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Summary

This RFC suggests stabilizing a reduced-scope Duration type that is appropriate for interoperating with various system calls that require timeouts. It does not stabilize a large number of conversion methods in Duration that have subtle caveats, with the intent of revisiting those conversions more holistically in the future.

Motivation

There are a number of different notions of "time", each of which has a different set of caveats, and each of which can be designed for optimal ergonomics for its domain. This proposal focuses on one particular one: an amount of time in high-precision units.

Eventually, there are a number of concepts of time that deserve fleshed out APIs. Using the terminology from the popular Java time library JodaTime:

  • Duration: an amount of time, described in terms of a high precision unit.
  • Period: an amount of time described in human terms ("5 minutes, 27 seconds"), and which can only be resolved into a Duration relative to a moment in time.
  • Instant: a moment in time represented in terms of a Duration since some epoch.

Human complications such as leap seconds, days in a month, and leap years, and machine complications such as NTP adjustments make these concepts and their full APIs more complicated than they would at first appear. This proposal focuses on fleshing out a design for Duration that is sufficient for use as a timeout, leaving the other concepts of time to a future proposal.

For the most part, the system APIs that this type is used to communicate with either use timespec (u64 seconds plus u32 nanos) or take a timeout in milliseconds (u32 on Windows).

For example, GetQueuedCompletionStatus, one of the primary APIs in the Windows IOCP API, takes a dwMilliseconds parameter as a DWORD, which is a u32. Some Windows APIs use "ticks" or 100-nanosecond units.

In light of that, this proposal has two primary goals:

  • to define a type that can describe portable timeouts for cross- platform APIs
  • to describe what should happen if a large Duration is passed into an API that does not accept timeouts that large

In general, this proposal considers it acceptable to reduce the granularity of timeouts (eliminating nanosecond granularity if only milliseconds are supported) and to truncate very large timeouts.

This proposal retains the two fields in the existing Duration:

  • a u64 of seconds
  • a u32 of additional nanosecond precision

Timeout APIs defined in terms of milliseconds will truncate Durations that are more than u32::MAX in milliseconds, and will reduce the granularity of the nanosecond field.

A u32 of milliseconds supports a timeout longer than 45 days.

Future APIs to support a broader set of Durations APIs, a Period and Instant type, as well as coercions between these types, would be useful, compatible follow-ups to this RFC.

Detailed design

A Duration represents a period of time represented in terms of nanosecond granularity. It has u64 seconds and an additional u32 nanoseconds. There is no concept of a negative Duration.

A negative Duration has no meaning for many APIs that may wish to take a Duration, which means that all such APIs would need to decide what to do when confronted with a negative Duration. As a result, this proposal focuses on the predominant use-cases for Duration, where unsigned types remove a number of caveats and ambiguities.

pub struct Duration {
  secs: u64,
  nanos: u32 // may not be more than 1 billion
}

impl Duration {
    /// create a Duration from a number of seconds and an
    /// additional nanosecond precision. If nanos is one
    /// billion or greater, it carries into secs.
    pub fn new(secs: u64, nanos: u32) -> Timeout;

    /// create a Duration from a number of seconds
    pub fn from_secs(secs: u64) -> Timeout;

    /// create a Duration from a number of milliseconds
    pub fn from_millis(millis: u64) -> Timeout;

    /// the number of seconds represented by the Duration
    pub fn secs(self) -> u64;

    /// the number of additional nanosecond precision
    pub fn nanos(self) -> u32;
}

When Duration is used with a system API that expects u32 milliseconds, the Duration's precision is coarsened to milliseconds, and, and the number is truncated to u32::MAX.

In general, this RFC assumes that timeout APIs permit spurious updates (see, for example, pthread_cond_timedwait, "Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur").

Duration implements:

  • Add, Sub, Mul, Div which follow the overflow and underflow rules for u64 when applied to the secs field (in particular, Sub will panic if the result would be negative). Nanoseconds must be less than 1 billion and great than or equal to 0, and carry into the secs field.
  • Display, which prints a number of seconds, milliseconds and nanoseconds (if more than 0). For example, a Duration would be represented as "15 seconds, 306 milliseconds, and 13 nanoseconds"
  • Debug, Ord (and PartialOrd), Eq (and PartialEq), Copy and Clone, which are derived.

This proposal does not, at this time, include mechanisms for instantiating a Duration from weeks, days, hours or minutes, because there are caveats to each of those units. In particular, the existence of leap seconds means that it is only possible to properly understand them relative to a particular starting point.

The Joda-Time library in Java explains the problem well in their documentation:

A duration in Joda-Time represents a duration of time measured in milliseconds. The duration is often obtained from an interval. Durations are a very simple concept, and the implementation is also simple. They have no chronology or time zone, and consist solely of the millisecond duration.

A period in Joda-Time represents a period of time defined in terms of fields, for example, 3 years 5 months 2 days and 7 hours. This differs from a duration in that it is inexact in terms of milliseconds. A period can only be resolved to an exact number of milliseconds by specifying the instant (including chronology and time zone) it is relative to.

In short, this is saying that people expect "23:50:00 + 10 minutes" to equal "00:00:00", but it's impossible to know for sure whether that's true unless you know the exact starting point so you can take leap seconds into consideration.

In order to address this confusion, Joda-Time's Duration has methods like standardDays/toStandardDays and standardHours/toStandardHours, which are meant to indicate to the user that the number of milliseconds is based on the standard number of milliseconds in an hour, rather than the colloquial notion of an "hour".

An approach like this could work for Rust, but this RFC is intentionally limited in scope to areas without substantial tradeoffs in an attempt to allow a minimal solution to progress more quickly.

This proposal does not include a method to get a number of milliseconds from a Duration, because the number of milliseconds could exceed u64, and we would have to decide whether to return an Option, panic, or wait for a standard bignum. In the interest of limiting this proposal to APIs with a straight-forward design, this proposal defers such a method.

Drawbacks

The main drawback to this proposal is that it is significantly more minimal than the existing Duration API. However, this API is quite sufficient for timeouts, and without the caveats in the existing Duration API.

Alternatives

We could stabilize the existing Duration API. However, it has a number of serious caveats:

  • The caveats described above about some of the units it supports.
  • It supports converting a Duration into a number of microseconds or nanoseconds. Because that cannot be done reliably, those methods return Options, and APIs that need to convert Duration into nanoseconds have to re-surface the Option (unergonomic) or panic.
  • More generally, it has a fairly large API surface area, and almost every method has some caveat that would need to be explored in order to stabilize it.

We could also include a number of convenience APIs that convert from other units into Durations. This proposal assumes that some of those conveniences will eventually be added. However, the design of each of those conveniences is ambiguous, so they are not included in this initial proposal.

Finally, we could avoid any API for timeouts, and simply take milliseconds throughout the standard library. However, this has two drawbacks.

First, it does not allow us to represent higher-precision timeouts on systems that could support them.

Second, while this proposal does not yet include conveniences, it assumes that some conveniences should be added in the future once the design space is more fully explored. Starting with a simple type gives us space to grow into.

Unresolved questions

  • Should we implement all of the listed traits? Others?