[agent] de-initialize `Application` and OpenThread instance (#1183)

Background:

- OpenThread Instance was the last member to destruct in Application,
  and otSysDeinit will be called when destructing OpenThread
  Instance. However, otSysDeinit may call methods of other Application
  members, which have been destructed at the moment.

This commit introduces the Deinit() method to ControllerOpenThread and
other classes to make sure OpenThread are properly de-initialized
before destructing Application. The advantages are:

- ControllerOpenThread initializes OT instance in Init, so adding
  Deinit makes it symmetrical with Init.
  
- Provide a predictable runtime environment (Application object fully
  constructed) for OT instance from otSysInit to otSysDeinit, making
  sure OT can safely access Application members during its lifetime.

2 files changed, 16 insertions, 1 deletions

diff --git a/src/ncp/ncp_openthread.cpp b/src/ncp/ncp_openthread.cpp
index 22a6e162..42882cc7 100644
--- a/src/ncp/ncp_openthread.cpp
+++ b/src/ncp/ncp_openthread.cpp
@@ -83,7 +83,8 @@ ControllerOpenThread::ControllerOpenThread(const char *                     aInt
 
 ControllerOpenThread::~ControllerOpenThread(void)
 {
-    otSysDeinit();
+    // Make sure OpenThread Instance was gracefully de-initialized.
+    assert(mInstance == nullptr);
 }
 
 void ControllerOpenThread::Init(void)
@@ -141,6 +142,14 @@ exit:
     SuccessOrDie(error, "Failed to initialize NCP!");
 }
 
+void ControllerOpenThread::Deinit(void)
+{
+    assert(mInstance != nullptr);
+
+    otSysDeinit();
+    mInstance = nullptr;
+}
+
 void ControllerOpenThread::HandleStateChanged(otChangedFlags aFlags)
 {
     if (aFlags & OT_CHANGED_THREAD_ROLE)
diff --git a/src/ncp/ncp_openthread.hpp b/src/ncp/ncp_openthread.hpp
index b30dc295..e8ce0c6e 100644
--- a/src/ncp/ncp_openthread.hpp
+++ b/src/ncp/ncp_openthread.hpp
@@ -82,6 +82,12 @@ public:
     void Init(void);
 
     /**
+     * This method deinitialize the NCP controller.
+     *
+     */
+    void Deinit(void);
+
+    /**
      * This method get mInstance pointer.
      *
      * @retval The pointer of mInstance.