ncp-uart: Fix for platforms which call otPlatUartSendDone() reentrantly (#306)

This change makes the `NcpUart` class behave more robustly against
various sorts of `otPlatUart` API implementations; specifically those
that might call `otPlatUartSendDone()` directly from
`otPlatUartSend()`.

Without this change, all NCP frames after the first few at startup
will be squelched.

src/ncp/ncp_base.cpp

@@ -298,6 +298,7 @@ static spinel_status_t ResetReasonToSpinelStatus(otPlatResetReason reason)
 // ----------------------------------------------------------------------------
 
 NcpBase::NcpBase():
+    mSendDoneTask(&SendDoneTask, this),
     mUpdateChangedPropsTask(&UpdateChangedProps, this)
 {
     mSupportedChannelMask = kPhySupportedChannelMask;
@@ -569,27 +570,40 @@ void NcpBase::HandleReceive(const uint8_t *buf, uint16_t bufLength)
     }
 }
 
-void NcpBase::HandleSendDone()
+void NcpBase::SendDoneTask(void *context)
 {
-    if (mSendQueue.GetHead() != NULL)
-    {
-        Message &message(*mSendQueue.GetHead());
-        mSendQueue.Dequeue(message);
-        HandleDatagramFromStack(message);
-    }
+    NcpBase *obj = reinterpret_cast<NcpBase *>(context);
+    obj->SendDoneTask();
+}
 
-    if (mQueuedGetHeader != 0)
-    {
-        HandleCommandPropertyGet(mQueuedGetHeader, mQueuedGetKey);
-        mQueuedGetHeader = 0;
-    }
+void NcpBase::SendDoneTask(void)
+{
+    if (!mSending) {
+        if (mSendQueue.GetHead() != NULL)
+        {
+            Message &message(*mSendQueue.GetHead());
+            mSendQueue.Dequeue(message);
+            HandleDatagramFromStack(message);
+        }
 
-    if (!mSending)
-    {
-        UpdateChangedProps();
+        if (mQueuedGetHeader != 0)
+        {
+            HandleCommandPropertyGet(mQueuedGetHeader, mQueuedGetKey);
+            mQueuedGetHeader = 0;
+        }
+
+        if (!mSending)
+        {
+            UpdateChangedProps();
+        }
     }
 }
 
+void NcpBase::HandleSendDone(void)
+{
+    mSendDoneTask.Post();
+}
+
 
 // ----------------------------------------------------------------------------
 // MARK: Inbound Command Handlers

src/ncp/ncp_base.hpp

@@ -94,6 +94,12 @@ private:
 
     void UpdateChangedProps(void);
 
+    /**
+     * Trampoline for SendDoneTask().
+     */
+    static void SendDoneTask(void *context);
+
+    void SendDoneTask(void);
 
     static void HandleNetifStateChanged(uint32_t flags, void *context);
 
@@ -309,6 +315,8 @@ private:
 
     spinel_prop_key_t mQueuedGetKey;
 
+    Tasklet mSendDoneTask;
+
     Tasklet mUpdateChangedPropsTask;
 
     MessageQueue mSendQueue;

src/ncp/ncp_uart.cpp

@@ -145,12 +145,17 @@ NcpUart::OutboundFrameSend(void)
 
     if (errorCode == kThreadError_None)
     {
+        // We go ahead and set this to `true` here in case
+        // `otPlatUartSend()` ends up directly calling
+        // `otPlatUartSendDone()`.
+        mSending = true;
+
         errorCode = otPlatUartSend(mSendFrame.GetBuffer(), mSendFrame.GetLength());
     }
 
-    if (errorCode == kThreadError_None)
+    if (errorCode != kThreadError_None)
     {
-        mSending = true;
+        mSending = false;
     }
 
     return errorCode;