Call taskYIELD after calling FreeRTOS_send in Plaintext_FreeRTOS_send (#491)

FreeRTOS_send adds the packet to be sent to the IP task's queue for
later processing. The packet is sent later by the IP task. When
FreeRTOS is used in collaborative mode (i.e. configUSE_PREEMPTION is 0),
the Plaintext_FreeRTOS_send function returns without actually sending
the packet as the IP task never gets a chance to run.

The fact that Plaintext_FreeRTOS_send returns without actually sending
the packet causes an issue in the MQTT_Connect which expects the CONNECT
packet to be actually sent and waits for CONNACK.

This commit adds a taskYIELD call after calling FreeRTOS_send to ensure
that the IP task gets a chance to run and send the packet before the
Plaintext_FreeRTOS_send function returns.

FreeRTOS-Plus/Source/Application-Protocols/network_transport/freertos_plus_tcp/using_plaintext/using_plaintext.c

@@ -29,6 +29,9 @@
 
 /* FreeRTOS includes. */
 #include "FreeRTOS.h"
+#if ( configUSE_PREEMPTION == 0 )
+    #include "task.h"
+#endif
 
 /* FreeRTOS+TCP includes. */
 #include "FreeRTOS_IP.h"
@@ -42,8 +45,8 @@
 
 /*-----------------------------------------------------------*/
 
-/** 
- * @brief Each compilation unit that consumes the NetworkContext must define it. 
+/**
+ * @brief Each compilation unit that consumes the NetworkContext must define it.
  * It should contain a single pointer as seen below whenever the header file
  * of this transport implementation is included to your project.
  *
@@ -134,14 +137,14 @@ int32_t Plaintext_FreeRTOS_recv( NetworkContext_t * pNetworkContext,
 
     pPlaintextTransportParams = pNetworkContext->pParams;
 
-    /* The TCP socket may have a receive block time.  If bytesToRecv is greater 
-     * than 1 then a frame is likely already part way through reception and 
+    /* The TCP socket may have a receive block time.  If bytesToRecv is greater
+     * than 1 then a frame is likely already part way through reception and
      * blocking to wait for the desired number of bytes to be available is the
-     * most efficient thing to do.  If bytesToRecv is 1 then this may be a 
-     * speculative call to read to find the start of a new frame, in which case 
-     * blocking is not desirable as it could block an entire protocol agent 
-     * task for the duration of the read block time and therefore negatively 
-     * impact performance.  So if bytesToRecv is 1 then don't call recv unless 
+     * most efficient thing to do.  If bytesToRecv is 1 then this may be a
+     * speculative call to read to find the start of a new frame, in which case
+     * blocking is not desirable as it could block an entire protocol agent
+     * task for the duration of the read block time and therefore negatively
+     * impact performance.  So if bytesToRecv is 1 then don't call recv unless
      * it is known that bytes are already available. */
     if( bytesToRecv == 1 )
     {
@@ -182,5 +185,15 @@ int32_t Plaintext_FreeRTOS_send( NetworkContext_t * pNetworkContext,
         socketStatus = 0;
     }
 
+    #if ( configUSE_PREEMPTION == 0 )
+        {
+            /* FreeRTOS_send adds the packet to be sent to the IP task's queue for later processing.
+             * The packet is sent later by the IP task. When FreeRTOS is used in collaborative
+             * mode (i.e. configUSE_PREEMPTION is 0), call taskYIELD to give IP task a chance to run
+             * so that the packet is actually sent before this function returns. */
+            taskYIELD();
+        }
+    #endif
+
     return socketStatus;
 }