[tasklet] use circular linked list (#4309)

This commit contains the following changes:
- Changes `TaskletScheduler` to use a circular linked list.
- Adds `Tasklet::IsPosted()` which indicates whether a tasklet
  is posted or not (by checking `mNext` pointer against NULL).
- Simplifies  processing of tasklets from `ProcessQueuedTasklets()`
  by creating a copy of current list and clearing the main list
  (this way, a newly posted tasklet would automatically trigger
  a call to `otTaskletsSignalPending()`).

src/core/common/tasklet.cpp

@@ -50,78 +50,66 @@ Tasklet::Tasklet(Instance &aInstance, Handler aHandler, void *aOwner)
 }
 
 otError Tasklet::Post(void)
-{
-    return Get<TaskletScheduler>().Post(*this);
-}
-
-TaskletScheduler::TaskletScheduler(void)
-    : mHead(NULL)
-    , mTail(NULL)
-{
-}
-
-otError TaskletScheduler::Post(Tasklet &aTasklet)
 {
     otError error = OT_ERROR_NONE;
 
-    VerifyOrExit(mTail != &aTasklet && aTasklet.mNext == NULL, error = OT_ERROR_ALREADY);
-
-    VerifyOrExit(&aTasklet.Get<TaskletScheduler>() == this);
-
-    if (mTail == NULL)
-    {
-        mHead = &aTasklet;
-        mTail = &aTasklet;
-        otTaskletsSignalPending(&aTasklet.GetInstance());
-    }
-    else
-    {
-        mTail->mNext = &aTasklet;
-        mTail        = &aTasklet;
-    }
+    VerifyOrExit(!IsPosted(), error = OT_ERROR_ALREADY);
+    Get<TaskletScheduler>().PostTasklet(*this);
 
 exit:
     return error;
 }
 
-Tasklet *TaskletScheduler::PopTasklet(void)
+TaskletScheduler::TaskletScheduler(void)
+    : mTail(NULL)
 {
-    Tasklet *task = mHead;
+}
 
-    if (task != NULL)
+void TaskletScheduler::PostTasklet(Tasklet &aTasklet)
+{
+    // Tasklets are saved in a circular singly linked list.
+
+    if (mTail == NULL)
     {
-        mHead = mHead->mNext;
-
-        if (mHead == NULL)
-        {
-            mTail = NULL;
-        }
-
-        task->mNext = NULL;
+        mTail        = &aTasklet;
+        mTail->mNext = mTail;
+        otTaskletsSignalPending(&aTasklet.GetInstance());
+    }
+    else
+    {
+        aTasklet.mNext = mTail->mNext;
+        mTail->mNext   = &aTasklet;
+        mTail          = &aTasklet;
     }
-
-    return task;
 }
 
 void TaskletScheduler::ProcessQueuedTasklets(void)
 {
     Tasklet *tail = mTail;
-    Tasklet *cur;
 
-    while ((cur = PopTasklet()) != NULL)
+    // This method processes all tasklets queued when this is called. We
+    // keep a copy the current list and then clear the main list by
+    // setting `mTail` to NULL. A newly posted tasklet while processing
+    // the currently queued tasklets will then trigger a call to
+    // `otTaskletsSignalPending()`.
+
+    mTail = NULL;
+
+    while (tail != NULL)
     {
-        cur->RunTask();
+        Tasklet *tasklet = tail->mNext;
 
-        // only process tasklets that were queued at the time this method was called
-        if (cur == tail)
+        if (tasklet == tail)
         {
-            if (mHead != NULL)
-            {
-                otTaskletsSignalPending(&mHead->GetInstance());
-            }
-
-            break;
+            tail = NULL;
         }
+        else
+        {
+            tail->mNext = tasklet->mNext;
+        }
+
+        tasklet->mNext = NULL;
+        tasklet->RunTask();
     }
 }
 

src/core/common/tasklet.hpp

@@ -84,11 +84,20 @@ public:
     Tasklet(Instance &aInstance, Handler aHandler, void *aOwner);
 
     /**
-     * This method puts the tasklet on the run queue.
+     * This method puts the tasklet on the tasklet scheduler run queue.
      *
      */
     otError Post(void);
 
+    /**
+     * This method indicates whether the tasklet is posted or not.
+     *
+     * @retval TRUE  The tasklet is posted.
+     * @retval FALSE The tasklet is not posted.
+     *
+     */
+    bool IsPosted(void) const { return (mNext != NULL); }
+
 private:
     void RunTask(void) { mHandler(*this); }
 
@@ -140,6 +149,8 @@ private:
  */
 class TaskletScheduler
 {
+    friend class Tasklet;
+
 public:
     /**
      * This constructor initializes the object.
@@ -147,16 +158,6 @@ public:
      */
     TaskletScheduler(void);
 
-    /**
-     * This method enqueues a tasklet into the run queue.
-     *
-     * @param[in]  aTasklet  A reference to the tasklet to enqueue.
-     *
-     * @retval OT_ERROR_NONE     Successfully enqueued the tasklet.
-     * @retval OT_ERROR_ALREADY  The tasklet was already enqueued.
-     */
-    otError Post(Tasklet &aTasklet);
-
     /**
      * This method indicates whether or not there are tasklets pending.
      *
@@ -164,7 +165,7 @@ public:
      * @retval FALSE  If there are no tasklets pending.
      *
      */
-    bool AreTaskletsPending(void) const { return mHead != NULL; }
+    bool AreTaskletsPending(void) const { return mTail != NULL; }
 
     /**
      * This method processes all tasklets queued when this is called.
@@ -173,9 +174,9 @@ public:
     void ProcessQueuedTasklets(void);
 
 private:
-    Tasklet *PopTasklet(void);
-    Tasklet *mHead;
-    Tasklet *mTail;
+    void PostTasklet(Tasklet &aTasklet);
+
+    Tasklet *mTail; // A circular singly linked-list
 };
 
 /**