status_command doesn’t handle partial lines

[alyssais]

• Sway version: 1.0-rc3

Consider the following as a status_command:

#include <stdio.h>
#include <unistd.h>

int main() {
    for (;;) {
        printf("hello ");
        fflush(stdout);
        sleep(1);
        printf("world\n");
        fflush(stdout);
        sleep(10);
    }
}

The status bar will only show world, because AFAICT, status_handle_readable reads into a buffer, but then just reads into that same buffer again without displaying it, unless the bar happens to repaint for some other reason.

No documentation I can see for getline explains what it does to the buffer it’s given if it’s reading from a non-blocking fd — it doesn’t seem to be designed for that use case. So I think something else might need to be used instead.

[ddevault]

This is by design.

[alyssais]

sway-bar(5) says

Each line of text printed to stdout from this command will be displayed in the status area of the bar.

But that’s not what this is at all. This is “The first write to stdout from this command, up to the first newline, will be displayed in the status area of the bar”.

Should the manual be updated?

I’m also told that this is not what i3bar does.

[alyssais]

Sorry, I mean “the first write that ends in a newline”. Any write that doesn’t end in a newline will be ignored.

[ddevault]

Oh, I misunderstood your bug report - apologies.

[ianyfan]

I compiled your program and used it as the status_command, and it displayed "hello" followed by "world" a second later as expected, so I am not sure what is going on.

[alyssais]

I compiled your program and used it as the status_command, and it displayed "hello" followed by "world" a second later as expected, so I am not sure what is going on.

Just to confirm: it showed "hello ", and then "hello world" all at once, but never "world"?

[ianyfan]

No, it showed "hello" then "world", separately, never "hello world".

[emersion]

Okay, so that's a bug. If we receive one message without \n and one message with \n, we should concat both. It should be possible to check the last character returned by getline, and if it's not \n, append instead of replace the status.

[ianyfan]

I'm not sure we should, because the behaviour I'm getting matches the behaviour I'm getting with i3

[alyssais]

I'm not sure we should, because the behaviour I'm getting matches the behaviour I'm getting with i3

I think this warrants fixing even if it doesn't match i3's behaviour, because it's such a patently bad idea. How many writes a program uses to write a line is, surely, an implementation detail of that program, but if only the first write is displayed, then as a user I suddenly care how many writes a program is doing. As is, the only way to be sure that the output you want will actually be displayed correctly by swaybar is to wrap the whole command in echo "$(...)". Surely that can't be desirable behaviour?

[ianyfan]

You make a good point; I'm happy for this change to be made. And perhaps you should bring this up with the i3 team too if it hasn't already.

[alyssais]

And perhaps you should bring this up with the i3 team too if it hasn't already.

Are you aware of any documentation for i3bar that says the output will be shown line at a time? I couldn't find any (or really anything about the non-JSON format at all).

[ianyfan]

This is the only reference I can find: https://i3wm.org/docs/userguide.html#status_command