Migrate System::Timer to BitMapObjectPool (project-chip#11487)

#### Problem

The multiple object pool implementations are being consolidated,
but `System::Timer` isn't ready.

#### Change overview

Transitionally, convert `System::Timer` to use `BitMapObjectPool`
instead of `System::ObjectPool`, so that it doesn't obstruct
refactoring.

Any platform using both `System::Timer` and
`CHIP_SYSTEM_CONFIG_USE_HEAP` will show a `.bss` increase from this PR;
this will be undone by a later step in pool conversion.

#### Testing

CI; no changes to functionality.

src/system/SystemTimer.cpp

@@ -74,28 +74,45 @@ namespace System {
  *******************************************************************************
  */
 
-ObjectPool<Timer, CHIP_SYSTEM_CONFIG_NUM_TIMERS> Timer::sPool;
+BitMapObjectPool<Timer, CHIP_SYSTEM_CONFIG_NUM_TIMERS> Timer::sPool;
+Stats::count_t Timer::mNumInUse      = 0;
+Stats::count_t Timer::mHighWatermark = 0;
 
 Timer * Timer::New(System::Layer & systemLayer, System::Clock::Timeout delay, TimerCompleteCallback onComplete, void * appState)
 {
-    Timer * timer = Timer::sPool.TryCreate();
+    Timer * timer = Timer::sPool.CreateObject();
     if (timer == nullptr)
     {
         ChipLogError(chipSystemLayer, "Timer pool EMPTY");
     }
     else
     {
-        timer->AppState     = appState;
+        timer->mAppState    = appState;
         timer->mSystemLayer = &systemLayer;
         timer->mAwakenTime  = SystemClock().GetMonotonicTimestamp() + delay;
         if (!__sync_bool_compare_and_swap(&timer->mOnComplete, nullptr, onComplete))
         {
             chipDie();
         }
+#if CHIP_SYSTEM_CONFIG_PROVIDE_STATISTICS
+        static_assert(CHIP_SYSTEM_CONFIG_NUM_TIMERS < CHIP_SYS_STATS_COUNT_MAX, "Stats count is too small");
+        if (++mNumInUse > mHighWatermark)
+        {
+            mHighWatermark = mNumInUse;
+        }
+#endif // CHIP_SYSTEM_CONFIG_PROVIDE_STATISTICS
     }
     return timer;
 }
 
+void Timer::Release()
+{
+    Timer::sPool.ReleaseObject(this);
+#if CHIP_SYSTEM_CONFIG_PROVIDE_STATISTICS
+    --mNumInUse;
+#endif // CHIP_SYSTEM_CONFIG_PROVIDE_STATISTICS
+}
+
 void Timer::Clear()
 {
     TimerCompleteCallback lOnComplete = this->mOnComplete;
@@ -107,7 +124,7 @@ void Timer::Clear()
     VerifyOrReturn(__sync_bool_compare_and_swap(&mOnComplete, lOnComplete, nullptr));
 
     // Since this thread changed the state of mOnComplete, release the timer.
-    AppState     = nullptr;
+    mAppState    = nullptr;
     mSystemLayer = nullptr;
 }
 
@@ -116,15 +133,15 @@ void Timer::HandleComplete()
     // Save information needed to perform the callback.
     Layer * lLayer                          = this->mSystemLayer;
     const TimerCompleteCallback lOnComplete = this->mOnComplete;
-    void * lAppState                        = this->AppState;
+    void * lAppState                        = this->mAppState;
 
     // Check if timer is armed
     VerifyOrReturn(lOnComplete != nullptr, );
     // Atomically disarm if the value has not changed.
     VerifyOrReturn(__sync_bool_compare_and_swap(&this->mOnComplete, lOnComplete, nullptr), );
 
     // Since this thread changed the state of mOnComplete, release the timer.
-    AppState     = nullptr;
+    mAppState    = nullptr;
     mSystemLayer = nullptr;
     this->Release();
 
@@ -193,7 +210,7 @@ Timer * Timer::List::Remove(TimerCompleteCallback aOnComplete, void * aAppState)
     Timer * previous = nullptr;
     for (Timer * timer = mHead; timer != nullptr; timer = timer->mNextTimer)
     {
-        if (timer->mOnComplete == aOnComplete && timer->AppState == aAppState)
+        if (timer->mOnComplete == aOnComplete && timer->mAppState == aAppState)
         {
             if (previous == nullptr)
             {

src/system/SystemTimer.h

@@ -32,11 +32,12 @@
 
 // Include dependent headers
 #include <lib/support/DLLUtil.h>
+#include <lib/support/Pool.h>
 
 #include <system/SystemClock.h>
 #include <system/SystemError.h>
+#include <system/SystemLayer.h>
 #include <system/SystemMutex.h>
-#include <system/SystemObject.h>
 #include <system/SystemStats.h>
 
 #if CHIP_SYSTEM_CONFIG_USE_DISPATCH
@@ -57,7 +58,7 @@ using TimerCompleteCallback = void (*)(Layer * aLayer, void * appState);
 /**
  * This is an Object-pool based class that System::Layer implementations can use to assist in providing timer functions.
  */
-class DLL_EXPORT Timer : public Object
+class DLL_EXPORT Timer
 {
 public:
     /**
@@ -185,11 +186,16 @@ class DLL_EXPORT Timer : public Object
     Timer() = default;
 
     /**
-     * Obtain a new timer from the system object pool.
+     * Obtain a new timer from the object pool.
      */
     static Timer * New(System::Layer & systemLayer, System::Clock::Timeout delay, TimerCompleteCallback onComplete,
                        void * appState);
 
+    /**
+     * Return a timer to the object pool.
+     */
+    void Release();
+
     /**
      * Return the expiration time.
      */
@@ -215,20 +221,24 @@ class DLL_EXPORT Timer : public Object
     /**
      * Read timer pool statistics.
      */
-    static void GetStatistics(chip::System::Stats::count_t & aNumInUse, chip::System::Stats::count_t & aHighWatermark)
+    static void GetStatistics(Stats::count_t & aNumInUse, Stats::count_t & aHighWatermark)
     {
-        sPool.GetStatistics(aNumInUse, aHighWatermark);
+        aNumInUse      = mNumInUse;
+        aHighWatermark = mHighWatermark;
     }
 
 private:
     friend class LayerImplLwIP;
-    static ObjectPool<Timer, CHIP_SYSTEM_CONFIG_NUM_TIMERS> sPool;
+    static BitMapObjectPool<Timer, CHIP_SYSTEM_CONFIG_NUM_TIMERS> sPool;
+    static Stats::count_t mNumInUse;
+    static Stats::count_t mHighWatermark;
 
     TimerCompleteCallback mOnComplete;
     Clock::Timestamp mAwakenTime;
     Timer * mNextTimer;
 
     Layer * mSystemLayer;
+    void * mAppState;
 
 #if CHIP_SYSTEM_CONFIG_USE_DISPATCH
     friend class LayerImplSelect;