Shouldn't TimeSpec::from_duration preserve monotonicity?

[sirhcel]

When creating a TimeSpec from a std::time::Duration with more than libc::time_t seconds, the resulting value might be negative and will no longer preserve monotonicity with respect to the input duration. This happens to the integer casting in TimeSpec::from_duration.

See this example on Rust Playground.

I learned about this behavior through errors for negative timeouts from ppoll on Linux at runtime (serialport/serialport-rs#207). I did not expect this for the "porcellain" type Duration and the docs did not hint at it. And likely other users of the "porcellain" as well. When manually converting to the "plumbing" typetime_t I wold have looked deeper into the nitty gritty integer casts and cared for this case by saturating.

So shouldn't TimeSpec::from_duration preserve monotonicity? Isn't an inaccurate but still large TimeSpec less surprising?

[asomers]

The problem is that a Unix Timespec can't represent times greater than i64::MAX seconds. Saturating the conversion would be bad, because then subtracting two TimeSpecs that were produced by TimeSpec::from_duration wouldn't be guaranteed to give the correct answer. Arguably, what Nix ought to do would be to provide TimeSpec::from_instant(i: std::time::Instant) instead of from_duration, because std::time::Instant actually uses a TimeSpec under the hood. And perhaps a TryFrom<Duration> impl, too.

[sirhcel]

The problem is that a Unix Timespec can't represent times greater than i64::MAX seconds. Saturating the conversion would be bad, because then subtracting two TimeSpecs that were produced by TimeSpec::from_duration wouldn't be guaranteed to give the correct answer.

This is a fair point against saturating.

And perhaps a TryFrom<Duration> impl, too.

What about transitioning towards this impl over some time? I could prepare PRs for adding TryFrom<Duration> and once this is released the successor for deprecating From<Duration> so that other young players could get safely around this.

[asomers]

What about transitioning towards this impl over some time? I could prepare PRs for adding TryFrom<Duration> and once this is released the successor for deprecating From<Duration> so that other young players could get safely around this.

Yes, that would be good.

[sirhcel]

I started #2522 for asking if this in going in the right direction @asomers.

I also had a first look into std::time::Instant - but as far as I can tell this type is completely opaque and I don't see how to access its underlying time data. Am I missing the obvious here? Could you give me a hint?