Ncp: Ensure to always send the "end of scan" indicator spinel message (#396)

This commit changes the way we send out the "end of scan" indicator
message to ensure that it is always sent out and not dropped. If
there is no buffer space availabe when scan finishes, we remember
and send it out when buffer space becomes available later.

src/ncp/ncp_base.cpp

@@ -346,6 +346,7 @@ NcpBase::NcpBase():
     mSupportedChannelMask = kPhySupportedChannelMask;
     mChannelMask = mSupportedChannelMask;
     mScanPeriod = 200; // ms
+    mShouldSignalEndOfScan = false;
     sNcpContext = this;
     mChangedFlags = NCP_PLAT_RESET_REASON;
     mAllowLocalNetworkDataChange = false;
@@ -430,6 +431,8 @@ void NcpBase::HandleActiveScanResult_Jump(otActiveScanResult *result)
 
 void NcpBase::HandleActiveScanResult(otActiveScanResult *result)
 {
+    ThreadError errorCode;
+
     if (result)
     {
         uint8_t flags = static_cast<uint8_t>(result->mVersion << SPINEL_BEACON_THREAD_FLAG_VERSION_SHIFT);
@@ -466,13 +469,21 @@ void NcpBase::HandleActiveScanResult(otActiveScanResult *result)
     {
         // We are finished with the scan, so send out
         // a property update indicating such.
-        SendPropertyUpdate(
+        errorCode = SendPropertyUpdate(
             SPINEL_HEADER_FLAG | SPINEL_HEADER_IID_0,
             SPINEL_CMD_PROP_VALUE_IS,
             SPINEL_PROP_MAC_SCAN_STATE,
             SPINEL_DATATYPE_UINT8_S,
             SPINEL_SCAN_STATE_IDLE
         );
+
+        // If we could not send the end of scan inidciator message now (no
+        // buffer space), we set `mShouldSignalEndOfScan` to true to send
+        // it out when buffer space becomes available.
+        if (errorCode != kThreadError_None)
+        {
+            mShouldSignalEndOfScan = true;
+        }
     }
 }
 
@@ -651,6 +662,20 @@ void NcpBase::HandleSpaceAvailableInTxBuffer(void)
         while ((mDroppedReplyTidBitSet & (1 << mDroppedReplyTid)) == 0);
     }
 
+    if (mShouldSignalEndOfScan)
+    {
+        SuccessOrExit(
+            SendPropertyUpdate(
+                SPINEL_HEADER_FLAG | SPINEL_HEADER_IID_0,
+                SPINEL_CMD_PROP_VALUE_IS,
+                SPINEL_PROP_MAC_SCAN_STATE,
+                SPINEL_DATATYPE_UINT8_S,
+                SPINEL_SCAN_STATE_IDLE
+        ));
+
+        mShouldSignalEndOfScan = false;
+    }
+
     UpdateChangedProps();
 
 exit:
@@ -2238,6 +2263,7 @@ ThreadError NcpBase::SetPropertyHandler_MAC_SCAN_STATE(uint8_t header, spinel_pr
 
         case SPINEL_SCAN_STATE_BEACON:
             gActiveScanContextHack = this;
+            mShouldSignalEndOfScan = false;
             errorCode = otActiveScan(
                             mChannelMask,
                             mScanPeriod,

src/ncp/ncp_base.hpp

@@ -397,6 +397,8 @@ private:
 
     uint32_t mChangedFlags;
 
+    bool mShouldSignalEndOfScan;
+
     spinel_tid_t mDroppedReplyTid;
 
     uint16_t mDroppedReplyTidBitSet;