`asyncio.start_unix_server` does not exist on Windows (no AF_UNIX event-loop
support in asyncio), so arming the loop-tick witness in
`loop_heartbeat_forever` raised AttributeError on every native-Windows
gateway start. The broad except swallowed it and recorded
`loop_tick_socket=False`, so every stale-heartbeat probe classified the
gateway as UNKNOWN — never WEDGED, never ALIVE-with-stalled-write. The
two-witness interlock from a1c83ef9 (issue #90502 follow-up) has been
effectively disabled on Windows since it landed: a wedged native-Windows
gateway could never be detected, and an alive one could never be
distinguished from a stalled heartbeat write.
On non-POSIX platforms the witness now arms over a TCP loopback server on
127.0.0.1 (OS-assigned dynamic port) instead:
- same protocol — connect, read one byte "1"
- same semantics — pure in-memory, zero disk I/O, answered only while the
loop is dispatching, armed by the loop task itself (an awaited
`asyncio.start_server` is structurally loop-owned exactly like the Unix
variant, so a wedged loop cannot keep answering pings)
- the assigned port is published in the heartbeat payload as
`loop_tick_tcp_port`, and `probe_gateway_loop_liveness` prefers the TCP
witness when the producer published a port, falling back to the AF_UNIX
socket for POSIX/legacy producers
POSIX behavior is unchanged: the AF_UNIX arm (including the stale-node
sweep) stays gated behind `os.name == "posix"` so the missing attribute can
never raise on Windows again. Legacy heartbeats without `loop_tick_tcp_port`
keep the existing socket-node contract untouched.
Tested end-to-end on native Windows: witness arms, port is published,
`_probe_loop_tick_tcp` answers from an external thread while the loop
dispatches, and the existing loop-liveness suite passes unchanged (the
AF_UNIX structural test still passes — the Unix arm text is preserved
inside the POSIX branch).
Adds two tests pinning the new behavior: an E2E test that arms the TCP
witness and probes it (skipped on POSIX, where the Unix arm is the real
witness), and a structural test that the TCP arm stays awaited on the loop
task and the AF_UNIX arm stays POSIX-gated.
OmniaZ1
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