Implement I/O timeouts that specify the REALTIME clock. (#4777)

POSIX specifies that functions like `nanosleep` use the REALTIME clock,
so allow WASI `poll_oneoff` calls to use the REALTIME clock, at least
for non-absolute intervals. POSIX specifies that the timeouts should not
be affected by subsequent `clock_settime` calls, so they behave the same
way as MONOTONIC clock requests, so we can implement them as monotonic
requests.

crates/wasi-common/src/snapshots/preview_1.rs

@@ -1090,6 +1090,33 @@ impl wasi_snapshot_preview1::WasiSnapshotPreview1 for WasiCtx {
  sub.userdata.into(),
  )
  }
+ types::Clockid::Realtime => {
+ // POSIX specifies that functions like `nanosleep` and others use the
+ // `REALTIME` clock. But it also says that `clock_settime` has no effect
+ // on threads waiting in these functions. MONOTONIC should always have
+ // resolution at least as good as REALTIME, so we can translate a
+ // non-absolute `REALTIME` request into a `MONOTONIC` request.
+ let clock = self.clocks.monotonic.deref();
+ let precision = Duration::from_nanos(clocksub.precision);
+ let duration = Duration::from_nanos(clocksub.timeout);
+ let deadline = if clocksub
+ .flags
+ .contains(types::Subclockflags::SUBSCRIPTION_CLOCK_ABSTIME)
+ {
+ return Err(Error::not_supported());
+ } else {
+ clock
+ .now(precision)
+ .checked_add(duration)
+ .ok_or_else(|| Error::overflow().context("deadline"))?
+ };
+ poll.subscribe_monotonic_clock(
+ clock,
+ deadline,
+ precision,
+ sub.userdata.into(),
+ )
+ }
  _ => Err(Error::invalid_argument()
  .context("timer subscriptions only support monotonic timer"))?,
  },