Implement DTLSSocket helper

This is equivalent of TLSSocket class but for UDP and DTLS.

features/netsocket/DTLSSocket.cpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2018 ARM Limited
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "DTLSSocket.h"
+
+#define TRACE_GROUP "TLSS"
+#include "mbed-trace/mbed_trace.h"
+
+// This class requires Mbed TLS SSL/TLS client code
+#if defined(MBEDTLS_SSL_CLI_C)
+
+nsapi_error_t DTLSSocket::connect(const char *host, uint16_t port)
+{
+    if (!_remote_address) {
+        nsapi_error_t err = _stack->gethostbyname(host, &_remote_address);
+        if (err) {
+            return NSAPI_ERROR_DNS_FAILURE;
+        }
+
+        _remote_address.set_port(port);
+
+        set_hostname(host);
+        _udp_socket.connect(_remote_address); // UDPSocket::connect() cannot fail
+    }
+
+    return connect(_remote_address);
+}
+
+DTLSSocket::~DTLSSocket()
+{
+     /* Make sure that DTLSSocketWrapper::close() is called before the transport is
+     * destroyed.
+     */
+    close();
+}
+
+#endif // MBEDTLS_SSL_CLI_C

features/netsocket/DTLSSocket.h

@@ -0,0 +1,78 @@
+#ifndef DTLSSOCKET_H
+#define DTLSSOCKET_H
+
+#include "DTLSSocketWrapper.h"
+#include "SocketAddress.h"
+#include "UDPSocket.h"
+
+// This class requires Mbed TLS SSL/TLS client code
+#if defined(MBEDTLS_SSL_CLI_C)
+
+class DTLSSocket : public DTLSSocketWrapper {
+public:
+    /** Create an uninitialized DTLS socket
+     *
+     *  Must call open to initialize the socket on a network stack.
+     */
+    DTLSSocket() : DTLSSocketWrapper(&_udp_socket) {}
+
+    /** Destroy the DTLSSocket and closes the transport.
+     */
+    virtual ~DTLSSocket();
+
+    /** Create a socket on a network interface
+     *
+     *  Creates and opens a socket on the network stack of the given
+     *  network interface.
+     *  If hostname is also given, user is not required to call set_hostname() later.
+     *
+     *  @param stack    Network stack as target for socket
+     *  @param hostname Hostname used for certificate verification
+     */
+    template <typename S>
+    DTLSSocket(S *stack, const char *hostname = NULL) : DTLSSocketWrapper(&_udp_socket, hostname)
+    {
+        nsapi_error_t ret = _udp_socket.open(stack);
+        MBED_ASSERT(ret == NSAPI_ERROR_OK);
+    }
+
+    /** Opens a socket
+     *
+     *  Creates a network socket on the network stack of the given
+     *  network interface. Not needed if stack is passed to the
+     *  socket's constructor.
+     *
+     *  @param stack    Network stack as target for socket
+     *  @return         0 on success, negative error code on failure
+     */
+    virtual nsapi_error_t open(NetworkStack *stack) {
+        _stack = stack;
+        return _udp_socket.open(stack);
+    }
+
+    template <typename S>
+    nsapi_error_t open(S *stack) {
+        return open(nsapi_create_stack(stack));
+    }
+
+    using DTLSSocketWrapper::connect;
+
+    /** Connects TCP socket to a remote host
+     *
+     *  Initiates a connection to a remote server specified by either
+     *  a domain name or an IP address and a port.
+     *
+     *  @param host     Hostname of the remote host
+     *  @param port     Port of the remote host
+     *  @return         0 on success, negative error code on failure
+     */
+    nsapi_error_t connect(const char *host, uint16_t port);
+
+private:
+    UDPSocket _udp_socket;
+    NetworkStack *_stack;
+    SocketAddress _remote_address;
+};
+
+#endif
+#endif