Summary

Add sockopt-style timeouts to std::net types.

Motivation

Currently, operations on various socket types in std::net block indefinitely (i.e., until the connection is closed or data is transferred). But there are many contexts in which timing out a blocking call is important.

The goal of the current IO system is to gradually expose cross-platform, blocking APIs for IO, especially APIs that directly correspond to the underlying system APIs. Sockets are widely available with nearly identical system APIs across the platforms Rust targets, and this includes support for timeouts via sockopts.

So timeouts are well-motivated and well-suited to std::net.

Detailed design

The proposal is to directly expose the timeout functionality provided by setsockopt, in much the same way we currently expose functionality like set_nodelay:

impl TcpStream {
    pub fn set_read_timeout(&self, dur: Option<Duration>) -> io::Result<()> { ... }
    pub fn read_timeout(&self) -> io::Result<Option<Duration>>;

    pub fn set_write_timeout(&self, dur: Option<Duration>) -> io::Result<()> { ... }
    pub fn write_timeout(&self) -> io::Result<Option<Duration>>;
}

impl UdpSocket {
    pub fn set_read_timeout(&self, dur: Option<Duration>) -> io::Result<()> { ... }
    pub fn read_timeout(&self) -> io::Result<Option<Duration>>;

    pub fn set_write_timeout(&self, dur: Option<Duration>) -> io::Result<()> { ... }
    pub fn write_timeout(&self) -> io::Result<Option<Duration>>;
}

The setter methods take an amount of time in the form of a Duration, which is undergoing stabilization. They are implemented via straightforward calls to setsockopt. The Option is used to signify no timeout (for both setting and getting). Consequently, Some(Duration::new(0, 0)) is a possible argument; the setter methods will return an IO error of kind InvalidInput in this case. (See Alternatives for other approaches.)

The corresponding socket options are SO_RCVTIMEO and SO_SNDTIMEO.

Drawbacks

One potential downside to this design is that the timeouts are set through direct mutation of the socket state, which can lead to composition problems. For example, a socket could be passed to another function which needs to use it with a timeout, but setting the timeout clobbers any previous values. This lack of composability leads to defensive programming in the form of "callee save" resets of timeouts, for example. An alternative design is given below.

The advantage of binding the mutating APIs directly is that we keep a close correspondence between the std::net types and their underlying system types, and a close correspondence between Rust APIs and system APIs. It's not clear that this kind of composability is important enough in practice to justify a departure from the traditional API.

Alternatives

Taking `Duration` directly

Using an Option<Duration> introduces a certain amount of complexity -- it raises the issue of Some(Duration::new(0, 0)), and it's slightly more verbose to set a timeout.

An alternative would be to take a Duration directly, and interpret a zero length duration as "no timeout" (which is somewhat traditional in C APIs). That would make the API somewhat more familiar, but less Rustic, and it becomes somewhat easier to pass in a zero value by accident (without thinking about this possibility).

Note that both styles of API require code that does arithmetic on durations to check for zero in advance.

Aside from fitting Rust idioms better, the main proposal also gives a somewhat stronger indication of a bug when things go wrong (rather than simply failing to time out, for example).

Combining with nonblocking support

Another possibility would be to provide a single method that can choose between blocking indefinitely, blocking with a timeout, and nonblocking mode:

enum BlockingMode {
    Nonblocking,
    Blocking,
    Timeout(Duration)
}

This enum makes clear that it doesn't make sense to have both a timeout and put the socket in nonblocking mode. On the other hand, it would relinquish the one-to-one correspondence between Rust configuration APIs and underlying socket options.

Wrapping for compositionality

A different approach would be to wrap socket types with a "timeout modifier", which would be responsible for setting and resetting the timeouts:

struct WithTimeout<T> {
    timeout: Duration,
    inner: T
}

impl<T> WithTimeout<T> {
    /// Returns the wrapped object, resetting the timeout
    pub fn into_inner(self) -> T { ... }
}

impl TcpStream {
    /// Wraps the stream with a timeout
    pub fn with_timeout(self, timeout: Duration) -> WithTimeout<TcpStream> { ... }
}

impl<T: Read> Read for WithTimeout<T> { ... }
impl<T: Write> Write for WithTimeout<T> { ... }

A previous RFC spelled this out in more detail.

Unfortunately, such a "wrapping" API has problems of its own. It creates unfortunate type incompatibilities, since you cannot store a timeout-wrapped socket where a "normal" socket is expected. It is difficult to be "polymorphic" over timeouts.

Ultimately, it's not clear that the extra complexities of the type distinction here are worth the better theoretical composability.

Unresolved questions

Should we consider a preliminary version of this RFC that introduces methods like set_read_timeout_ms, similar to wait_timeout_ms on Condvar? These methods have been introduced elsewhere to provide a stable way to use timeouts prior to Duration being stabilized.

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