top: Fix refresh hangs by correctly configuring non-blocking TTY

[NoobZang]

Setting O_NONBLOCK via fcntl is not sufficient for TTYs in non-canonical mode. A subsequent tcsetattr call would override this, as the default VMIN and VTIME settings cause the driver to block.

Explicitly set VMIN and VTIME to ensure a true non-blocking read.

Fixes #26166.

[nico]

Do you know what caused the regression?

[NoobZang]

I hadn't dug into the history. Top code seems good. I guess a deeper change of IO/TTY/fcntl/tcsetattr caused the regression. Explicitly setting VMIN and VTIME makes top more robust.

[spholz]

I bisected this to 96675e6.

VMIN is by default 1:

serenity/Kernel/API/ttydefaults.h

Line 24 in 6bcd011

#define CMIN 1

(This also matches behavior with other unixes, e.g. OpenBSD: https://github.com/openbsd/src/blob/cfc59e797c2eed7e722f5fc0f94f22ef692897c1/sys/sys/ttydefaults.h#L65. glibc's version is very similar and actually has a BSD license header.)

Though I'm confused why this affects reading from stdin. VMIN and VTIME seem to about reading: http://unixwiz.net/techtips/termios-vmin-vtime.html. And we also only set those values for stdout. So why does this fix the bug?

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[oskar-skog]

Though I'm confused why this affects reading from stdin. VMIN and VTIME seem to about reading: http://unixwiz.net/techtips/termios-vmin-vtime.html. And we also only set those values for stdout. So why does this fix the bug?

In normal use, both stdin and stdout are connected to the same file: the terminal. So I think that's how it's working.

[spholz]

Yeah, but I think it would still be better to set it for stdin.

[NoobZang]

I found swapping the fcntl and tcsetattr call order works without setting VMIN=0 (because fcntl sets it back to non-blocking), but I think explicitly setting VMIN=0 and VTIME=0 is safer and less brittle.

I've also moved the config to STDIN_FILENO.

[spholz]

Sorry that it took so long for me to come back to this!