_read_loop opened with 'assert self._ws is not None' — an exit that
escaped BEFORE the finally that fails pending futures, contradicting
the 'fails pending on ANY exit path' invariant the hardening commit
established. Production currently assigns _ws before scheduling the
reader, so this was latent, but any future lifecycle change hitting it
would strand every in-flight waiter for the full outbound timeout with
only an AssertionError in the logs.
Turn it into a guarded early-return INSIDE the try: the reader logs
the lifecycle bug and unwinds through the same finally as every other
exit, settling all waiters. Regression test drives _read_loop with
_ws=None against a registered pending future.
Ben Barclay
2f77ca1993