netstack: update goroutine save / restore safety comments.

PiperOrigin-RevId: 204930314
Change-Id: Ifc4c41ed28616cd57fafbf7c92e87141a945c41f

pkg/tcpip/link/fdbased/endpoint.go

@@ -108,7 +108,10 @@ func New(opts *Options) tcpip.LinkEndpointID {
 // dispatches them via the provided dispatcher.
 func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
 	e.attached = true
-	go e.dispatchLoop(dispatcher) // S/R-FIXME
+	// Link endpoints are not savable. When transportation endpoints are
+	// saved, they stop sending outgoing packets and all incoming packets
+	// are rejected.
+	go e.dispatchLoop(dispatcher) // S/R-SAFE: See above.
 }
 
 // IsAttached implements stack.LinkEndpoint.IsAttached.

pkg/tcpip/link/sharedmem/sharedmem.go

@@ -142,7 +142,10 @@ func (e *endpoint) Attach(dispatcher stack.NetworkDispatcher) {
 	if !e.workerStarted && atomic.LoadUint32(&e.stopRequested) == 0 {
 		e.workerStarted = true
 		e.completed.Add(1)
-		go e.dispatchLoop(dispatcher) // S/R-FIXME
+		// Link endpoints are not savable. When transportation endpoints
+		// are saved, they stop sending outgoing packets and all
+		// incoming packets are rejected.
+		go e.dispatchLoop(dispatcher) // S/R-SAFE: see above.
 	}
 	e.mu.Unlock()
 }

pkg/tcpip/sample/tun_tcp_connect/main.go

@@ -187,7 +187,7 @@ func main() {
 
 	// Start the writer in its own goroutine.
 	writerCompletedCh := make(chan struct{})
-	go writer(writerCompletedCh, ep) // S/R-FIXME
+	go writer(writerCompletedCh, ep) // S/R-SAFE: sample code.
 
 	// Read data and write to standard output until the peer closes the
 	// connection from its side.

pkg/tcpip/sample/tun_tcp_echo/main.go

@@ -187,6 +187,6 @@ func main() {
 			log.Fatal("Accept() failed:", err)
 		}
 
-		go echo(wq, n) // S/R-FIXME
+		go echo(wq, n) // S/R-SAFE: sample code.
 	}
 }

pkg/tcpip/stack/linkaddrcache.go

@@ -262,7 +262,7 @@ func (c *linkAddrCache) startAddressResolution(k tcpip.FullAddress, linkRes Link
 	e := c.makeAndAddEntry(k, "")
 	e.addWaker(waker)
 
-	go func() { // S/R-FIXME
+	go func() { // S/R-SAFE: link non-savable; wakers dropped synchronously.
 		for i := 0; ; i++ {
 			// Send link request, then wait for the timeout limit and check
 			// whether the request succeeded.

pkg/tcpip/transport/tcp/forwarder.go

@@ -90,7 +90,7 @@ func (f *Forwarder) HandlePacket(r *stack.Route, id stack.TransportEndpointID, v
 	// Launch a new goroutine to handle the request.
 	f.inFlight[id] = struct{}{}
 	s.incRef()
-	go f.handler(&ForwarderRequest{ // S/R-FIXME
+	go f.handler(&ForwarderRequest{ // S/R-SAFE: not used by Sentry.
 		forwarder:  f,
 		segment:    s,
 		synOptions: opts,