device: set peer to expire unconditionally (#73)

e3ac4a0afb4e introduced a lightweight API that can be used instead of UAPI
to reconfigure peers. Peer state created via the new PeerLookupFunc
is not set to expire until the handshake succeeds, making device leak two
goroutines and a set of buffers for each failed handshake.

This change arms the expiry timer before the handshake gets to proceed.

Updates tailscale/tailscale#20183

Change-Id: Ibc0abb6eec97aca0a10f50515dea9e0d6a6a6964
Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>

device/peer.go

@@ -254,6 +254,14 @@ func (peer *Peer) Start() {
 	go peer.RoutineSequentialReceiver(batchSize)
 
 	peer.isRunning.Store(true)
+
+	// A lazily-created peer that never completes a handshake otherwise never
+	// arms its reaping timer. Arm it here, while running under state.Lock, so
+	// it's reclaimed after RejectAfterTime*3 of no session and is guaranteed to
+	// be torn down by a matching Stop. A completed handshake re-Mods it.
+	if peer.deleteOnIdle {
+		peer.timers.zeroKeyMaterial.Mod(RejectAfterTime * 3)
+	}
 }
 
 func (peer *Peer) ZeroAndFlushAll() {