This is in preparation for landing the Tokio upgrade.
In order to be generic over Tokio's current thread and threadpool executors,
a number of types in Conduit which were not previously `Send` are now required
to be `Send`. A majority of this work will be done in the main Tokio upgrade
PR, as it is in many cases not possible to make these types `Send` _without_
using the new Tokio API (in order to remove `Handle`s, etc.); however, I'm
factoring out everything possible and trying to land it in separate PRs.
The p2c load balancer constructed in `Outbound` is currently parameterized
over a random number generator. We currently construct it by getting the
thread-local RNG, and passing it to the load balancer constructor. However,
the thread-local RNG is not `Send`. I've fixed this issue by creating a new
zero-sized empty struct type which implements `rand::Rng` simply by calling
`thread_rng()` every time its' called, and passing that to
`choose::power_of_two_choices` instead. Since this is an empty type which
contains no data, and the correct thread-local RNG is accessed whenever
the methods are called, this new type can trivially be `Send`. According to
the `rand` crate's documentation, this is the correct way to use `ThreadRng`
anyway:
> Retrieve the lazily-initialized thread-local random number generator, seeded
> by the system. Intended to be used in method chaining style, e.g.
> `thread_rng().gen::<i32>()`.
> (from https://docs.rs/rand/0.4.2/rand/fn.thread_rng.html)
This shouldn't lead to any functional changes.
Signed-off-by: Eliza Weisman <eliza@buoyant.io>
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