quad: don't block for client on virtual quad

Problem

The virt-quad requires the ground station to be run, due to the blocking while loop for accepting socket clients. Not every quad test will require the ground station.

Solution

Make the virt-quad more flexible by allowing it to connect to clients on the fly, enabling us to test/simulate other aspects of the quad that don't require the ground station.

Disclaimer

The git-diffs don't give Kris justice. Most of the code is still his, just with a tab adjustment because I haven't bothered to change my editor config. The only part that has actually changed is the accepting of clients (and then the addition of checking for clients in the write/read functions).

Review:

• @burneykb
• @dawehr

quad/src/virt_quad/hw_impl_unix_uart.c

@@ -13,7 +13,7 @@
 int check_socket_client();
 
 static int backendSocket;
-static int client;
+static int client = -1;
 
 int unix_uart_reset(struct UARTDriver *self) {
   char * backend_socket_path = DEFAULT_SOCKET;
@@ -70,6 +70,14 @@ int check_socket_client() {
       return -1;
     }
     printf("backend connection found on socket %d.\n", client);
+  } else {
+    char c;
+    ssize_t x;
+    if ((x = recv(client, &c, 1, MSG_PEEK)) <= 0) {
+      client = -1;
+      printf("backend connection lost\n");
+      return -1;
+    }
   }
   return 0;
 }