This adds initial, unstable, support for the wasm32-wasi target. Not all of Tokio's
features are supported yet as WASI's non-blocking APIs are still limited.
Refs: tokio-rs/tokio#4827
Instead of using sleep in time::advance, this fixes the root of the issue. When futures passed
to `Runtime::block_on` are woken, it bypassed all the machinery around advancing time. By
intercepting wakes in the time driver, we know when the block_on task is woken and skip
advancing time in that case.
Fixes#3837
Previously, `time::advance` would set the mocked clock forward the
requested amount, then yield. However, if there was no work ready to
perform immediately, this would result in advancing to the next expiring
sleep.
Now, `time::advance(...)` will unblock at the requested time. The
difference between `time::advance(...)` and `time::sleep(...)` is a bit
fuzzy. The main difference is `time::sleep(...)` operates on the current
task and `time::advance(...)` operates at the runtime level.
Fixes#3710
The time driver stores an Instant internally used as a "base" for future
time calculations. Since this is generated as the Runtime is being
constructed, it previously always happened before the user had a chance
to pause time. The fractional-millisecond variations in the timing
around the runtime construction and time pause cause tests running
entirely in paused time to be very slightly deterministic, with the time
driver advancing time by 1 millisecond more or less depending on how the
sub-millisecond components of the `Instant`s involved compared.
To avoid this, there is now a new option on `runtime::Builder` which
will create a `Runtime` with time "instantly" paused. This, along with a
small change to have the time driver use the provided clock as the
source for its start time allow totally deterministic tests with paused
time.
Pausing time is a capability added to assist with testing Tokio code
dependent on time. Currently, the capability implicitly requires the
current_thread runtime.
This change enforces the requirement by panicking if called from a
multi-threaded runtime.
Satyam1Vishwakarma