initctl: finit may be busy, wait for it ...

When calling initctl early in the boot process, e.g. just when Finit is
switching to the final runlevel, it may take a few milliseconds for it
to actually be ready to process our request.

This patch adds poll() to both the write(REQ) and the read(RSP) steps
to prevent from getting weird errors when Finit is busy.  The timeout
is currently 2 sec in both directions, which should be enough.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/client.c

@@ -23,6 +23,7 @@
 
 #include <err.h>
 #include <errno.h>
+#include <poll.h>
 #include <sys/socket.h>
 #include <sys/un.h>
 
@@ -64,27 +65,44 @@ int client_disconnect(void)
 
 int client_send(struct init_request *rq, ssize_t len)
 {
+	struct pollfd pfd = { 0 };
 	int sd, result = 255;
 
 	sd = client_connect();
 	if (-1 == sd)
 		return -1;
 
-	if (write(sd, rq, len) != len)
-		goto error;
+	pfd.fd     = sd;
+	pfd.events = POLLOUT;
+	if (poll(&pfd, 1, 2000) <= 0) {
+		warn("Timed out waiting for Finit, errno %d", errno);
+		goto exit;
+	}
 
-	if (read(sd, rq, len) != len)
-		goto error;
+	if (write(sd, rq, len) != len) {
+		warn("Failed communicating with Finit, errno %d", errno);
+		goto exit;
+	}
+
+	pfd.fd = sd;
+	pfd.events = POLLIN;
+	if (poll(&pfd, 1, 2000) <= 0) {
+		warn("Timed out waiting for reply from Finit, errno %d", errno);
+		goto exit;
+	}
+
+	if (read(sd, rq, len) != len) {
+		warn("Failed reading reply from Finit, errno %d", errno);
+		goto exit;
+	}
 
 	if (rq->cmd == INIT_CMD_NACK)
 		result = 1;
 	else
 		result = 0;
-	goto exit;
-error:
-	perror("Failed communicating with finit");
 exit:
 	client_disconnect();
+
 	return result;
 }