Add polling function for network contexts

This commit adds a function `mbedtls_net_poll` to the network module
allowing to check if a network context is available for read or write.

include/mbedtls/net_sockets.h

@@ -45,12 +45,17 @@
 #define MBEDTLS_ERR_NET_UNKNOWN_HOST                      -0x0052  /**< Failed to get an IP address for the given hostname. */
 #define MBEDTLS_ERR_NET_BUFFER_TOO_SMALL                  -0x0043  /**< Buffer is too small to hold the data. */
 #define MBEDTLS_ERR_NET_INVALID_CONTEXT                   -0x0045  /**< The context is invalid, eg because it was free()ed. */
+#define MBEDTLS_ERR_NET_POLL_FAILED                       -0x0047  /**< Polling the net context failed. */
+#define MBEDTLS_ERR_NET_BAD_INPUT_DATA                    -0x0049  /**< Input invalid. */
 
 #define MBEDTLS_NET_LISTEN_BACKLOG         10 /**< The backlog that listen() should use. */
 
 #define MBEDTLS_NET_PROTO_TCP 0 /**< The TCP transport protocol */
 #define MBEDTLS_NET_PROTO_UDP 1 /**< The UDP transport protocol */
 
+#define MBEDTLS_NET_POLL_READ  1 /**< Used in \c mbedtls_net_poll to check for pending data  */
+#define MBEDTLS_NET_POLL_WRITE 2 /**< Used in \c mbedtls_net_poll to check if write possible */
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -131,6 +136,29 @@ int mbedtls_net_accept( mbedtls_net_context *bind_ctx,
                         mbedtls_net_context *client_ctx,
                         void *client_ip, size_t buf_size, size_t *ip_len );
 
+/**
+ * \brief          Check and wait for the context to be ready for read/write
+ *
+ * \param ctx      Socket to check
+ * \param rw       Bitflag composed of MBEDTLS_NET_POLL_READ and
+ *                 MBEDTLS_NET_POLL_WRITE specifying the events
+ *                 to wait for:
+ *                 - If MBEDTLS_NET_POLL_READ is set, the function
+ *                   will return as soon as the net context is available
+ *                   for reading.
+ *                 - If MBEDTLS_NET_POLL_WRITE is set, the function
+ *                   will return as soon as the net context is available
+ *                   for writing.
+ * \param timeout  Maximal amount of time to wait before returning,
+ *                 in milliseconds. If \c timeout is zero, the
+ *                 function returns immediately. If \c timeout is
+ *                 -1u, the function blocks potentially indefinitely.
+ *
+ * \return         Bitmask composed of MBEDTLS_NET_POLL_READ/WRITE
+ *                 on success or timeout, or a negative return code otherwise.
+ */
+int mbedtls_net_poll( mbedtls_net_context *ctx, uint32_t rw, uint32_t timeout );
+
 /**
  * \brief          Set the socket blocking
  *

library/error.c

@@ -654,6 +654,10 @@ void mbedtls_strerror( int ret, char *buf, size_t buflen )
         mbedtls_snprintf( buf, buflen, "NET - Buffer is too small to hold the data" );
     if( use_ret == -(MBEDTLS_ERR_NET_INVALID_CONTEXT) )
         mbedtls_snprintf( buf, buflen, "NET - The context is invalid, eg because it was free()ed" );
+    if( use_ret == -(MBEDTLS_ERR_NET_POLL_FAILED) )
+        mbedtls_snprintf( buf, buflen, "NET - Polling the net context failed" );
+    if( use_ret == -(MBEDTLS_ERR_NET_BAD_INPUT_DATA) )
+        mbedtls_snprintf( buf, buflen, "NET - Input invalid" );
 #endif /* MBEDTLS_NET_C */
 
 #if defined(MBEDTLS_OID_C)

library/net_sockets.c

@@ -433,6 +433,58 @@ int mbedtls_net_set_nonblock( mbedtls_net_context *ctx )
 #endif
 }
 
+/*
+ * Check if data is available on the socket
+ */
+
+int mbedtls_net_poll( mbedtls_net_context *ctx, uint32_t rw, uint32_t timeout )
+{
+    int ret;
+    struct timeval tv;
+
+    fd_set read_fds;
+    fd_set write_fds;
+
+    int fd = ctx->fd;
+
+    if( fd < 0 )
+        return( MBEDTLS_ERR_NET_INVALID_CONTEXT );
+
+    FD_ZERO( &read_fds );
+    if( rw & MBEDTLS_NET_POLL_READ )
+    {
+        rw &= ~MBEDTLS_NET_POLL_READ;
+        FD_SET( fd, &read_fds );
+    }
+
+    FD_ZERO( &write_fds );
+    if( rw & MBEDTLS_NET_POLL_WRITE )
+    {
+        rw &= ~MBEDTLS_NET_POLL_WRITE;
+        FD_SET( fd, &write_fds );
+    }
+
+    if( rw != 0 )
+        return( MBEDTLS_ERR_NET_BAD_INPUT_DATA );
+
+    tv.tv_sec  = timeout / 1000;
+    tv.tv_usec = ( timeout % 1000 ) * 1000;
+
+    ret = select( fd + 1, &read_fds, &write_fds, NULL,
+                  timeout == (uint32_t) -1u ? NULL : &tv );
+
+    if( ret < 0 )
+        return( MBEDTLS_ERR_NET_POLL_FAILED );
+
+    ret = 0;
+    if( FD_ISSET( fd, &read_fds ) )
+        ret |= MBEDTLS_NET_POLL_READ;
+    if( FD_ISSET( fd, &write_fds ) )
+        ret |= MBEDTLS_NET_POLL_WRITE;
+
+    return( ret );
+}
+
 /*
  * Portable usleep helper
  */
@@ -492,8 +544,8 @@ int mbedtls_net_recv( void *ctx, unsigned char *buf, size_t len )
 /*
  * Read at most 'len' characters, blocking for at most 'timeout' ms
  */
-int mbedtls_net_recv_timeout( void *ctx, unsigned char *buf, size_t len,
-                      uint32_t timeout )
+int mbedtls_net_recv_timeout( void *ctx, unsigned char *buf,
+                              size_t len, uint32_t timeout )
 {
     int ret;
     struct timeval tv;