Test Details Overview

Guard `async::task` joins and move assignment (#5266)

Joining a running task from an io thread of its own io_context, or after
that context stopped, never returns. The debug asserts against this lived
in `tcp_server`, `tcp_socket` and `tcp_channel` `stop ()`, one copy each,
and the listener's join sites had none. `task::join ()` now release
asserts both through the strand's inner executor whenever it would have
to wait, so a release node aborts with a message instead of hanging, and
the copies are gone. A finished task still joins from any thread, any
number of times, even after shutdown.

Move assignment overwrote the target's future unchecked, which detached a
running coroutine from the only handle that can cancel or join it. It now
asserts the target is not running, as the destructor does.

Death tests cover the join and move guards, the destructor, the empty
task and a moved-from condition; plain tests cover joining finished tasks
after the context stopped and from an io thread.

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Type: commit

Hash: 8c173d9ecabd301755979bcfb7ac0267993d7ce2

Created: yesterday

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gr0v1ty/nano-node:8c173d9ecabd301755979bcfb7ac0267993d7ce2