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Thanks to eta's refactoring, these tests no longer need to be async.
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Previously, the reactor would use an `UnboundedSender` to send things to
the `RawCellStream`, in order that the reactor wouldn't block if you
failed to read from the latter. This is bad, though, since it means
people can just run us out of memory by sending lots of things.
To fix this, we make the new `StreamReader` type (which does the reading
parts from `RawCellStream`) keep track of the stream's receive window
and issue SENDMEs once *it* has consumed enough data to require it, thus
meaning that we shouldn't get sent enough data to fill the channel
between reactor and `StreamReader` (and, if we do, that's someone trying
to flood us, and we abort the circuit).
As hinted to above, the `RawCellStream` was removed and its reading
functionalities replaced by `StreamReader`; its writing functionalities
are handled by `StreamTarget` anyway, so we just give out one of those
for the write end. This now means we don't need any mutexes!
note: this commit introduces a known issue, arti#230
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Rather like e8e9699c3c239d6c30f9ad414f15d3bad6ec03fd ("Get rid of
tor-proto's ChannelImpl, and use the reactor more instead"), this
admittedly rather large commit refactors the way circuits in `tor-proto`
work, centralising all of the logic in one large nonblocking reactor
which other things send messages into and out of, instead of having a
bunch of `-Impl` types that are protected by mutexes.
Congestion control becomes a lot simpler with this refactor, since the
reactor can manage both stream- and circuit-level congestion control
unilaterally without having to share this information with consumers,
meaning we can get rid of some locks.
The way streams work also changes, in order to facilitate better
handling of backpressure / fairness between streams: each stream now has
a set of channels to send and receive messages over, instead of sending
relay cells directly onto the channel (now, the reactor pulls messages
off each stream in each map, and tries to avoid doing so if it won't be
able to forward them yet).
Additionally, a lot of "close this circuit / stream" messages aren't
required any more, since that state is simply indicated by one end of a
channel going away. This should make cleanup a lot less brittle.
Getting all of this to work involved writing a fair deal of intricate
nonblocking code in Reactor::run_once that tries very hard to be mindful
of making backpressure work correctly (and congestion control); the old
code could get away with having tasks .await on things, but the new
reactor can't really do this (as it'd lock the reactor up), so has to do
everything in a nonblocking manner.
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Instead of awkwardly sharing the internals of a `tor-proto` `Channel`
between the reactor task and any other tasks, move most of the internals
into the reactor and have other tasks communicate with the reactor via
message-passing to allocate circuits and send cells.
This makes a lot of things simple, and has convenient properties like
not needing to wrap the `Channel` in an `Arc` (though some places in the
code still do this for now).
A lot of test code required tweaking in order to deal with the refactor;
in fact, fixing the tests probably took longer than writing the mainline
code (!). Importantly, we now use `tokio`'s `tokio::test` annotation
instead of `async_test`, so that we can run things in the background
(which is required to have reactors running for the circuit tests).
This is an instance of #205, and also kind of #217.
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Basically the same thing as 371437d3384ed73520e7141e66874be3d85f1df0
("Refactor tor_proto::channel::Reactor to use an UnboundedSender"), but
for tor_proto::circuit's Reactor instead.
(part of arti#217)
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This will cause some pain for now, but now is really the best time
to do this kind of thing.
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