PR #269 added the main-thread executor (call-on-main-thread, run-main-pump,
stop-main-pump) so the nREPL accept loop can run on a worker thread while the
primordial thread owns the GUI main loop. Two problems made stop-main-pump
unusable as a graceful-shutdown or external API.
run-main-pump set jolt-main-pump-active to #t on entry but never cleared it on
exit, so after the pump returned the flag stayed #t. call-on-main-thread also
read that flag outside the queue mutex, so even with a reset there was a window
where a job could be enqueued just as the pump left, then block forever on a
pump that was gone.
Both are now decided under jolt-main-queue-mu. The pump clears active in the
same critical section where it sees the stop flag and an empty queue, and
call-on-main-thread reads active and enqueues atomically under that lock. A
caller that loses the race sees the pump inactive and runs the thunk inline,
the same fallback used when no pump is running, rather than blocking. A
dynamic-wind around the loop also clears active on an abnormal exit so a later
run-main-pump starts clean.
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