[PATCH v2] libvchan: tidy up usages of fcntl in select-type sample application

[Matthew Daley <mattjd@gmail.com>]

Namely, don't overwrite all the other flags when twiddling O_NONBLOCK,
and add basic error handling.

Coverity-ID: 1055041
Signed-off-by: Matthew Daley <mattjd@gmail.com>
---
v2: Adjust other fcntl usage further down as well. Use F_GETFL to modify
only the O_NONBLOCK flag. Both suggested by Daniel De Graaf.
Improve commit message.

 tools/libvchan/node-select.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/tools/libvchan/node-select.c b/tools/libvchan/node-select.c
index 6c6c19e..acd9bae 100644
--- a/tools/libvchan/node-select.c
+++ b/tools/libvchan/node-select.c
@@ -90,6 +90,7 @@ int main(int argc, char **argv)
 {
 	int ret;
 	int libxenvchan_fd;
+	int flags;
 	if (argc < 4 || argv[3][0] != '/')
 		usage(argv);
 	if (!strcmp(argv[1], "server")) {
@@ -105,8 +106,13 @@ int main(int argc, char **argv)
 		exit(1);
 	}
 
-	fcntl(0, F_SETFL, O_NONBLOCK);
-	fcntl(1, F_SETFL, O_NONBLOCK);
+	if ((flags = fcntl(0, F_GETFL)) == -1 ||
+		fcntl(0, F_SETFL, flags | O_NONBLOCK) == -1 ||
+		(flags = fcntl(1, F_GETFL)) == -1 ||
+		fcntl(1, F_SETFL, flags | O_NONBLOCK) == -1) {
+		perror("fcntl");
+		exit(1);
+	}
 
 	libxenvchan_fd = libxenvchan_fd_for_select(ctrl);
 	for (;;) {
@@ -153,7 +159,11 @@ int main(int argc, char **argv)
 			stdout_wr();
 		}
 		if (!libxenvchan_is_open(ctrl)) {
-			fcntl(1, F_SETFL, 0);
+			if ((flags = fcntl(1, F_GETFL)) == -1 ||
+				fcntl(1, F_SETFL, flags & ~O_NONBLOCK) == -1) {
+				perror("fcntl");
+				exit(1);
+			}
 			while (outsiz)
 				stdout_wr();
 			return 0;
-- 
1.7.10.4