feat: add an OptionalDuration type

[marten-seemann]

This is needed for #146, so we can deal with optional durations the same way we deal with optional integers.
Disadvantage: omitempty doesn't work any more. Instead, we'll have a null in the JSON represenation.

The Duration is only used in the AutoNATThrottleConfig, so this is minimal change. This go-ipfs branch is the only change that's required.

[lidel]

Looks good, but as this is something we will add to other places, we may want to tackle the awkward "null" notation used for indicating a default value. See comment inline.

[lidel]

Small UX issue after testing it in go-ipfs's optional-duration branch

Setting works as expected via ipfs config AutoNAT.Throttle.Interval 1s, but to unset I had to execute ipfs config AutoNAT.Throttle.Interval null which produced an awkward JSON with "null" (a string), not an actual null (without quotes):

 "AutoNAT": {
    "Throttle": {
      "GlobalLimit": 0,
      "Interval": "null",
      "PeerLimit": 0
    }
  },

Not the best way of indicating "default" value.

I've added more tests and support for "" (empty) string in c9b7984 so users don't break their nodes when they remove custom value by hand, but when done via CLI getting the "null" is awkward.

Is there a way to get "" or null or "default" instead of "null" ?
The "default" is the most user-friendly and would be preferable in my mind.

I'll look into this and push more tests. We should be liberal with values that we interpret as "use default" + serialize "default" to something more intuitive than "null" (with quotes).

[marten-seemann]

Can we change the logic for marshaling the default in MarshalText:

func (d Duration) MarshalText() ([]byte, error) {
	if d.value != nil {
		return []byte(d.value.String()), nil
	}
	return []byte("default"), nil
}

[aschmahmann]

Perhaps switching to (Un)MarshalJSON rather than text would help you out here.

@Stebalien any context for why you went with Text rather than JSON originally?

[lidel]

I switched to *JSON in 16af6f8 – *Text was probably used to avoid handling double quotes, but now that we want explicit string as the default (instead of null) *JSON is easier.

[lidel]

Anyway, the mentioned change removed some paper cuts.

Took it for a spin and now, no matter how user tries to restore default and ruin their day..

$ ipfs config AutoNAT.Throttle.Interval ""
$ ipfs config AutoNAT.Throttle.Interval null
$ ipfs config --json AutoNAT.Throttle.Interval '"null"'
$ ipfs config AutoNAT.Throttle.Interval default

..for all the above (done via CLI or a manual edit of JSON config) the Duration will correctly serialize to:

 "AutoNAT": {
    "Throttle": {
      "Interval": "default",

[lidel]

Ugh.. so I've played with this a bit while testing Swarm.RelayService config from ipfs/kubo#8522 and we will have Field: null all over the place due to OptionalInteger and Flag anyway.
We can't have "default" string for those, so I think its better to unify and switch Duration to use null as well. At least it will be consistent and easy to document.
It is also what OptionalString from #149 will do.

The default will now look like this:

 "AutoNAT": {
   "Throttle": {
     "Interval": null,

[lidel]

With recently added UX improvements described in #148 (comment) this LGTM.

[lidel]

Switched defaults to use JSON null (matching existing behavior in OptionalInteger, Flag and other types)
848e479 made it even smarter, when used as *OptionalDuration json:",omitempty" works again.