0005: The isolated context process is a gen_statem

Since 5.0.0. Code: src/py_isolated.erl, entered from init/4 in py_context with gen_statem:enter_loop/5.

Situation

The embedded context process is a hand-written receive loop in py_context. The isolated process has more to track: a request in flight, requests to hold while the child restarts, a running loop with a grace period, an interrupt with a kill backstop bound to one request id, and callback processes whose nested requests must pass while others wait. A first version as a receive loop mirrored py_context but every wait needed its own selective receive and its own timer bookkeeping.

Decision

py_isolated is a gen_statem (handle_event_function, state enter calls) with states idle, {busy, Id}, looping, stopping_loop and {restarting, Reason}. Requests that must wait are postponed and replayed by the behaviour in arrival order; timers are state_timeout and named generic timeouts ({timeout, kill}) cancelled by the state change that makes them moot. It is spawned with proc_lib so it keeps the process identity and message protocol of py_context.

Not chosen: sharing the receive loop with py_context (the two have different failure models: a child can die and restart, a thread cannot), or a gen_server with a state field (postpone and state timeouts would have to be reimplemented).

Consequences

• sys:get_state/1 shows what a context is doing and sys:trace/2 prints every event; the state machines page is a transcription of the callback module.
• Callers do not see the behaviour: messages and replies are those of py_context.
• The two context processes are different code. A change to the message protocol touches both.