io: add io_flush_sync().

This is needed for emergency handling in lightningd: we want to output
a (fatal) error packet on the socket, but we don't want to do so in the middle
of another packet.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/io/io.c

@@ -483,3 +483,37 @@ struct io_plan *io_set_plan(struct io_conn *conn, enum io_direction dir,
 
 	return plan;
 }
+
+bool io_flush_sync(struct io_conn *conn)
+{
+	struct io_plan *plan = &conn->plan[IO_OUT];
+	bool ok;
+
+	/* Not writing?  Nothing to do. */
+	if (plan->status != IO_POLLING)
+		return true;
+
+	/* Synchronous please. */
+	set_blocking(io_conn_fd(conn), true);
+
+again:
+	switch (plan->io(conn->fd.fd, &plan->arg)) {
+	case -1:
+		ok = false;
+		break;
+	/* Incomplete, try again. */
+	case 0:
+		goto again;
+	case 1:
+		ok = true;
+		/* In case they come back. */
+		set_always(conn, IO_OUT, plan->next, plan->next_arg);
+		break;
+	default:
+		/* IO should only return -1, 0 or 1 */
+		abort();
+	}
+
+	set_blocking(io_conn_fd(conn), false);
+	return ok;
+}

ccan/io/io.h

@@ -673,6 +673,24 @@ void *io_loop(struct timers *timers, struct timer **expired);
  */
 int io_conn_fd(const struct io_conn *conn);
 
+/**
+ * io_flush_sync - (synchronously) complete any outstanding output.
+ * @conn: the connection.
+ *
+ * This is generally used as an emergency escape, for example when we
+ * want to write an error message on a socket before terminating, but it may
+ * be in the middle of existing I/O.  We don't want to service any other
+ * IO, either.
+ *
+ * This returns true if all pending output is complete, false on error.
+ * The next callback is not called on the conn, but will be as soon as
+ * io_loop() is called.
+ *
+ * See Also:
+ *	io_close_taken_fd
+ */
+bool io_flush_sync(struct io_conn *conn);
+
 /**
  * io_time_override - override the normal call for time.
  * @nowfn: the function to call.

ccan/io/test/run-30-io_flush_sync.c

+#include <ccan/io/io.h>
+/* Include the C files directly. */
+#include <ccan/io/poll.c>
+#include <ccan/io/io.c>
+#include <ccan/tap/tap.h>
+#include <sys/wait.h>
+#include <stdio.h>
+
+static size_t bytes_written;
+
+/* Should be called multiple times, since only writes 1 byte. */
+static int do_controlled_write(int fd, struct io_plan_arg *arg)
+{
+	ssize_t ret;
+
+	ret = write(fd, arg->u1.cp, 1);
+	if (ret < 0)
+		return -1;
+	bytes_written += ret;
+	arg->u1.cp += ret;
+	arg->u2.s -= ret;
+	return arg->u2.s == 0;
+}
+
+static int do_error(int fd, struct io_plan_arg *arg)
+{
+	errno = 1001;
+	return -1;
+}
+
+static struct io_plan *conn_wait(struct io_conn *conn, void *unused)
+{
+	return io_wait(conn, conn, io_never, NULL);
+}
+
+static struct io_plan *init_conn_writer(struct io_conn *conn, const char *str)
+{
+	struct io_plan_arg *arg = io_plan_arg(conn, IO_OUT);
+
+	arg->u1.const_vp = str;
+	arg->u2.s = strlen(str);
+
+	return io_set_plan(conn, IO_OUT, do_controlled_write, conn_wait, NULL);
+}
+
+static struct io_plan *init_conn_reader(struct io_conn *conn, void *dst)
+{
+	/* Never actually succeeds. */
+	return io_read(conn, dst, 1000, io_never, NULL);
+}
+
+static struct io_plan *init_conn_error(struct io_conn *conn, void *unused)
+{
+	io_plan_arg(conn, IO_OUT);
+	return io_set_plan(conn, IO_OUT, do_error, io_never, NULL);
+}
+
+int main(void)
+{
+	int fd = open("/dev/null", O_RDWR);
+	const tal_t *ctx = tal(NULL, char);
+	struct io_conn *conn;
+
+	/* This is how many tests you plan to run */
+	plan_tests(9);
+
+	conn = io_new_conn(ctx, fd, init_conn_writer, "hello");
+	ok1(bytes_written == 0);
+
+	ok1(io_flush_sync(conn));
+	ok1(bytes_written == strlen("hello"));
+
+	/* This won't do anything */
+	ok1(io_flush_sync(conn));
+	ok1(bytes_written == strlen("hello"));
+
+	/* It's reading, this won't do anything. */
+	conn = io_new_conn(ctx, fd, init_conn_reader, ctx);
+	ok1(io_flush_sync(conn));
+	ok1(bytes_written == strlen("hello"));
+
+	/* Now test error state. */
+	conn = io_new_conn(ctx, fd, init_conn_error, ctx);
+	ok1(!io_flush_sync(conn));
+	ok1(errno == 1001);
+
+	tal_free(ctx);
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}