Another face palm that was causing serious issues for code that is using
the `.shielded` feature..
Add a bunch more detailed comments for all this subtlety and hopefully
get it right once and for all. Also aggregated the `trio` errors that
should trigger closure inside `.aclose()`, hopefully that's right too.
Revert this change since it really is poking at internals and doesn't
make a lot of sense. If the context is going to be cancelled then the
msg loop will tear down the feed memory channel when ready, we don't
need to be clobbering it and confusing the runtime machinery lol.
Add clear teardown semantics for `Context` such that the remote side
cancellation propagation happens only on error or if client code
explicitly requests it (either by exit flag to `Portal.open_context()`
or by manually calling `Context.cancel()`). Add `Context.result()`
to wait on and capture the final result from a remote context function;
any lingering msg sequence will be consumed/discarded.
Changes in order to make this possible:
- pass the runtime msg loop's feeder receive channel in to the context
on the calling (portal opening) side such that a final 'return' msg
can be waited upon using `Context.result()` which delivers the final
return value from the callee side `@tractor.context` async function.
- always await a final result from the target context function in
`Portal.open_context()`'s `__aexit__()` if the context has not
been (requested to be) cancelled by client code on block exit.
- add an internal `Context._cancel_called` for context "cancel
requested" tracking (much like `trio`'s cancel scope).
- allow flagging a stream as terminated using an internal
`._eoc` flag which will mark the stream as stopped for iteration.
- drop `StopAsyncIteration` catching in `.receive()`; it does
nothing.
This mostly adds the api described in
https://github.com/goodboy/tractor/issues/53#issuecomment-806258798
The first draft summary:
- formalize bidir steaming using the `trio.Channel` style interface
which we derive as a `MsgStream` type.
- add `Portal.open_context()` which provides a `trio.Nursery.start()`
remote task invocation style for setting up and tearing down tasks
contexts in remote actors.
- add a distinct `'started'` message to the ipc protocol to facilitate
`Context.start()` with a first return value.
- for our `ReceiveMsgStream` type, don't cancel the remote task in
`.aclose()`; this is now done explicitly by the surrounding `Context`
usage: `Context.cancel()`.
- streams in either direction still use a `'yield'` message keeping the
proto mostly symmetric without having to worry about which side is the
caller / portal opener.
- subtlety: only allow sending a `'stop'` message during a 2-way
streaming context from `ReceiveStream.aclose()`, detailed comment
with explanation is included.
Relates to #53
Since we currently have no real "discovery protocol" between process
trees, the current naive approach is to check via a connect and drop to
see if a TCP server is bound to a particular address during root actor
startup. This was a historical decision and had no real grounding beyond
taking a simple approach to get something working when the project
was first started.
This is obviously problematic from an error handling perspective since
we need to be able to avoid such quick connect-and-drops from cancelling
an "arbiter"'s (registry actor's) channel-msg loop machinery (which
would propagate and cancel the actor).
For now we map this particular TCP error, which gets remapped by `trio`
as a `trio.BrokenResourceError` to our own internal `TransportClosed`
which is swallowed by channel message loop processing and indicates
a graceful teardown of the far end actor.
This change some super old (and bad) code from the project's very early
days. For some redic reason i must have thought masking `trio`'s
internal stream / transport errors and a TCP EOF as `StopAsyncIteration`
somehow a good idea. The reality is you probably
want to know the difference between an unexpected transport error
and a simple EOF lol. This begins to resolve that by adding our own
special `TransportClosed` error to signal the "graceful" termination of
a channel's underlying transport. Oh, and this builds on the `msgspec`
integration which helped shed light on the core issues here B)
It's clear now that special attention is needed to handle the case where
a spawned `multiprocessing` proc is started but then the parent is
cancelled before the child can connect back; in this case we need to be
sure to kill the near-zombie child asap. This may end up being the
solution to other resiliency issues seen around mp with nested process
trees too. More testing is needed to be sure.
Relates to #84#89#134#146
Tyler Goodlet