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;