iiod-responder: Handle BINARY command in binary mode

Unlike with the network and USB backends, IIOD is unable to detect when
a client disconnected. A new client would then send the BINARY command,
which would not be understood by IIOD if it already was in binary mode.

Use the convenient fact that the BINARY string is exactly the same size
as our command opcodes, and compare each opcode read with the
"BINARY\r\n" string. In case of a match, simply send a code 0 to tell
the client that we are in binary mode, and continue as usual.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

iiod-responder.c

@@ -240,12 +240,14 @@ static int iiod_responder_reader_thrd(void *d)
 {
 	struct iiod_responder *priv = d;
 	struct iiod_command cmd;
-	struct iiod_buf cmd_buf;
+	struct iiod_buf cmd_buf, ok_buf;
 	struct iiod_io *io;
 	ssize_t ret = 0;
 
 	cmd_buf.ptr = &cmd;
 	cmd_buf.size = sizeof(cmd);
+	ok_buf.ptr = "0\r\n";
+	ok_buf.size = 3;
 
 	iio_mutex_lock(priv->lock);
 
@@ -254,6 +256,18 @@ static int iiod_responder_reader_thrd(void *d)
 
 		ret = iiod_rw_all(priv, NULL, &cmd_buf, 1, sizeof(cmd), true);
 
+		if (!strncmp((char *)&cmd, "BINARY\r\n", 8)) {
+			/* If we receive again the "BINARY\r\n" string, send a
+			 * return code of zero and continue as usual.
+			 * This can happen with the serial backend when the
+			 * client disconnects and a new client appears.
+			 * Conveniently, the string is exactly 8 bytes, which is
+			 * the size of a iio_command. */
+
+			iiod_rw_all(priv, NULL, &ok_buf, 1, ok_buf.size, false);
+			continue;
+		}
+
 		iio_mutex_lock(priv->lock);
 		if (ret <= 0)
 			break;