", "string"})->GetText();
+      auto method_name_member = element_access(child, {"struct", "member"});
+      auto member_name        = method_name_member->FirstChildElement("name");
+
+      if (member_name == nullptr || member_name->GetText() == nullptr ||
+          std::strncmp(member_name->GetText(), "methodName", sizeof("methodName")) != 0)
+        throw rpc_error(XMLRPC_PARSE_ERROR, "multicall struct's first member must be methodName");
+
+      auto sub_method_name = element_access(method_name_member, {"value", "string"})->GetText();
 
       if (sub_method_name == nullptr)
         throw rpc_error(XMLRPC_PARSE_ERROR, "multicall methodName element is empty");
       // If sub_params ends up a nullptr at the end of this if-chian,
       // execute_command will turn it into an empty list
-      auto sub_params = element_access(child, {"struct", "member"});
-      if (sub_params != nullptr)
-        sub_params = sub_params->NextSiblingElement("member");
+      auto sub_params = method_name_member->NextSiblingElement("member");
       if (sub_params != nullptr)
         sub_params = sub_params->FirstChildElement("value");
       if (sub_params != nullptr)

test/rpc/test_xmlrpc.cc

@@ -192,12 +192,54 @@ TestXmlrpc::test_response_size_limit() {
   CPPUNIT_ASSERT_EQUAL(expected, output);
 }
 
+namespace {
+
+const std::string multicall_method_name =
+  "<member><name>methodName</name><value><string>xmlrpc_reflect</string></value></member>";
+const std::string multicall_params =
+  "<member><name>params</name><value><array><data>"
+  "<value><string></string></value><value><string>a</string></value>"
+  "</data></array></value></member>";
+
+std::string
+multicall_request(const std::string& members) {
+  return "<?xml version=\"1.0\"?><methodCall><methodName>system.multicall</methodName>"
+         "<params><param><value><array><data><value><struct>" + members +
+         "</struct></value></data></array></value></param></params></methodCall>";
+}
+
+}
+
+void
+TestXmlrpc::test_multicall_member_order() {
+  auto call = [this](const std::string& input) {
+    std::string output;
+    m_xmlrpc.process(input.c_str(), input.size(), [&output](const char* c, uint32_t l){ output.append(c, l); return true;});
+    return output;
+  };
+
+  // methodName first is the only order accepted; the positive control proves
+  // the harness drives the real code path rather than a stub.
+  std::string ordered = call(multicall_request(multicall_method_name + multicall_params));
+  CPPUNIT_ASSERT(ordered.find("faultCode") == std::string::npos);
+
+  // params before methodName is rejected with a clear top-level fault,
+  // instead of the "could not find expected element string" of a positional read.
+  std::string expected_fault =
+    "<?xml version=\"1.0\"?><methodResponse><fault><value><struct>"
+    "<member><name>faultCode</name><value><i8>-503</i8></value></member>"
+    "<member><name>faultString</name><value><string>multicall struct's first member must be methodName</string></value></member>"
+    "</struct></value></fault></methodResponse>";
+  CPPUNIT_ASSERT_EQUAL(expected_fault, call(multicall_request(multicall_params + multicall_method_name)));
+}
+
 #else
 
 void TestXmlrpc::test_invalid_utf8() {}
 void TestXmlrpc::test_basics() {}
 void TestXmlrpc::test_size_limit() {}
 void TestXmlrpc::test_response_size_limit() {}
+void TestXmlrpc::test_multicall_member_order() {}
 void TestXmlrpc::setUp() {}
 void TestXmlrpc::tearDown() {}
 

test/rpc/test_xmlrpc.h

@@ -11,6 +11,7 @@ class TestXmlrpc : public test_fixture {
   CPPUNIT_TEST(test_invalid_utf8);
   CPPUNIT_TEST(test_size_limit);
   CPPUNIT_TEST(test_response_size_limit);
+  CPPUNIT_TEST(test_multicall_member_order);
 
   CPPUNIT_TEST_SUITE_END();
 
@@ -24,6 +25,7 @@ public:
   void test_invalid_utf8();
   void test_size_limit();
   void test_response_size_limit();
+  void test_multicall_member_order();
 
 private:
   std::unique_ptr<TestMainThread> m_test_main_thread;