Send ended an unsendable session after releasing SessionGate, and the
goodbye was built under the gate but sent outside it. If the polling
thread admitted a new Connect from the same endpoint in that window, the
ended generation's goodbye reached the new client: it saw Data, Accept,
Disconnect and believed itself disconnected while the listener held it
connected.
Every path that finds a permanent send failure now reads the session's
generation under the gate at the send, and EndUnsendableSession builds
and sends the goodbye under the gate only while that generation is
unchanged. The generation check replaces the UnsendableReason once-guard:
a second failure in the same generation finds it already ended.
The test orders the new Connect inside the send, before the fragment
that can never be sent, and asserts the new client hears no Disconnect
and stays connected on both sides. It fails on the pre-fix code
(Data, Accept, Disconnect) and kills the mutant that drops the check.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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