Debug builds crash on subtraction overflow on non-linux/macos/windows platforms

[nortti0]

I've been working on porting Servo to FreeBSD. My debug builds were crashing in Webrender, since this subtraction ends up underflowing. The fix is to instead use .saturating_sub(), but I'm unsure where exactly I should send a fix for this, for it to land in Servo.

Why this only hits FreeBSD and not others is because zeitstempel's fallback implementation starts timestamps off at 0, while other platforms seem to use a timestamp relative to the system bootup – I presume a similar bug would manifest if Servoshell debug builds were to be run on Linux less than one second after boot.

Workaround: build with any profile other than debug, such as --profile medium or --release

[sagudev]

While we should fix the WR (both in upstream and here) I think one solution would be to use linux impl of zeitstempel on freebsd too (as far as I can tell all syscalls exist on frebsd). This should be only matter of changing this cfg: https://github.com/badboy/zeitstempel/blob/990c8b9c92b056fc5dd84277e9de3937733c3d9f/src/lib.rs#L73

[nortti0]

On FreeBSD the semantics of CLOCK_BOOTTIME and CLOCK_MONOTONIC are identical. The equivalent to Linux's CLOCK_MONOTONIC on FreeBSD is CLOCK_UPTIME, which, on the other hand, does not exist on Linux.

OTOH, the fallback implementation is also identical between now_awake() and now_including_suspend(), so it'd not be any worse than the current situation.

[nortti0]

I have reported the bug upstream.