fixed the issue of sending on closed channel

This commit fixes the issue of headscale crashing after sending on a
closed channel by moving the channel close to the sender side, instead
of the creator. closeChanWithLog is also implemented with generics now.

Fixes: https://github.com/juanfont/headscale/issues/342
Signed-off-by: Moritz Poldrack <git@moritz.sh>

poll.go

@@ -175,32 +175,13 @@ func (h *Headscale) PollNetMapHandler(ctx *gin.Context) {
 		Str("machine", machine.Name).
 		Msg("Loading or creating update channel")
 
-	// TODO: could probably remove all that duplication once generics land.
-	closeChanWithLog := func(channel interface{}, name string) {
-		log.Trace().
-			Str("handler", "PollNetMap").
-			Str("machine", machine.Name).
-			Str("channel", "Done").
-			Msg(fmt.Sprintf("Closing %s channel", name))
-
-		switch c := channel.(type) {
-		case (chan struct{}):
-			close(c)
-
-		case (chan []byte):
-			close(c)
-		}
-	}
-
 	const chanSize = 8
 	updateChan := make(chan struct{}, chanSize)
-	defer closeChanWithLog(updateChan, "updateChan")
 
 	pollDataChan := make(chan []byte, chanSize)
-	defer closeChanWithLog(pollDataChan, "pollDataChan")
+	defer closeChanWithLog(pollDataChan, machine.Name, "pollDataChan")
 
 	keepAliveChan := make(chan []byte)
-	defer closeChanWithLog(keepAliveChan, "keepAliveChan")
 
 	if req.OmitPeers && !req.Stream {
 		log.Info().
@@ -273,7 +254,27 @@ func (h *Headscale) PollNetMapStream(
 	updateChan chan struct{},
 ) {
 	{
-		ctx, cancel := context.WithCancel(ctx.Request.Context())
+		machine, err := h.GetMachineByMachineKey(machineKey)
+		if err != nil {
+			if errors.Is(err, gorm.ErrRecordNotFound) {
+				log.Warn().
+					Str("handler", "PollNetMap").
+					Msgf("Ignoring request, cannot find machine with key %s", machineKey.String())
+				ctx.String(http.StatusUnauthorized, "")
+
+				return
+			}
+			log.Error().
+				Str("handler", "PollNetMap").
+				Msgf("Failed to fetch machine from the database with Machine key: %s", machineKey.String())
+			ctx.String(http.StatusInternalServerError, "")
+
+			return
+		}
+
+		ctx := context.WithValue(ctx.Request.Context(), "machineName", machine.Name)
+
+		ctx, cancel := context.WithCancel(ctx)
 		defer cancel()
 
 		go h.scheduledPollWorker(
@@ -564,8 +565,8 @@ func (h *Headscale) PollNetMapStream(
 
 func (h *Headscale) scheduledPollWorker(
 	ctx context.Context,
-	updateChan chan<- struct{},
+	updateChan chan struct{},
-	keepAliveChan chan<- []byte,
+	keepAliveChan chan []byte,
 	machineKey key.MachinePublic,
 	mapRequest tailcfg.MapRequest,
 	machine *Machine,
@@ -573,6 +574,17 @@ func (h *Headscale) scheduledPollWorker(
 	keepAliveTicker := time.NewTicker(keepAliveInterval)
 	updateCheckerTicker := time.NewTicker(updateCheckInterval)
 
+	defer closeChanWithLog(
+		updateChan,
+		fmt.Sprint(ctx.Value("machineName")),
+		"updateChan",
+	)
+	defer closeChanWithLog(
+		keepAliveChan,
+		fmt.Sprint(ctx.Value("machineName")),
+		"updateChan",
+	)
+
 	for {
 		select {
 		case <-ctx.Done():
@@ -606,3 +618,13 @@ func (h *Headscale) scheduledPollWorker(
 		}
 	}
 }
+
+func closeChanWithLog[C chan []byte | chan struct{}](channel C, machine, name string) {
+	log.Trace().
+		Str("handler", "PollNetMap").
+		Str("machine", machine).
+		Str("channel", "Done").
+		Msg(fmt.Sprintf("Closing %s channel", name))
+
+	close(channel)
+}