WIP lib.duration: add basic duration functions

[rycee]

Motivation for this change

I wanted a more structured and unified way to represent various time durations (e.g. timeouts and such things) in Home Manager and figured it was general enough that there might be an interest to add it to Nixpkgs instead of just HM.

While the majority of code is centered around a few utility functions, my main interest is the included duration type for the module system. Using it you can have a module along the lines

{ config, lib, ... }:

with lib;

{
  options = {
    value = mkOption {
      type = types.duration;
      apply = durations.parse;
    };

    result = mkOption {
      type = types.str;
    };
  };

  config = {
    value = "1d 12h";
    result = "In millis: ${toString (durations.toMillis config.value)}";
  };
}

and result would then contain "In millis: 129600000".

I don't consider this ready for merge yet, this is mainly for discussion whether it is of any interest for inclusion in Nixpkgs.

Things done

The included tests pass.

[rycee]

Yeah, the current format is (intentionally) a subset of the systemd.time format and in principle it shouldn't be too tricky to expand to the full format (as far as it is demonstrated in the man page). But looking at the systemd code it does seem like it supports things like "5.5s 0.5ms" which makes things more complicated… For my own purposes a subset of the systemd format is sufficient.

Another difficulty I'm facing is that I would like to be able to pretty print the duration. I.e., I'd like to have

toString (parse "150h") == "6d 6h"

but this is a bit tricky without having a remainder operator 😕

Also I wish there were some convenient way to convert from a Nix float value to an int value.

[rycee]

@volth Of course! Thanks, I hadn't thought of it 🙂

I've updated the PR to more closely match the systemd format and also extended the toString support. With the changes we have

toString (parse "8d 30h 0m 177s 1ms") == "1w 2d 6h 2m 57s 1ms"

[rycee]

I'm also planning to add a corresponding durationWithIntFallback type.

[zimbatm]

it looks useful! Are there any places where it could already be used in nixpkgs?

[zimbatm]

rename to mkDuration to indicate that it's a constructor?

[rycee]

Nice idea, fixed.

[rycee]

@zimbatm I think many NixOS options that contains the string "second" or "minute" are candidates for using this type (well the one with int fallback for backwards compatibility). Taking two random examples from the manual:

boot.initrd.luks.devices.<name?>.yubikey.gracePeriod could be

boot.initrd.luks.devices.<name?>.yubikey.gracePeriod

    Duration of time to wait before attempting to find the Yubikey.

    Type: duration

    Default: 2 seconds

and

services.factorio.autosave-interval could be

services.factorio.autosave-interval

    Autosave interval.

    Type: null or duration

    Default: null

    Example: 10 minutes

[basvandijk]

This might also be used in the strongswan-swanctl module which also uses durations. I even added a TODO to check for the correct duration format.

[edolstra]

Nix is not a general purpose language. We absolutely should not be parsing durations in Nix code since this sort of thing slows down NixOS/Nixpkgs evaluation substantially. For specifying durations in NixOS options, I don't think we need much more than a set of multipliers (e.g. timeout = 123 * lib.durations.minute), or if you want to get superfancy, a duration type (e.g. timeout = lib.durationFromMinutes 123).

[rycee]

Closing a few left-over issues.