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Closing the same tunnel from several clients at once could crash the daemon with "close of closed channel": Close only checked Status, which stays Open until the run loop has shut the tunnel down. The stop channel is now closed through a sync.Once, so Close is safe to call repeatedly. Status and LastConn were written by the tunnel goroutines and read by the daemon for 'list' without synchronization. They are now guarded by a mutex in Tunnel, and the daemon lists tunnels through Snapshot. Goroutines tracked by the tunnel's WaitGroup called Add from inside the new goroutine, racing with Wait. goWait (formerly waitFor) now calls Add before starting the goroutine. Opening the same tunnel concurrently let every client past the 'already running' check, so all but one failed with 'address already in use'. The name is now reserved while Open is in progress. Other clients wait for that attempt and report the tunnel as already running, or retry if it failed. A closed tunnel was removed from the daemon by name, so a tunnel that was reopened quickly under the same name could be dropped from the map while still running. Removal now checks it is the same tunnel, and closeTunnel removes it before responding. Covered by unit tests for Tunnel.Close, Snapshot and the daemon's name reservation, plus e2e tests that open and close the same tunnel from several clients at once.
A client can also lose the race after the tunnel has shut down but before it was removed from the daemon, which is reported as 'trying to close a closed tunnel'. That is as correct as the other two outcomes, so the test must not fail on it.
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