Weird stdout/stderr indefinite loop with vala async exec / pipe example

Question

I tried out the example about exec and spawning, when running this with vala or compiling with valac, I get an endless loop of the output below. Is that normal? Incidentally, this is on OS X, normally I am on Linux and I can test it later on Linux. Vala version 0.40.9 installed with brew, using glib version 2.58.0 also installed with brew.

out: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stderr: (null)stdout: (null)stde

Edit: When running the same example on Ubuntu Xenial it worked as expected, but it is also using vala 0.30.1 by default. Is that just a way too old version? But at least now I saw what the example should look like.

Edit2: The code used is 'Spawn with pipes, async' from https://valadoc.org/glib-2.0/GLib.Process.spawn_async_with_pipes.html without any modifications, just copy pasted.

private static bool process_line (IOChannel channel, IOCondition condition, string stream_name) {
    if (condition == IOCondition.HUP) {
        print ("%s: The fd has been closed.\n", stream_name);
        return false;
    }

    try {
        string line;
        channel.read_line (out line, null, null);
        print ("%s: %s", stream_name, line);
    } catch (IOChannelError e) {
        print ("%s: IOChannelError: %s\n", stream_name, e.message);
        return false;
    } catch (ConvertError e) {
        print ("%s: ConvertError: %s\n", stream_name, e.message);
        return false;
    }

    return true;
}

public static int main (string[] args) {
    MainLoop loop = new MainLoop ();
    try {
        string[] spawn_args = {"ls", "-l", "-h"};
        string[] spawn_env = Environ.get ();
        Pid child_pid;

        int standard_input;
        int standard_output;
        int standard_error;

        Process.spawn_async_with_pipes ("/",
            spawn_args,
            spawn_env,
            SpawnFlags.SEARCH_PATH | SpawnFlags.DO_NOT_REAP_CHILD,
            null,
            out child_pid,
            out standard_input,
            out standard_output,
            out standard_error);

        // stdout:
        IOChannel output = new IOChannel.unix_new (standard_output);
        output.add_watch (IOCondition.IN | IOCondition.HUP, (channel, condition) => {
            return process_line (channel, condition, "stdout");
        });

        // stderr:
        IOChannel error = new IOChannel.unix_new (standard_error);
        error.add_watch (IOCondition.IN | IOCondition.HUP, (channel, condition) => {
            return process_line (channel, condition, "stderr");
        });

        ChildWatch.add (child_pid, (pid, status) => {
            // Triggered when the child indicated by child_pid exits
            Process.close_pid (pid);
            loop.quit ();
        });

        loop.run ();
    } catch (SpawnError e) {
        print ("Error: %s\n", e.message);
    }
    return 0;
}

The bug report is here: https://gitlab.gnome.org/GNOME/glib/issues/1512