Implement DTLSSocketWrapper and fix non-blocking connections on TLSSocket

DTLSSocketWrapper is equivalent of TLSSocketWrapper but uses datagram mode
and timers for handling Mbed TLS timeouts.

Non-blocking connections were not working earlier, now fixed for both
secure socket modes.

Author: Seppo Takalo

features/netsocket/DTLSSocketWrapper.cpp

@@ -0,0 +1,55 @@
+#include "DTLSSocketWrapper.h"
+#include "platform/Callback.h"
+#include "drivers/Timer.h"
+#include "events/mbed_events.h"
+
+#if defined(MBEDTLS_SSL_CLI_C)
+
+DTLSSocketWrapper::DTLSSocketWrapper(Socket *transport, const char *hostname, control_transport control)
+    : TLSSocketWrapper(transport, hostname, control)
+{
+    mbedtls_ssl_conf_transport( get_ssl_config(), MBEDTLS_SSL_TRANSPORT_DATAGRAM);
+    mbedtls_ssl_set_timer_cb( get_ssl_context(), this, timing_set_delay, timing_get_delay);
+    _timer_event_id = 0;
+    _timer_expired = false;
+}
+
+void DTLSSocketWrapper::timing_set_delay(void *ctx, uint32_t int_ms, uint32_t fin_ms)
+{
+    DTLSSocketWrapper *context = static_cast<DTLSSocketWrapper *>(ctx);
+
+    if (context->_timer_event_id) {
+        mbed::mbed_event_queue()->cancel(context->_timer_event_id);
+        context->_timer_expired = false;
+    }
+
+    if (fin_ms == 0) {
+        context->_timer_event_id = 0;
+        return;
+    }
+
+    context->_timer_event_id = mbed::mbed_event_queue()->call_in(fin_ms, context, &DTLSSocketWrapper::timer_event);
+}
+
+int DTLSSocketWrapper::timing_get_delay(void *ctx)
+{
+    DTLSSocketWrapper *context = static_cast<DTLSSocketWrapper *>(ctx);
+
+    /* See documentation of "typedef int mbedtls_ssl_get_timer_t( void * ctx );" from ssl.h */
+
+    if (context->_timer_event_id == 0) {
+        return -1;
+    } else if (context->_timer_expired) {
+        return 2;
+    } else {
+        return 0;
+    }
+}
+
+void DTLSSocketWrapper::timer_event(void)
+{
+    _timer_expired = true;
+    event();
+}
+
+#endif /* MBEDTLS_SSL_CLI_C */

features/netsocket/DTLSSocketWrapper.h

@@ -0,0 +1,21 @@
+#ifndef DTLSSOCKETWRAPPER_H
+#define DTLSSOCKETWRAPPER_H
+
+#include "TLSSocketWrapper.h"
+
+// This class requires Mbed TLS SSL/TLS client code
+#if defined(MBEDTLS_SSL_CLI_C)
+
+class DTLSSocketWrapper : public TLSSocketWrapper {
+public:
+    DTLSSocketWrapper(Socket *transport, const char *hostname = NULL, control_transport control = TRANSPORT_CONNECT_AND_CLOSE);
+private:
+    static void timing_set_delay(void *ctx, uint32_t int_ms, uint32_t fin_ms);
+    static int timing_get_delay(void *ctx);
+    void timer_event();
+    int _timer_event_id;
+    bool _timer_expired:1;
+};
+
+#endif
+#endif

features/netsocket/TLSSocket.cpp

@@ -25,14 +25,17 @@
 
 nsapi_error_t TLSSocket::connect(const char *host, uint16_t port)
 {
-    set_hostname(host);
-
-    nsapi_error_t ret = tcp_socket.connect(host, port);
-    if (ret) {
-        return ret;
+    nsapi_error_t ret = NSAPI_ERROR_OK;
+    if (!is_handshake_started()) {
+        ret = tcp_socket.connect(host, port);
+        if (ret == NSAPI_ERROR_OK || ret == NSAPI_ERROR_IN_PROGRESS) {
+            set_hostname(host);
+        }
+        if (ret != NSAPI_ERROR_OK && ret != NSAPI_ERROR_IS_CONNECTED) {
+            return ret;
+        }
     }
-
-    return TLSSocketWrapper::do_handshake();
+    return TLSSocketWrapper::start_handshake(ret == NSAPI_ERROR_OK);
 }
 
 TLSSocket::~TLSSocket()