[utils] make generic VendorServer (#1995)

This commit changes VendorServer to an abstract class so that it
can be extended to have different implementations.

Fulfills request from #1967 (comment)

src/agent/application.cpp

@@ -77,7 +77,7 @@ Application::Application(const std::string &aInterfaceName,
  , mDBusAgent(mNcp, mBorderAgent.GetPublisher())
 #endif
 #if OTBR_ENABLE_VENDOR_SERVER
- , mVendorServer(mNcp)
+ , mVendorServer(vendor::VendorServer::newInstance(mNcp))
 #endif
 {
  OTBR_UNUSED_VARIABLE(aRestListenAddress);
@@ -104,7 +104,7 @@ void Application::Init(void)
  mDBusAgent.Init();
 #endif
 #if OTBR_ENABLE_VENDOR_SERVER
- mVendorServer.Init();
+ mVendorServer->Init();
 #endif
 }
 

src/agent/application.hpp

@@ -147,7 +147,7 @@ class Application : private NonCopyable
  DBus::DBusAgent mDBusAgent;
 #endif
 #if OTBR_ENABLE_VENDOR_SERVER
- vendor::VendorServer mVendorServer;
+ std::shared_ptr<vendor::VendorServer> mVendorServer;
 #endif
 
  static std::atomic_bool sShouldTerminate;

src/agent/vendor.hpp

@@ -41,29 +41,30 @@
 namespace otbr {
 namespace vendor {
 
+/**
+ * An interface for customized behavior depending on OpenThread API and state.
+ */
 class VendorServer
 {
 public:
+ virtual ~VendorServer(void) = default;
+
  /**
- * The constructor of vendor server.
- *
- * @param[in] aNcp A reference to the NCP controller.
+ * Creates a new instance of VendorServer.
  *
+ * Custom vendor servers should implement this method to return an object of the derived class.
  */
- VendorServer(otbr::Ncp::ControllerOpenThread &aNcp)
- : mNcp(aNcp)
- {
- }
+ static std::shared_ptr<VendorServer> newInstance(otbr::Ncp::ControllerOpenThread &aNcp);
 
  /**
- * This method initializes the vendor server.
+ * Initializes the vendor server.
  *
+ * This will be called by `Application::Init()` after OpenThread instance and other built-in
+ * servers have been created and initialized.
  */
- void Init(void);
-
-private:
- otbr::Ncp::ControllerOpenThread &mNcp;
+ virtual void Init(void) = 0;
 };
+
 } // namespace vendor
 } // namespace otbr
 #endif // OTBR_AGENT_VENDOR_HPP_

src/ncp/ncp_openthread.hpp

@@ -97,14 +97,13 @@ class ControllerOpenThread : public MainloopProcessor
  void Deinit(void);
 
  /**
- * This method get mInstance pointer.
- *
- * @retval The pointer of mInstance.
+ * Returns an OpenThread instance.
  *
+ * @retval The OpenThread instance if `ControllerOpenThread::Init()` has been called,
+ * otherwise `null`
  */
  otInstance *GetInstance(void)
  {
- assert(mInstance != nullptr);
  return mInstance;
  }