Features: #310 — reaper no longer stops a just-delivered worker

11-Features.md

@@ -3715,3 +3715,27 @@ becomes the lead. The range matters because on Linux the whole `/8` is bound to
 `127.0.0.2` is bindable; measured on the fleet host with `curl --interface 127.0.0.2` returning exit
 7 (connect refused) rather than 45 (bind failed). On macOS the same command fails at the bind, which
 is why no test binds a real `127.0.0.2` source — it would fail for every developer on a Mac.
+
+## The idle reaper no longer stops a worker that just got a turn
+
+**What.** `reapIdle` decided a session was idle from a roster snapshot and then released it without
+re-reading. A delivery landing in between made the session BUSY, and it was torn down anyway —
+contrary to the method's own javadoc, which promised it never reaps a BUSY worker. The reap is now a
+compare-and-release: it tears the session down only while the registry still holds the exact record
+it checked.
+
+**On.** Always on.
+
+**Why it exists.** `onDelivered` flips exactly the two states the reaper accepts (READY, DONE) to
+BUSY, and `bumpTurn` refreshes the activity clock — so the very act that should save a session from
+the reaper is the one that raced it. The worker was killed, the turn never ran, and the release
+reason said nothing about a race. It was never a silent loss: the release listener still fails a
+blocked send with a reason, so the lead is told something went wrong, just not what.
+
+**One thing to know for maintenance.** The compare is `ConcurrentHashMap.remove(key, value)`, and
+that is the linearization point — do not "simplify" it to a re-read of the state followed by a plain
+remove, which shrinks the window without closing it and leaves the code looking correct. The public
+`release` path stays unconditional on purpose: an explicit `fleet_stop` must not be refused because
+the worker happens to have just become busy. A skipped reap logs one debug line naming the pane;
+without it the race is unobservable by construction, and a reaper that quietly stops reaping is very
+hard to diagnose.