[mac] ensure MAC operations get started from a tasklet (#2427)

This commit changes how MAC operations (frame tx, active/energy scan,
etc.) get started by adding a new tasklet `mOperationTask` which
is used to start/perform any pending operation. This ensures that
callbacks related to an operation are invoked after the method which
initiates the operation returns. For example, when a new `Mac::Sender`
is registered with a call to `Mac::SendFrameRequest()`, its callback
`FrameRequestHandler` would be invoked after the `SendFrameRequest()`
call itself returns.

The `mOperationTask` serves two purposes. It's mainly used for
starting a new operation. It is also used during Energy Scan to take
an RSSI sample.

src/core/mac/mac.cpp

@@ -89,6 +89,7 @@ Mac::Mac(Instance &aInstance):
 #if OPENTHREAD_CONFIG_STAY_AWAKE_BETWEEN_FRAGMENTS
     mDelaySleep(false),
 #endif
+    mOperationTask(aInstance, &Mac::PerformOperation, this),
     mMacTimer(aInstance, &Mac::HandleMacTimer, this),
     mBackoffTimer(aInstance, &Mac::HandleBeginTransmit, this),
     mReceiveTimer(aInstance, &Mac::HandleReceiveTimer, this),
@@ -109,7 +110,6 @@ Mac::Mac(Instance &aInstance):
     mEnergyScanCurrentMaxRssi(kInvalidRssiValue),
     mScanContext(NULL),
     mActiveScanHandler(NULL), // Initialize `mActiveScanHandler` and `mEnergyScanHandler` union
-    mEnergyScanSampleRssiTask(aInstance, &Mac::HandleEnergyScanSampleRssi, this),
     mPcapCallback(NULL),
     mPcapCallbackContext(NULL),
 #if OPENTHREAD_ENABLE_MAC_FILTER
@@ -273,7 +273,7 @@ void Mac::PerformEnergyScan(void)
     {
         RadioReceive(mScanChannel);
         mEnergyScanCurrentMaxRssi = kInvalidRssiValue;
-        mEnergyScanSampleRssiTask.Post();
+        mOperationTask.Post();
 
         if (mScanDuration != 0)
         {
@@ -335,17 +335,10 @@ void Mac::EnergyScanDone(int8_t aRssi)
     PerformEnergyScan();
 }
 
-void Mac::HandleEnergyScanSampleRssi(Tasklet &aTasklet)
-{
-    aTasklet.GetOwner<Mac>().HandleEnergyScanSampleRssi();
-}
-
-void Mac::HandleEnergyScanSampleRssi(void)
+void Mac::SampleRssi(void)
 {
     int8_t rssi;
 
-    VerifyOrExit(mOperation == kOperationEnergyScan);
-
     rssi = otPlatRadioGetRssi(&GetInstance());
 
     if (rssi != kInvalidRssiValue)
@@ -362,11 +355,8 @@ void Mac::HandleEnergyScanSampleRssi(void)
     }
     else
     {
-        mEnergyScanSampleRssiTask.Post();
+        mOperationTask.Post();
     }
-
-exit:
-    return;
 }
 
 otError Mac::RegisterReceiver(Receiver &aReceiver)
@@ -577,6 +567,30 @@ void Mac::StartOperation(Operation aOperation)
         break;
     }
 
+    if (mOperation == kOperationIdle)
+    {
+        mOperationTask.Post();
+    }
+}
+
+void Mac::PerformOperation(Tasklet &aTasklet)
+{
+    aTasklet.GetOwner<Mac>().PerformOperation();
+}
+
+void Mac::PerformOperation(void)
+{
+    // The `mOperationTask` tasklet serves two purposes:
+    //
+    // (a) it is used to start a pending operation,
+    // (b) while performing Energy Scan, it is used to take RSSI samples.
+
+    if (mOperation == kOperationEnergyScan)
+    {
+        SampleRssi();
+        ExitNow();
+    }
+
     VerifyOrExit(mOperation == kOperationIdle);
 
     // `WaitingForData` should be checked before any other pending
@@ -634,7 +648,13 @@ void Mac::FinishOperation(void)
     otLogDebgMac(GetInstance(), "Finishing operation \"%s\"", OperationToString(mOperation));
 
     mOperation = kOperationIdle;
-    StartOperation(kOperationIdle);
+
+    // Note that we do not want to post the `mOperationTask` here and
+    // instead we do a direct call to `PerformOperation()`. This helps
+    // ensure that if there is no pending operation, the radio is
+    // switched to idle mode immediately.
+
+    PerformOperation();
 }
 
 void Mac::GenerateNonce(const ExtAddress &aAddress, uint32_t aFrameCounter, uint8_t aSecurityLevel, uint8_t *aNonce)

src/core/mac/mac.hpp

@@ -644,8 +644,8 @@ private:
     void HandleBeginTransmit(void);
     static void HandleReceiveTimer(Timer &aTimer);
     void HandleReceiveTimer(void);
-    static void HandleEnergyScanSampleRssi(Tasklet &aTasklet);
-    void HandleEnergyScanSampleRssi(void);
+    static void PerformOperation(Tasklet &aTasklet);
+    void PerformOperation(void);
 
     void StartCsmaBackoff(void);
 
@@ -654,6 +654,7 @@ private:
     void PerformActiveScan(void);
     void PerformEnergyScan(void);
     void ReportEnergyScanResult(int8_t aRssi);
+    void SampleRssi(void);
 
     otError RadioTransmit(Frame *aSendFrame);
     otError RadioReceive(uint8_t aChannel);
@@ -674,6 +675,8 @@ private:
     bool mDelaySleep              : 1;
 #endif
 
+    Tasklet mOperationTask;
+
     TimerMilli mMacTimer;
 #if OPENTHREAD_CONFIG_ENABLE_PLATFORM_USEC_TIMER
     TimerMicro mBackoffTimer;
@@ -708,7 +711,6 @@ private:
         ActiveScanHandler mActiveScanHandler;
         EnergyScanHandler mEnergyScanHandler;
     };
-    Tasklet mEnergyScanSampleRssiTask;
 
     otLinkPcapCallback mPcapCallback;
     void *mPcapCallbackContext;