[child-supervision] change how child supervision gets started/stopped (#2627)

This commit removes the `Start()/Stop()` APIs from `ChildSupervisor`
class. Instead `ChildSupervisor` class itself would decide when to
start/stop. It registers a `Notifier` callback to be notified when the
Thread role changes and/or when a child is added or removed. If MLE
operation is enabled and there is at least one "valid" child in the
child table, child supervision starts, otherwise it is stopped.

src/core/common/notifier.hpp

@@ -80,7 +80,7 @@ public:
          *
          * @param[in] aCallback    A reference to callback instance.
          * @param[in] aFlags       A bit-field indicating specific state that has changed. See `OT_CHANGED_<STATE>`
-         *                         definitions in `types.h`.
+         *                         definitions in `instance.h`.
          *
          */
         typedef void (*Handler)(Callback &aCallback, uint32_t aFlags);

src/core/thread/thread_netif.cpp

@@ -140,7 +140,6 @@ otError ThreadNetif::Up(void)
 #if OPENTHREAD_ENABLE_CHANNEL_MONITOR
         GetInstance().GetChannelMonitor().Start();
 #endif
-        mChildSupervisor.Start();
         mMleRouter.Enable();
         mIsUp = true;
     }
@@ -157,7 +156,6 @@ otError ThreadNetif::Down(void)
 #if OPENTHREAD_ENABLE_CHANNEL_MONITOR
     GetInstance().GetChannelMonitor().Stop();
 #endif
-    mChildSupervisor.Stop();
     mMleRouter.Disable();
     mMeshForwarder.Stop();
     GetIp6().RemoveNetif(*this);

src/core/utils/child_supervision.cpp

@@ -53,28 +53,15 @@ ChildSupervisor::ChildSupervisor(Instance &aInstance)
     : InstanceLocator(aInstance)
     , mSupervisionInterval(kDefaultSupervisionInterval)
     , mTimer(aInstance, &ChildSupervisor::HandleTimer, this)
+    , mNotifierCallback(&ChildSupervisor::HandleStateChanged, this)
 {
-}
-
-void ChildSupervisor::Start(void)
-{
-    VerifyOrExit(mSupervisionInterval != 0);
-    VerifyOrExit(!mTimer.IsRunning());
-    mTimer.Start(kOneSecond);
-
-exit:
-    return;
-}
-
-void ChildSupervisor::Stop(void)
-{
-    mTimer.Stop();
+    aInstance.GetNotifier().RegisterCallback(mNotifierCallback);
 }
 
 void ChildSupervisor::SetSupervisionInterval(uint16_t aInterval)
 {
     mSupervisionInterval = aInterval;
-    Start();
+    CheckState();
 }
 
 Child *ChildSupervisor::GetDestination(const Message &aMessage) const
@@ -147,7 +134,7 @@ void ChildSupervisor::HandleTimer(void)
 
     for (uint8_t i = 0; i < numChildren; i++, child++)
     {
-        if (!child->IsStateValidOrRestoring())
+        if (child->GetState() != Child::kStateValid)
         {
             continue;
         }
@@ -166,6 +153,52 @@ exit:
     return;
 }
 
+void ChildSupervisor::CheckState(void)
+{
+    bool            shouldRun   = false;
+    uint8_t         numChildren = 0;
+    Mle::MleRouter &mle         = GetInstance().Get<Mle::MleRouter>();
+
+    // Child Supervision should run if `mSupervisionInterval` is not
+    // zero, Thread MLE operation is enabled, and there is at least one
+    // "valid" child in the child table.
+
+    VerifyOrExit(mSupervisionInterval != 0);
+    VerifyOrExit(mle.GetRole() != OT_DEVICE_ROLE_DISABLED);
+
+    for (Child *child = mle.GetChildren(&numChildren); numChildren > 0; child++, numChildren--)
+    {
+        VerifyOrExit(child->GetState() != Child::kStateValid, shouldRun = true);
+    }
+
+exit:
+
+    if (shouldRun && !mTimer.IsRunning())
+    {
+        mTimer.Start(kOneSecond);
+        otLogInfoUtil(GetInstance(), "Starting Child Supervision");
+    }
+
+    if (!shouldRun && mTimer.IsRunning())
+    {
+        mTimer.Stop();
+        otLogInfoUtil(GetInstance(), "Stopping Child Supervision");
+    }
+}
+
+void ChildSupervisor::HandleStateChanged(Notifier::Callback &aCallback, uint32_t aFlags)
+{
+    aCallback.GetOwner<ChildSupervisor>().HandleStateChanged(aFlags);
+}
+
+void ChildSupervisor::HandleStateChanged(uint32_t aFlags)
+{
+    if ((aFlags & (OT_CHANGED_THREAD_ROLE | OT_CHANGED_THREAD_CHILD_ADDED | OT_CHANGED_THREAD_CHILD_REMOVED)) != 0)
+    {
+        CheckState();
+    }
+}
+
 #endif // #if OPENTHREAD_FTD
 
 SupervisionListener::SupervisionListener(Instance &aInstance)

src/core/utils/child_supervision.hpp

@@ -39,6 +39,7 @@
 
 #include "common/locator.hpp"
 #include "common/message.hpp"
+#include "common/notifier.hpp"
 #include "common/timer.hpp"
 #include "mac/mac_frame.hpp"
 #include "thread/topology.hpp"
@@ -155,11 +156,15 @@ private:
     };
 
     void        SendMessage(Child &aChild);
+    void        CheckState(void);
     static void HandleTimer(Timer &aTimer);
     void        HandleTimer(void);
+    static void HandleStateChanged(Notifier::Callback &aCallback, uint32_t aFlags);
+    void        HandleStateChanged(uint32_t aFlags);
 
-    uint16_t   mSupervisionInterval;
-    TimerMilli mTimer;
+    uint16_t           mSupervisionInterval;
+    TimerMilli         mTimer;
+    Notifier::Callback mNotifierCallback;
 };
 
 #else // #if OPENTHREAD_ENABLE_CHILD_SUPERVISION && OPENTHREAD_FTD