Add a ``tractor._portal.StreamReceiveChannel.shield_channel()`` context
manager which allows for avoiding the closing of an IPC stream's
underlying channel for the purposes of task re-spawning. Sometimes you
might want to cancel a task consuming a stream but not tear down the IPC
between actors (the default). A common use can might be where the task's
"setup" work might need to be redone but you want to keep the
established portal / channel in tact despite the task restart.
Includes a test.
This appears to demonstrate the same bug found in #156. It looks like
cancelling a subactor with a child, while that child is running sync code,
can result in the child never getting cancelled due to some strange
condition where the internal nurseries aren't being torn down as
expected when a `trio.Cancelled` is raised.
The real issue is if the root nursery gets cancelled prior to
de-registration with the arbiter. This doesn't seem easy to
reproduce by side effect of a KBI however that is how it was
discovered in practise.
There was code from the last de-registration fix PR that I had commented
(to do with shielding arbiter dereg steps in `Actor._async_main()`) because
the block didn't seem to make a difference under infinite streaming
tests. Turns out it **for sure** is needed under certain conditions (likely
if the actor's root nursery is cancelled prior to actor nursery exit).
This was an attempt to simulate the failure mode if you manually close the
stream **before** cancelling the containing **actor**.
More tests to come I guess.
This truly reproduces #141. It turns out the problem only occurs when
we're cancelled in the middle of consuming "infinite streams".
Good news is this tests a lot of edge cases :)
Tyler Goodlet
Guillermo Rodriguez