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.
The runtime is inherently multi-threaded, so it's going to have to deal
with synchronization when submitting new tasks anyway. This allows a
runtime to be shared by multiple threads more easily when e.g. building
a blocking facade over a tokio-based API.
Steven Fackler