The write gate was one mutex the writer held across every synchronous transport
poll, and close_conn took that same mutex before reclaiming a connection's
owners. Under a transport that accepts a byte per poll the writer re-acquired it
hundreds of times while teardown queued behind it, so teardown could be starved
for a whole frame and the delivery completed just before its owner was
reclaimed. bus-v1 section 9 puts release and route-health control out of reach of
telemetry starvation; tests/sol_rereview_regressions.rs's poll-gate regression
failed 6 release runs out of 6 alone, and about one debug run in five.
The gate now separates "teardown has begun" from "a poll is in progress": a
mutex plus a condvar. begin_close sets closing once without waiting, then waits
for the poll already in progress and records whether it left a frame half
written. The writer brackets each poll with enter_poll (refused once closing)
and leave_poll(bytes) instead of holding a lock across it, so teardown's window
is one poll rather than a frame, and no byte follows it. close_conn no longer
holds the gate across reclamation. Every item is pub(crate): no public
signature changed.
The regression test no longer synchronises with a sleep. The shutdown thread
announces itself on a channel, and what follows is structural: teardown cannot
pass the gate until the held poll returns, which only release() allows. Its
delivery is 50 KB now, which a writer resuming a byte per poll cannot finish
inside the window, so the test observes the ordering instead of a coin toss and
asserts the two legal shapes of the stream: cut short with nothing appended, or
never begun with exactly the closing notices following.
