wgengine/magicsock: re-shape relayManager to use an event loop (#15935)

The event loop removes the need for growing locking complexities and
synchronization. Now we simply use channels. The event loop only runs
while there is active work to do.

relayManager remains no-op inside magicsock for the time being.
endpoints are never 'relayCapable' and therefore endpoint & Conn will
not feed CallMeMaybeVia or allocation events into it.

A number of relayManager events remain unimplemented, e.g.
CallMeMaybeVia reception and relay handshaking.

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -95,6 +95,7 @@ type endpoint struct {
 
 	expired         bool // whether the node has expired
 	isWireguardOnly bool // whether the endpoint is WireGuard only
+	relayCapable    bool // whether the node is capable of speaking via a [tailscale.com/net/udprelay.Server]
 }
 
 func (de *endpoint) setBestAddrLocked(v addrQuality) {
@@ -1249,6 +1250,13 @@ func (de *endpoint) sendDiscoPingsLocked(now mono.Time, sendCallMeMaybe bool) {
 		// sent so our firewall ports are probably open and now
 		// would be a good time for them to connect.
 		go de.c.enqueueCallMeMaybe(derpAddr, de)
+
+		// Schedule allocation of relay endpoints. We make no considerations for
+		// current relay endpoints or best UDP path state for now, keep it
+		// simple.
+		if de.relayCapable {
+			go de.c.relayManager.allocateAndHandshakeAllServers(de)
+		}
 	}
 }
 
@@ -1863,6 +1871,7 @@ func (de *endpoint) resetLocked() {
 		}
 	}
 	de.probeUDPLifetime.resetCycleEndpointLocked()
+	de.c.relayManager.cancelOutstandingWork(de)
 }
 
 func (de *endpoint) numStopAndReset() int64 {