usock: add helper waiting for socket to be ready

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

usock.c

@@ -20,6 +20,7 @@
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <netdb.h>
+#include <poll.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <fcntl.h>
@@ -130,3 +131,29 @@ int usock(int type, const char *host, const char *service) {
 	usock_set_flags(sock, type);
 	return sock;
 }
+
+int usock_wait_ready(int fd, int msecs) {
+	struct pollfd fds[1];
+	int res;
+
+	fds[0].fd = fd;
+	fds[0].events = POLLOUT;
+
+	res = poll(fds, 1, msecs);
+	if (res < 0) {
+		return errno;
+	} else if (res == 0) {
+		return -ETIMEDOUT;
+	} else {
+		int err = 0;
+		socklen_t optlen = sizeof(err);
+
+		res = getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &optlen);
+		if (res)
+			return errno;
+		if (err)
+			return err;
+	}
+
+	return 0;
+}

usock.h

@@ -33,4 +33,15 @@
 const char *usock_port(int port);
 int usock(int type, const char *host, const char *service);
 
+/**
+ * Wait for a socket to become ready.
+ *
+ * This may be useful for users of USOCK_NONBLOCK to wait (with a timeout)
+ * for a socket.
+ *
+ * @param fd file descriptor of socket
+ * @param msecs timeout in microseconds
+ */
+int usock_wait_ready(int fd, int msecs);
+
 #endif /* USOCK_H_ */