tcpreuse: avoid deadlock between listen and close

[felix314159]

Description

ConnMgr.DemultiplexedListen held the connection-manager mutex while acquiring a multiplexed-listener mutex. Listener closure acquired those mutexes in the opposite order while removing itself from the manager.

Concurrent listener registration and closure could therefore deadlock, preventing listener reconfiguration or host shutdown from completing.

Cancel the multiplexed listener under its own mutex, then close and remove it without holding that mutex. Reject new demultiplexed listeners after closure begins and make closure idempotent.

Reproduction

A deterministic regression test pauses underlying listener closure while registering another demultiplexed listener. Before the fix, both operations wait indefinitely on the inverted locks.

Testing

• go test ./p2p/transport/tcpreuse -run TestConcurrentListenAndCloseDoesNotDeadlock -count=100
• go test -race ./p2p/transport/tcpreuse
• go vet ./p2p/transport/tcpreuse
• go test ./...