By re-purposing our `pexpect`-based console matching with a new
`debugging/shield_hang_in_sub.py` example, this tests a few "hanging
actor" conditions more formally:
- that despite a hanging actor's task we can dump
a `stackscope.extract()` tree on relay of `SIGUSR1`.
- the actor tree will terminate despite a shielded forever-sleep by our
"T-800" zombie reaper machinery activating and hard killing the
underlying subprocess.
Some test deats:
- simulates the expect actions of a real user by manually using
`os.kill()` to send both signals to the actor-tree program.
- `pexpect`-matches against `log.devx()` emissions under normal
`debug_mode == True` usage.
- ensure we get the actual "T-800 deployed" `log.error()` msg and
that the actor tree eventually terminates!
Surrounding (re-org/impl/test-suite) changes:
- allow disabling usage via a `maybe_enable_greenback: bool` to
`open_root_actor()` but enable by def.
- pretty up the actual `.devx()` content from `.devx._stackscope`
including be extra pedantic about the conc-primitives for each signal
event.
- try to avoid double handles of `SIGUSR1` even though it seems the
original (what i thought was a) problem was actually just double
logging in the handler..
|_ avoid double applying the handler func via `signal.signal()`,
|_ use a global to avoid double handle func calls and,
|_ a `threading.RLock` around handling.
- move common fixtures and helper routines from `test_debugger` to
`tests/devx/conftest.py` and import them for use in both test mods.
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