poll: stop stale map sessions through an explicit teardown hook

When stale-send cleanup prunes a connection from the batcher, the old serveLongPoll session needs an explicit stop signal. Pass a stop hook into AddNode and trigger it when that connection is removed, so the session exits through its normal cancel path instead of relying on channel closure from the batcher side.

hscontrol/mapper/batcher_lockfree.go

@@ -54,7 +54,13 @@ type LockFreeBatcher struct {
 // AddNode registers a new node connection with the batcher and sends an initial map response.
 // It creates or updates the node's connection data, validates the initial map generation,
 // and notifies other nodes that this node has come online.
-func (b *LockFreeBatcher) AddNode(id types.NodeID, c chan<- *tailcfg.MapResponse, version tailcfg.CapabilityVersion) error {
+// The stop function tears down the owning session if this connection is later declared stale.
+func (b *LockFreeBatcher) AddNode(
+	id types.NodeID,
+	c chan<- *tailcfg.MapResponse,
+	version tailcfg.CapabilityVersion,
+	stop func(),
+) error {
 	addNodeStart := time.Now()
 	nlog := log.With().Uint64(zf.NodeID, id.Uint64()).Logger()
 
@@ -68,6 +74,7 @@ func (b *LockFreeBatcher) AddNode(id types.NodeID, c chan<- *tailcfg.MapResponse
 		c:       c,
 		version: version,
 		created: now,
+		stop:    stop,
 	}
 	// Initialize last used timestamp
 	newEntry.lastUsed.Store(now.Unix())
@@ -511,6 +518,7 @@ type connectionEntry struct {
 	c        chan<- *tailcfg.MapResponse
 	version  tailcfg.CapabilityVersion
 	created  time.Time
+	stop     func()
 	lastUsed atomic.Int64 // Unix timestamp of last successful send
 	closed   atomic.Bool  // Indicates if this connection has been closed
 }
@@ -556,27 +564,29 @@ func (mc *multiChannelNodeConn) close() {
 	defer mc.mutex.Unlock()
 
 	for _, conn := range mc.connections {
-		mc.closeConnection(conn)
+		mc.stopConnection(conn)
 	}
 }
 
-// closeConnection closes connection channel at most once, even if multiple cleanup
-// paths race to tear the same session down.
-func (mc *multiChannelNodeConn) closeConnection(conn *connectionEntry) {
+// stopConnection marks a connection as closed and tears down the owning session
+// at most once, even if multiple cleanup paths race to remove it.
+func (mc *multiChannelNodeConn) stopConnection(conn *connectionEntry) {
 	if conn.closed.CompareAndSwap(false, true) {
-		close(conn.c)
+		if conn.stop != nil {
+			conn.stop()
+		}
 	}
 }
 
 // removeConnectionAtIndexLocked removes the active connection at index.
-// If closeChannel is true, it also closes that session's map-response channel.
+// If stopConnection is true, it also stops that session.
 // Caller must hold mc.mutex.
-func (mc *multiChannelNodeConn) removeConnectionAtIndexLocked(i int, closeChannel bool) *connectionEntry {
+func (mc *multiChannelNodeConn) removeConnectionAtIndexLocked(i int, stopConnection bool) *connectionEntry {
 	conn := mc.connections[i]
 	mc.connections = append(mc.connections[:i], mc.connections[i+1:]...)
 
-	if closeChannel {
-		mc.closeConnection(conn)
+	if stopConnection {
+		mc.stopConnection(conn)
 	}
 
 	return conn