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.
A context is the natural fit (vs. a receive stream) for locking the root
proc's tty usage via it's `.started()` sync point. Simplify the
`_breakpoin()` routine to be a simple async func instead of all this
"returning a coroutine" stuff from before we decided that
`tractor.breakpoint()` must be async. Use `runtime` level for locking
logging making it easier to trace.
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
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)
Tyler Goodlet
goodboy