[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)

Change-Id: Ic1b41b8b60e5953142854f761e377e080f1937fc

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 +41,34 @@
 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.
+     * Creates a new instance of VendorServer.
      *
-     * @param[in] aNcp  A reference to the NCP controller.
+     * Custom vendor servers should implement this method to return an object of the derived class.
      *
+     * @param[in]  aNcp  The OpenThread controller object.
+     *
+     * @returns  New derived VendorServer instance.
      */
-    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

@@ -97,16 +97,12 @@ class ControllerOpenThread : public MainloopProcessor
     void Deinit(void);
 
     /**
-     * This method get mInstance pointer.
-     *
-     * @retval The pointer of mInstance.
+     * Returns an OpenThread instance.
      *
+     * @retval Non-null OpenThread instance if `ControllerOpenThread::Init()` has been called.
+     *         Otherwise, it's guaranteed to be `null`
      */
-    otInstance *GetInstance(void)
-    {
-        assert(mInstance != nullptr);
-        return mInstance;
-    }
+    otInstance *GetInstance(void) { return mInstance; }
 
     /**
      * This method gets the thread functionality helper.