Two pre-existing issues in the nrepl command, exposed when #269 moved the accept
loop into a future.
jolt.nrepl/start was invoked inside (future ...), so binding the socket happened
on a background thread. A bind failure such as the port already being in use was
captured into a future that nothing derefs and silently swallowed, leaving the
main thread parked in run-main-pump forever with no server and no error. start
now binds the socket synchronously before returning, so that failure propagates
to the caller and the process exits with a visible message. Only the blocking
accept loop runs on a worker thread.
There was also no shutdown path: the accept loop ran forever and the listen
socket was never closed. start now returns a stop fn that breaks the accept
loop, closes the socket to free the port, and removes .nrepl-port. main.clj runs
run-main-pump and then calls the stop fn, so a stop-main-pump (from a glimmer app
on quit, or from nrepl-evaluated code) shuts the server down cleanly and the
process exits.
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