serial: Use non-blocking reads in a polling loop

Instead of doing blocking reads, which are not cancellable and therefore
cause deadlocks when the .shutdown callback is called, use a fast
polling loop with non-blocking reads.

This fixes the read thread not closing on shutdown.

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

serial.c

@@ -13,18 +13,21 @@
 #include <iio/iio-lock.h>
 #include <iio/iiod-client.h>
 
+#include <ctype.h>
 #include <errno.h>
 #include <libserialport.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <ctype.h>
+#include <time.h>
 
 struct iio_context_pdata {
 	struct sp_port *port;
 	struct iiod_client *iiod_client;
 
 	struct iio_context_params params;
+
+	bool shutdown;
 };
 
 struct iio_buffer_pdata {
@@ -149,17 +152,33 @@ static ssize_t serial_read_data(struct iiod_client_pdata *io_data,
 				char *buf, size_t len, unsigned int timeout_ms)
 {
 	struct iio_context_pdata *pdata = (struct iio_context_pdata *) io_data;
+	long long time_left_ns = (long long)timeout_ms * 1000 * 1000;
+	const struct timespec ts = {
+		/* One millisecond */
+		.tv_nsec = 1 * 1000 * 1000,
+	};
 	enum sp_return sp_ret;
 	ssize_t ret;
 
-	sp_ret = sp_blocking_read_next(pdata->port, buf, len, timeout_ms);
-	ret = (ssize_t) libserialport_to_errno(sp_ret);
+	while (true) {
+		if (timeout_ms && time_left_ns <= 0)
+			break;
+
+		sp_ret = sp_nonblocking_read(pdata->port, buf, len);
+		ret = (ssize_t) libserialport_to_errno(sp_ret);
+		if (ret || pdata->shutdown)
+			break;
+
+		nanosleep(&ts, NULL);
+		time_left_ns -= ts.tv_nsec;
+	}
 
 	prm_dbg(&pdata->params, "Read returned %li: %.*s\n",
 		(long) ret, (int) ret, buf);
 
 	if (ret == 0) {
-		prm_err(&pdata->params, "sp_blocking_read_next has timed out\n");
+		if (!pdata->shutdown)
+			prm_err(&pdata->params, "serial_read_data has timed out\n");
 		return -ETIMEDOUT;
 	}
 
@@ -170,6 +189,8 @@ static void serial_shutdown(struct iio_context *ctx)
 {
 	struct iio_context_pdata *ctx_pdata = iio_context_get_pdata(ctx);
 
+	ctx_pdata->shutdown = true;
+
 	iiod_client_destroy(ctx_pdata->iiod_client);
 	sp_close(ctx_pdata->port);
 	sp_free_port(ctx_pdata->port);