io: add io_fd_block() helper.

It's a common thing to want to do, so add helper here.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/io/benchmarks/run-different-speed.c

@@ -60,7 +60,7 @@ static void create_clients(struct sockaddr_un *addr, int waitfd)
 		if (connect(sock[i], (void *)addr, sizeof(*addr)) != 0)
 			err(1, "connecting socket");
 		/* Make nonblocking. */
-		fcntl(sock[i], F_SETFD, fcntl(sock[i], F_GETFD)|O_NONBLOCK);
+		io_fd_block(sock[i], false);
 		done[i] = 0;
 	}
 

ccan/io/benchmarks/run-length-prefix.c

@@ -66,7 +66,7 @@ static void create_clients(struct sockaddr_un *addr, int waitfd)
 		if (connect(sock[i], (void *)addr, sizeof(*addr)) != 0)
 			err(1, "connecting socket");
 		/* Make nonblocking. */
-		fcntl(sock[i], F_SETFD, fcntl(sock[i], F_GETFD)|O_NONBLOCK);
+		io_fd_block(sock[i], false);
 		done[i] = 0;
 	}
 

ccan/io/io.c

@@ -72,16 +72,19 @@ static bool next_plan(struct io_conn *conn, struct io_plan *plan)
 	return true;
 }
 
-static void set_blocking(int fd, bool block)
+bool io_fd_block(int fd, bool block)
 {
 	int flags = fcntl(fd, F_GETFL);
 
+	if (flags == -1)
+		return false;
+
 	if (block)
 		flags &= ~O_NONBLOCK;
 	else
 		flags |= O_NONBLOCK;
 
-	fcntl(fd, F_SETFL, flags);
+	return fcntl(fd, F_SETFL, flags) != -1;
 }
 
 struct io_conn *io_new_conn_(const tal_t *ctx, int fd,
@@ -103,7 +106,7 @@ struct io_conn *io_new_conn_(const tal_t *ctx, int fd,
 		return tal_free(conn);
 
 	/* Keep our I/O async. */
-	set_blocking(fd, false);
+	io_fd_block(fd, false);
 
 	/* We start with out doing nothing, and in doing our init. */
 	conn->plan[IO_OUT].status = IO_UNSET;
@@ -436,7 +439,7 @@ struct io_plan *io_close_cb(struct io_conn *conn, void *next_arg)
 
 struct io_plan *io_close_taken_fd(struct io_conn *conn)
 {
-	set_blocking(conn->fd.fd, true);
+	io_fd_block(conn->fd.fd, true);
 
 	cleanup_conn_without_close(conn);
 	return io_close(conn);
@@ -507,7 +510,7 @@ bool io_flush_sync(struct io_conn *conn)
 		return true;
 
 	/* Synchronous please. */
-	set_blocking(io_conn_fd(conn), true);
+	io_fd_block(io_conn_fd(conn), true);
 
 again:
 	switch (plan->io(conn->fd.fd, &plan->arg)) {
@@ -527,6 +530,6 @@ again:
 		abort();
 	}
 
-	set_blocking(io_conn_fd(conn), false);
+	io_fd_block(io_conn_fd(conn), false);
 	return ok;
 }

ccan/io/io.h

@@ -692,6 +692,16 @@ int io_conn_fd(const struct io_conn *conn);
  */
 bool io_flush_sync(struct io_conn *conn);
 
+/**
+ * io_fd_block - helper to set an fd blocking/nonblocking.
+ * @fd: the file descriptor
+ * @block: true to set blocking, false to set non-blocking.
+ *
+ * Generally only fails is @fd isn't a valid file descriptor, otherwise
+ * returns true.
+ */
+bool io_fd_block(int fd, bool block);
+
 /**
  * io_time_override - override the normal call for time.
  * @nowfn: the function to call.