3b9ffac2 claimed that the generation check subsumed the UnsendableReason
once-guard. It did not. A thread that reads the generation only after
the session has ended carries the ended session's own generation, and the
check lets it through. For example, the polling thread answers a Ping on a
session that a caller's send has just ended but that has not yet been
retired. When that Pong fails permanently, the thread says a second
goodbye and overwrites the first failure's reason (Soul pass 4, PR4).
EndUnsendableSession now also returns when the session has already ended.
Two tests:
- The polling thread answers a Ping on a session already ended, and the
Pong can never be sent. Exactly one goodbye goes out, and the first
failure's reason stands. The hook admits the Ping after the session
ended and before the peer is queued for retirement. The test kills the
mutant that drops the Ended check.
- A failed send's goodbye is sent under the gate. A polling thread
admitting a new Connect from the endpoint waits on the gate, so the old
generation's goodbye never follows the new session's Accept. The test
kills the mutant that sends the goodbye outside the gate.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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