Cellular socket improvement: read/write exit on remote closure. (#1070)

If a cellular socket write or read function (i.e. uCellSockWrite()/uCellSockRead()) were in progress when the remote-end of the connection closed the socket, the read/write processes would not notice, they would just continue what they were doing until time out. The functions will now exit if that occurs.

Thanks to arnoutdekimo for pointing this out.

cell/src/u_cell_sock.c

@@ -127,6 +127,7 @@ typedef struct {
     void (*pClosedCallback) (uDeviceHandle_t, int32_t); /**< Set to NULL
                                                      if socket is
                                                      not in use. */
+    bool closedByRemote; /**< Will be set to true if +UUSOCL lands. */
 } uCellSockSocket_t;
 
 /** Definition of a URC handler.
@@ -226,6 +227,7 @@ static uCellSockSocket_t *pSockCreate(int32_t sockHandle,
         pSock->pAsyncClosedCallback = NULL;
         pSock->pDataCallback = NULL;
         pSock->pClosedCallback = NULL;
+        pSock->closedByRemote = false;
     }
 
     return pSock;
@@ -249,6 +251,7 @@ static void sockFree(int32_t sockHandle)
             pSock->pAsyncClosedCallback = NULL;
             pSock->pDataCallback = NULL;
             pSock->pClosedCallback = NULL;
+            pSock->closedByRemote = false;
         }
     }
 }
@@ -367,6 +370,7 @@ static void UUSOCL_urc(const uAtClientHandle_t atHandle,
                                   closedCallback,
                                   U_INT32_TO_PTR(pSocket->sockHandle));
             }
+            pSocket->closedByRemote = true;
         }
     }
 }
@@ -1567,7 +1571,7 @@ int32_t uCellSockWrite(uDeviceHandle_t cellHandle,
                     while ((leftToSendSize > 0) &&
                            (negErrnoLocalOrSize == U_SOCK_ENONE) &&
                            (x < U_CELL_SOCK_TCP_RETRY_LIMIT) &&
-                           written) {
+                           written && !pSocket->closedByRemote) {
                         if (leftToSendSize < thisSendSize) {
                             thisSendSize = leftToSendSize;
                         }
@@ -1738,7 +1742,8 @@ int32_t uCellSockRead(uDeviceHandle_t cellHandle,
                     // pending data or room in the buffer
                     while ((dataSizeBytes > 0) &&
                            (pSocket->pendingBytes > 0) &&
-                           (negErrnoLocalOrSize == U_SOCK_ENONE)) {
+                           (negErrnoLocalOrSize == U_SOCK_ENONE) &&
+                           !pSocket->closedByRemote) {
                         thisWantedReceiveSize = dataLengthMax;
                         if (thisWantedReceiveSize > (int32_t) dataSizeBytes) {
                             thisWantedReceiveSize = (int32_t) dataSizeBytes;