struct Deadline [src]

Deadline is used to wait efficiently for a pointer's value to change using Futex and a fixed timeout. Futex's timedWait() api uses a relative duration which suffers from over-waiting when used in a loop which is often required due to the possibility of spurious wakeups. Deadline instead converts the relative timeout to an absolute one so that multiple calls to Futex timedWait() can block for and report more accurate error.Timeouts.

Fields

timeout: ?u64

started: std.time.Timer

Members

init (Function)
wait (Function)

Source

  pub const Deadline = struct {
    timeout: ?u64,
    started: std.time.Timer,

    /// Create the deadline to expire after the given amount of time in nanoseconds passes.
    /// Pass in `null` to have the deadline call `Futex.wait()` and never expire.
    pub fn init(expires_in_ns: ?u64) Deadline {
        var deadline: Deadline = undefined;
        deadline.timeout = expires_in_ns;

        // std.time.Timer is required to be supported for somewhat accurate reportings of error.Timeout.
        if (deadline.timeout != null) {
            deadline.started = std.time.Timer.start() catch unreachable;
        }

        return deadline;
    }

    /// Wait until either:
    /// - the `ptr`'s value changes from `expect`.
    /// - `Futex.wake()` is called on the `ptr`.
    /// - A spurious wake occurs.
    /// - The deadline expires; In which case `error.Timeout` is returned.
    pub fn wait(self: *Deadline, ptr: *const atomic.Value(u32), expect: u32) error{Timeout}!void {
        @branchHint(.cold);

        // Check if we actually have a timeout to wait until.
        // If not just wait "forever".
        const timeout_ns = self.timeout orelse {
            return Futex.wait(ptr, expect);
        };

        // Get how much time has passed since we started waiting
        // then subtract that from the init() timeout to get how much longer to wait.
        // Use overflow to detect when we've been waiting longer than the init() timeout.
        const elapsed_ns = self.started.read();
        const until_timeout_ns = std.math.sub(u64, timeout_ns, elapsed_ns) catch 0;
        return Futex.timedWait(ptr, expect, until_timeout_ns);
    }
}  