headscale/hscontrol/mapper/batcher.go

package mapper

import (
	"errors"
	"fmt"
	"slices"
	"sync"
	"sync/atomic"
	"time"

	"github.com/juanfont/headscale/hscontrol/state"
	"github.com/juanfont/headscale/hscontrol/types"
	"github.com/juanfont/headscale/hscontrol/types/change"
	"github.com/juanfont/headscale/hscontrol/util/zlog/zf"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"github.com/puzpuzpuz/xsync/v4"
	"github.com/rs/zerolog/log"
	"tailscale.com/tailcfg"
)

// Mapper errors.
var (
	ErrInvalidNodeID      = errors.New("invalid nodeID")
	ErrMapperNil          = errors.New("mapper is nil")
	ErrNodeConnectionNil  = errors.New("nodeConnection is nil")
	ErrNodeNotFoundMapper = errors.New("node not found")
)

// offlineNodeCleanupThreshold is how long a node must be disconnected
// before [Batcher.cleanupOfflineNodes] removes its in-memory state.
const offlineNodeCleanupThreshold = 15 * time.Minute

var mapResponseGenerated = promauto.NewCounterVec(prometheus.CounterOpts{
	Namespace: "headscale",
	Name:      "mapresponse_generated_total",
	Help:      "total count of mapresponses generated by response type",
}, []string{"response_type"})

// mapperChangesDropped tracks changes the batcher refused to fan out.
// Reason is bounded; "empty" covers IsEmpty changes dropped at ingress.
var mapperChangesDropped = promauto.NewCounterVec(prometheus.CounterOpts{
	Namespace: "headscale",
	Name:      "mapper_changes_dropped_total",
	Help:      "changes dropped by the batcher before recipient fan-out",
}, []string{"reason"})

func NewBatcher(batchTime time.Duration, workers int, mapper *mapper) *Batcher {
	return &Batcher{
		mapper:  mapper,
		workers: workers,
		tick:    time.NewTicker(batchTime),

		// The size of this channel is arbitrary chosen, the sizing should be revisited.
		workCh: make(chan work, workers*200),
		done:   make(chan struct{}),
		nodes:  xsync.NewMap[types.NodeID, *multiChannelNodeConn](),
	}
}

// NewBatcherAndMapper creates a new [Batcher] with its [mapper].
func NewBatcherAndMapper(cfg *types.Config, state *state.State) *Batcher {
	m := newMapper(cfg, state)
	b := NewBatcher(cfg.Tuning.BatchChangeDelay, cfg.Tuning.BatcherWorkers, m)
	m.batcher = b

	return b
}

// nodeConnection interface for different connection implementations.
type nodeConnection interface {
	nodeID() types.NodeID
	version() tailcfg.CapabilityVersion
	send(data *tailcfg.MapResponse) error
	// computePeerDiff returns peers that were previously sent but are no longer in the current list.
	computePeerDiff(currentPeers []tailcfg.NodeID) (removed []tailcfg.NodeID)
	// updateSentPeers updates the tracking of which peers have been sent to this node.
	updateSentPeers(resp *tailcfg.MapResponse)
}

// generateMapResponse generates the [tailcfg.MapResponse] values to send, in
// order, for the given [types.NodeID] based on the provided [change.Change].
func generateMapResponse(
	nc nodeConnection,
	mapper *mapper,
	r change.Change,
) ([]*tailcfg.MapResponse, error) {
	nodeID := nc.nodeID()
	version := nc.version()

	if r.IsEmpty() {
		return nil, nil
	}

	if nodeID == 0 {
		return nil, fmt.Errorf("%w: %d", ErrInvalidNodeID, nodeID)
	}

	if mapper == nil {
		return nil, fmt.Errorf("%w for nodeID %d", ErrMapperNil, nodeID)
	}

	// Handle self-only responses
	if r.IsSelfOnly() && r.TargetNode != nodeID {
		return nil, nil
	}

	// Check if this is a self-update (the changed node is the receiving node).
	// When true, ensure the response includes the node's self info so it sees
	// its own attribute changes (e.g., tags changed via admin API).
	isSelfUpdate := r.OriginNode != 0 && r.OriginNode == nodeID

	var (
		resp    *tailcfg.MapResponse
		removed []tailcfg.NodeID
		err     error
	)

	// Track metric using categorized type, not free-form reason
	mapResponseGenerated.WithLabelValues(r.Type()).Inc()

	// Check if this requires runtime peer visibility computation (e.g., policy changes)
	if r.RequiresRuntimePeerComputation {
		currentPeers := mapper.state.ListPeers(nodeID)

		currentPeerIDs := make([]tailcfg.NodeID, 0, currentPeers.Len())
		for _, peer := range currentPeers.All() {
			currentPeerIDs = append(currentPeerIDs, peer.ID().NodeID())
		}

		removed = nc.computePeerDiff(currentPeerIDs)
		resp, err = mapper.policyChangeResponse(nodeID, version, currentPeers)
	} else if isSelfUpdate {
		// Non-policy self-update: just send the self node info
		resp, err = mapper.selfMapResponse(nodeID, version)
	} else {
		resp, err = mapper.buildFromChange(nodeID, version, &r)
	}

	if err != nil {
		return nil, fmt.Errorf("generating map response for nodeID %d: %w", nodeID, err)
	}

	if resp == nil {
		return nil, nil
	}

	// A full peer list replaces the node's peer set, so peers missing from
	// it are removals. Clients take Peers as authoritative only when it is
	// non-empty, so a full update that isolates a node (e.g.
	// [Batcher.addToBatch] replacing a pending [change.PolicyChange] with a
	// [change.FullUpdate]) needs them sent explicitly.
	if resp.Peers != nil {
		peerIDs := make([]tailcfg.NodeID, 0, len(resp.Peers))
		for _, peer := range resp.Peers {
			peerIDs = append(peerIDs, peer.ID)
		}

		removed = nc.computePeerDiff(peerIDs)
	}

	return withRemovals(resp, removed), nil
}

// withRemovals returns resp and the peers removed from the node's view as the
// responses to send. The removals lead, in a response of their own: clients
// before Tailscale v1.104 only report removed peers to IPN bus watchers opted
// out of full netmaps (the Android app since 1.100) from responses they apply
// as deltas, and resp can carry DNSConfig, SSHPolicy, Node or Peers, which
// force a full rebuild.
//
// TODO(kradalby): when the tailscale/tailscale#15660 compat goes (see
// capver.CanOldCodeBeCleanedUp), set resp.PeersRemoved and return resp alone.
func withRemovals(resp *tailcfg.MapResponse, removed []tailcfg.NodeID) []*tailcfg.MapResponse {
	if len(removed) == 0 {
		return []*tailcfg.MapResponse{resp}
	}

	return []*tailcfg.MapResponse{{PeersRemoved: removed}, resp}
}

// handleNodeChange generates and sends the [tailcfg.MapResponse] values for a given node and [change.Change].
func handleNodeChange(nc nodeConnection, mapper *mapper, r change.Change) error {
	if nc == nil {
		return ErrNodeConnectionNil
	}

	nodeID := nc.nodeID()

	log.Debug().Caller().Uint64(zf.NodeID, nodeID.Uint64()).Str(zf.Reason, r.Reason).Msg("node change processing started")

	resps, err := generateMapResponse(nc, mapper, r)
	if err != nil {
		return fmt.Errorf("generating map response for node %d: %w", nodeID, err)
	}

	for _, resp := range resps {
		err = nc.send(resp)
		if err != nil {
			// If the node has no active connections, the data was not
			// delivered. Do not update lastSentPeers — recording phantom
			// peer state would corrupt future computePeerDiff calculations,
			// causing the node to miss peer additions or removals after
			// reconnection.
			if errors.Is(err, errNoActiveConnections) {
				return nil
			}

			return fmt.Errorf("sending map response to node %d: %w", nodeID, err)
		}

		// Update peer tracking only after confirmed delivery to at
		// least one active connection.
		nc.updateSentPeers(resp)
	}

	return nil
}

// workResult represents the result of processing a change.
type workResult struct {
	mapResponse *tailcfg.MapResponse
	err         error
}

// work represents a unit of work to be processed by workers.
// All pending changes for a node are bundled into a single work item
// so that one worker processes them sequentially. This prevents
// out-of-order [tailcfg.MapResponse] delivery and races on lastSentPeers
// that occur when multiple workers process changes for the same node.
type work struct {
	changes  []change.Change
	nodeID   types.NodeID
	resultCh chan<- workResult // optional channel for synchronous operations
}

// Batcher errors.
var (
	errConnectionClosed      = errors.New("connection channel already closed")
	ErrInitialMapSendTimeout = errors.New("sending initial map: timeout")
	ErrBatcherShuttingDown   = errors.New("batcher shutting down")
	ErrConnectionSendTimeout = errors.New("timeout sending to channel (likely stale connection)")
)

// Batcher batches and distributes map responses to connected nodes.
// It uses concurrent maps, per-node mutexes, and a worker pool.
//
// Lifecycle: Call [Batcher.Start] to spawn workers, then [Batcher.Close]
// to shut down. [Batcher.Close] blocks until all workers have exited.
// A [Batcher] must not be reused after [Batcher.Close].
type Batcher struct {
	tick    *time.Ticker
	mapper  *mapper
	workers int

	nodes *xsync.Map[types.NodeID, *multiChannelNodeConn]

	// Work queue channel
	workCh   chan work
	done     chan struct{}
	doneOnce sync.Once // Ensures done is only closed once

	// wg tracks the doWork and all worker goroutines so that Close()
	// can block until they have fully exited.
	wg sync.WaitGroup

	started atomic.Bool // Ensures Start() is only called once

	// Metrics
	totalNodes      atomic.Int64
	workQueuedCount atomic.Int64
	workProcessed   atomic.Int64
	workErrors      atomic.Int64
}

// 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.
// The stop function tears down the owning session if this connection is later declared stale.
func (b *Batcher) 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()

	// Generate connection ID
	connID := generateConnectionID()

	// Create new connection entry
	now := time.Now()
	newEntry := &connectionEntry{
		id:      connID,
		c:       c,
		version: version,
		created: now,
		stop:    stop,
	}
	// Block broadcast sends to this connection until its initial map
	// is delivered below, so a delta cannot become the stream's first
	// frame — clients reject streams whose first frame lacks the self
	// node.
	newEntry.pendingInitial.Store(true)
	// Initialize last used timestamp
	newEntry.lastUsed.Store(now.Unix())

	// Get or create the multiChannelNodeConn and register this connection in a
	// single Compute so the new connection is visible atomically. Doing the
	// LoadOrStore and addConnection as separate steps let cleanupOfflineNodes
	// (which deletes via Compute when hasActiveConnections() is false) observe a
	// reused offline conn with zero connections mid-reconnect and delete it,
	// orphaning the live connection.
	var nodeConn *multiChannelNodeConn

	b.nodes.Compute(
		id,
		func(existing *multiChannelNodeConn, loaded bool) (*multiChannelNodeConn, xsync.ComputeOp) {
			if !loaded || existing == nil {
				existing = newMultiChannelNodeConn(id, b.mapper)
				b.totalNodes.Add(1)
			}

			existing.addConnection(newEntry)
			nodeConn = existing

			return existing, xsync.UpdateOp
		},
	)

	// Use the worker pool for controlled concurrency instead of direct generation
	initialMap, err := b.MapResponseFromChange(id, change.FullSelf(id))
	if err != nil {
		nlog.Error().Err(err).Msg("initial map generation failed")
		nodeConn.detach(c)

		return fmt.Errorf("generating initial map for node %d: %w", id, err)
	}

	// Use a blocking send with timeout for initial map since the channel should be ready
	// and we want to avoid the race condition where the receiver isn't ready yet
	select {
	case c <- initialMap:
		// Still pendingInitial, so no broadcast can race this.
		newEntry.lastSSHPolicy.Store(initialMap.SSHPolicy)

		// Record sent peers only after confirmed delivery, mirroring the async
		// path, and under workMu so a concurrent async bundle for this node
		// cannot interleave its own lastSentPeers update.
		nodeConn.workMu.Lock()

		// Peers removed while the node had no stream: the initial map is a
		// full rebuild, so follow it with the removal as a delta, see
		// [withRemovals], and keep the removed peers tracked across the
		// baseline reset below.
		//
		// TODO(kradalby): delete with the tailscale/tailscale#15660 compat, see
		// capver.CanOldCodeBeCleanedUp.
		peerIDs := make([]tailcfg.NodeID, 0, len(initialMap.Peers))
		for _, peer := range initialMap.Peers {
			peerIDs = append(peerIDs, peer.ID)
		}

		removedIDs := nodeConn.computePeerDiff(peerIDs)

		removed := make([]types.NodeID, 0, len(removedIDs))
		for _, peerID := range removedIDs {
			removed = append(removed, types.NodeID(peerID)) //nolint:gosec // NodeID types are equivalent
		}

		if len(removed) > 0 {
			rm := change.PeersRemoved(removed...)
			rm.TargetNode = id
			b.AddWork(rm)
		}

		nodeConn.updateSentPeers(initialMap)

		// The client has not had these removals as a delta yet. Pending
		// may lose the queued one (a full supersedes it, or the node
		// leaves before it is sent), so keep them tracked until delivery
		// drops them: the next full or reconnect then re-derives them.
		for _, peerID := range removedIDs {
			nodeConn.lastSentPeers.Store(peerID, struct{}{})
		}

		nodeConn.workMu.Unlock()

		// Still pendingInitial, so no broadcast can race this.
		newEntry.lastSelf.Store(initialMap.Node)

		// Open the connection for broadcast sends now that the initial
		// map is the stream's first frame; send() requeued any changes
		// that arrived in the meantime.
		newEntry.pendingInitial.Store(false)
	case <-time.After(5 * time.Second): //nolint:mnd
		nlog.Error().Err(ErrInitialMapSendTimeout).Msg("initial map send timeout")
		nlog.Debug().Caller().Dur("timeout.duration", 5*time.Second). //nolint:mnd
										Msg("initial map send timed out because channel was blocked or receiver not ready")
		nodeConn.detach(c)

		return fmt.Errorf("%w for node %d", ErrInitialMapSendTimeout, id)
	}

	// Mark the node as connected now that the initial map was sent.
	nodeConn.markConnected()

	// Node will automatically receive updates through the normal flow
	// The initial full map already contains all current state

	nlog.Debug().Caller().Dur(zf.TotalDuration, time.Since(addNodeStart)).
		Int("active.connections", nodeConn.getActiveConnectionCount()).
		Msg("node connection established in batcher")

	return nil
}

// RemoveNode disconnects a node from the batcher, marking it as offline and cleaning up its state.
// It validates the connection channel matches one of the current connections, closes that specific connection,
// and keeps the node entry alive for rapid reconnections instead of aggressive deletion.
// Reports if the node still has active connections after removal.
func (b *Batcher) RemoveNode(id types.NodeID, c chan<- *tailcfg.MapResponse) bool {
	nlog := log.With().Uint64(zf.NodeID, id.Uint64()).Logger()

	nodeConn, exists := b.nodes.Load(id)
	if !exists || nodeConn == nil {
		nlog.Debug().Caller().Msg("removeNode called for non-existent node")
		return false
	}

	// Remove specific connection
	removed := nodeConn.removeConnectionByChannel(c)
	if !removed {
		nlog.Debug().Caller().Msg("removeNode: channel not found, connection already removed or invalid")
	}

	// Check if node has any remaining active connections
	if nodeConn.hasActiveConnections() {
		nlog.Debug().Caller().
			Int("active.connections", nodeConn.getActiveConnectionCount()).
			Msg("node connection removed but keeping online, other connections remain")

		return true // Node still has active connections
	}

	// No active connections - keep the node entry alive for rapid reconnections
	// The node will get a fresh full map when it reconnects
	nlog.Debug().Caller().Msg("node disconnected from batcher, keeping entry for rapid reconnection")
	nodeConn.markDisconnected()

	return false
}

// AddWork queues a change to be processed by the batcher. Empty changes
// carry no work for any recipient and are dropped here so they never create
// pending entries or in-flight work. A PeersRemoved change is not empty, so
// deletion cleanup in addToBatch still runs.
func (b *Batcher) AddWork(r ...change.Change) {
	if slices.ContainsFunc(r, change.Change.IsEmpty) {
		kept := slices.DeleteFunc(slices.Clone(r), change.Change.IsEmpty)
		mapperChangesDropped.WithLabelValues("empty").Add(float64(len(r) - len(kept)))
		r = kept
	}

	if len(r) == 0 {
		return
	}

	b.addToBatch(r...)
}

func (b *Batcher) Start() {
	if !b.started.CompareAndSwap(false, true) {
		return
	}

	b.wg.Go(b.doWork)
}

// Done is closed when batcher shutdown begins.
func (b *Batcher) Done() <-chan struct{} {
	return b.done
}

func (b *Batcher) Close() {
	// Signal shutdown to all goroutines, only once.
	// Workers and queueWork both select on done, so closing it
	// is sufficient for graceful shutdown. We intentionally do NOT
	// close workCh here because processBatchedChanges or
	// MapResponseFromChange may still be sending on it concurrently.
	b.doneOnce.Do(func() {
		close(b.done)
	})

	// Wait for all worker goroutines (and doWork) to exit before
	// tearing down node connections. This prevents workers from
	// sending on connections that are being closed concurrently.
	b.wg.Wait()

	// Stop the ticker to prevent resource leaks.
	b.tick.Stop()

	// Close the underlying channels supplying the data to the clients.
	b.nodes.Range(func(nodeID types.NodeID, conn *multiChannelNodeConn) bool {
		if conn == nil {
			return true
		}

		conn.close()

		return true
	})
}

func (b *Batcher) doWork() {
	for i := range b.workers {
		b.wg.Go(func() { b.worker(i + 1) })
	}

	// Create a cleanup ticker for removing truly disconnected nodes
	cleanupTicker := time.NewTicker(5 * time.Minute)
	defer cleanupTicker.Stop()

	for {
		select {
		case <-b.tick.C:
			// Process batched changes
			b.processBatchedChanges()
		case <-cleanupTicker.C:
			// Clean up nodes that have been offline for too long
			b.cleanupOfflineNodes()
		case <-b.done:
			log.Info().Msg("batcher done channel closed, stopping to feed workers")
			return
		}
	}
}

func (b *Batcher) worker(workerID int) {
	wlog := log.With().Int(zf.WorkerID, workerID).Logger()

	for {
		select {
		case w, ok := <-b.workCh:
			if !ok {
				wlog.Debug().Msg("worker channel closing, shutting down")
				return
			}

			b.workProcessed.Add(1)

			// Synchronous path: a caller is blocking on resultCh
			// waiting for a generated MapResponse (used by AddNode
			// for the initial map). Always contains a single change.
			if w.resultCh != nil {
				var result workResult

				if nc, exists := b.nodes.Load(w.nodeID); exists && nc != nil {
					// Hold workMu so concurrent async work for this
					// node waits until the initial map is sent.
					nc.workMu.Lock()

					resps, err := generateMapResponse(nc, b.mapper, w.changes[0])
					if len(resps) > 0 {
						// The initial map must be the stream's first frame; AddNode
						// sends any removals ahead of it after it instead.
						result.mapResponse = resps[len(resps)-1]
					}

					result.err = err
					if result.err != nil {
						b.workErrors.Add(1)
						wlog.Error().Err(result.err).
							Uint64(zf.NodeID, w.nodeID.Uint64()).
							Str(zf.Reason, w.changes[0].Reason).
							Msg("failed to generate map response for synchronous work")
					}
					// Peer tracking is recorded by the caller (AddNode) only
					// after the initial map is actually delivered; recording it
					// here, before delivery, would leave phantom lastSentPeers
					// if the send times out.

					nc.workMu.Unlock()
				} else {
					result.err = fmt.Errorf("%w: %d", ErrNodeNotFoundMapper, w.nodeID)

					b.workErrors.Add(1)
					wlog.Error().Err(result.err).
						Uint64(zf.NodeID, w.nodeID.Uint64()).
						Msg("node not found for synchronous work")
				}

				select {
				case w.resultCh <- result:
				case <-b.done:
					return
				}

				continue
			}

			// Async path: process all bundled changes sequentially.
			// workMu ensures that if another worker picks up the next
			// tick's bundle for the same node, it waits until we
			// finish — preventing out-of-order delivery and races
			// on lastSentPeers (Clear+Store vs Range).
			if nc, exists := b.nodes.Load(w.nodeID); exists && nc != nil {
				// Changes that found only connections still awaiting
				// their initial map are retried next tick: the
				// in-flight initial map may have been generated from
				// a snapshot older than the change, so dropping them
				// would lose updates. Collected and prepended as a
				// group to keep their order ahead of newer pending
				// changes — order matters for stateful patches like
				// online/offline.
				var retry []change.Change

				nc.workMu.Lock()
				for _, ch := range w.changes {
					err := nc.change(ch)
					if errors.Is(err, errNoReadyConnections) {
						retry = append(retry, ch)
						continue
					}

					if err != nil {
						b.workErrors.Add(1)
						wlog.Error().Err(err).
							Uint64(zf.NodeID, w.nodeID.Uint64()).
							Str(zf.Reason, ch.Reason).
							Msg("failed to apply change")
					}
				}
				nc.workMu.Unlock()

				if len(retry) > 0 {
					nc.prependPending(retry...)
				}

				// Bundle delivered; allow the next tick's bundle to queue.
				nc.inFlight.Store(false)
			}
		case <-b.done:
			wlog.Debug().Msg("batcher shutting down, exiting worker")
			return
		}
	}
}

// queueWork safely queues work.
func (b *Batcher) queueWork(w work) {
	b.workQueuedCount.Add(1)

	select {
	case b.workCh <- w:
		// Successfully queued
	case <-b.done:
		// Batcher is shutting down
		return
	}
}

// addToBatch adds changes to the pending batch.
func (b *Batcher) addToBatch(changes ...change.Change) {
	// Clean up any nodes being permanently removed from the system.
	//
	// This handles the case where a node is deleted from state but the batcher
	// still has it registered. By cleaning up here, we prevent "node not found"
	// errors when workers try to generate map responses for deleted nodes.
	//
	// [change.Change.DeletedNodes] is an explicit internal lifecycle signal;
	// [change.Change.PeersRemoved] remains only the protocol delta sent to clients
	// when peers disappear from their view.
	//
	// See: https://github.com/juanfont/headscale/issues/2924
	for _, ch := range changes {
		for _, removedID := range ch.DeletedNodes {
			if nc, existed := b.nodes.LoadAndDelete(removedID); existed {
				b.totalNodes.Add(-1)

				// Tear the node's map sessions down. Dropping the entry alone
				// leaves [mapSession.serveLongPoll] streaming to a node that no
				// longer exists: [Batcher.Close] ranges b.nodes and can no
				// longer reach it, so shutdown blocks on clientStreamsOpen, and
				// the client keeps polling a node it should be re-authenticating.
				// See: https://github.com/juanfont/headscale/issues/3410
				if nc != nil {
					nc.close()
				}

				log.Debug().
					Uint64(zf.NodeID, removedID.Uint64()).
					Msg("removed deleted node from batcher")
			}
		}
	}

	// A full update supersedes every state change pending or in this call,
	// but not the pings addressed to a node; those follow the full.
	if change.HasFull(changes) {
		b.nodes.Range(func(_ types.NodeID, nc *multiChannelNodeConn) bool {
			if nc == nil {
				return true
			}

			nc.collapsePendingToFull(changes)

			return true
		})

		return
	}

	broadcast, targeted := change.SplitTargetedAndBroadcast(changes)

	// Handle targeted changes - send only to the specific node
	for _, ch := range targeted {
		if nc, ok := b.nodes.Load(ch.TargetNode); ok && nc != nil {
			nc.appendPending(ch)
		}
	}

	// Broadcasts have no TargetNode, so every node receives the same changes.
	if len(broadcast) > 0 {
		b.nodes.Range(func(_ types.NodeID, nc *multiChannelNodeConn) bool {
			if nc == nil {
				return true
			}

			nc.appendPending(broadcast...)

			return true
		})
	}
}

// processBatchedChanges processes all pending batched changes.
func (b *Batcher) processBatchedChanges() {
	b.nodes.Range(func(nodeID types.NodeID, nc *multiChannelNodeConn) bool {
		if nc == nil {
			return true
		}

		// Only one batched bundle per node may be in flight at a time. If the
		// previous tick's bundle is still queued or processing, leave this
		// tick's changes in pending; they are picked up once it completes. This
		// keeps delivery ordered even when the worker pool is saturated.
		if nc.inFlight.Load() {
			return true
		}

		pending := nc.takePending()
		if len(pending) == 0 {
			return true
		}

		// One policy recompute rebuilds the whole netmap; drop same-tick
		// repeats. A frame per peer delta lets a join storm outgrow the
		// session channel within one tick, so render each run as one.
		pending = change.CoalescePeerDeltas(nodeID, change.DedupePolicyChanges(pending))

		// Queue a single work item containing all pending changes.
		nc.inFlight.Store(true)
		b.queueWork(work{changes: pending, nodeID: nodeID, resultCh: nil})

		return true
	})
}

// cleanupOfflineNodes removes nodes that have been offline for too long to prevent memory leaks.
// Uses xsync.Map.Compute for atomic check-and-delete to prevent TOCTOU races where a node
// reconnects between the hasActiveConnections check and the Delete call.
func (b *Batcher) cleanupOfflineNodes() {
	var nodesToCleanup []types.NodeID

	// Find nodes that have been offline for too long by scanning b.nodes
	// and checking each node's disconnectedAt timestamp.
	b.nodes.Range(func(nodeID types.NodeID, nc *multiChannelNodeConn) bool {
		if nc != nil && !nc.hasActiveConnections() && nc.offlineDuration() > offlineNodeCleanupThreshold {
			nodesToCleanup = append(nodesToCleanup, nodeID)
		}

		return true
	})

	// Clean up the identified nodes using Compute() for atomic check-and-delete.
	// This prevents a TOCTOU race where a node reconnects (adding an active
	// connection) between the hasActiveConnections() check and the Delete() call.
	cleaned := 0

	for _, nodeID := range nodesToCleanup {
		b.nodes.Compute(
			nodeID,
			func(conn *multiChannelNodeConn, loaded bool) (*multiChannelNodeConn, xsync.ComputeOp) {
				if !loaded || conn == nil || conn.hasActiveConnections() {
					return conn, xsync.CancelOp
				}

				// Perform all bookkeeping inside the Compute callback so
				// that a concurrent AddNode (which calls LoadOrStore on
				// b.nodes) cannot slip in between the delete and the
				// counter update.
				b.totalNodes.Add(-1)

				cleaned++

				log.Info().Uint64(zf.NodeID, nodeID.Uint64()).
					Dur("offline_duration", offlineNodeCleanupThreshold).
					Msg("cleaning up node that has been offline for too long")

				return conn, xsync.DeleteOp
			},
		)
	}

	if cleaned > 0 {
		log.Info().Int(zf.CleanedNodes, cleaned).
			Msg("completed cleanup of long-offline nodes")
	}
}

// IsConnected is a lock-free read that checks if a node is connected.
// A node is considered connected if it has active connections or has
// not been marked as disconnected.
func (b *Batcher) IsConnected(id types.NodeID) bool {
	nodeConn, exists := b.nodes.Load(id)
	if !exists || nodeConn == nil {
		return false
	}

	return nodeConn.isConnected()
}

// ConnectedMap returns a lock-free map of all known nodes and their
// connection status (true = connected, false = disconnected).
func (b *Batcher) ConnectedMap() *xsync.Map[types.NodeID, bool] {
	ret := xsync.NewMap[types.NodeID, bool]()

	b.nodes.Range(func(id types.NodeID, nc *multiChannelNodeConn) bool {
		if nc != nil {
			ret.Store(id, nc.isConnected())
		}

		return true
	})

	return ret
}

// MapResponseFromChange queues work to generate a map response and waits for the result.
// This allows synchronous map generation using the same worker pool.
func (b *Batcher) MapResponseFromChange(id types.NodeID, ch change.Change) (*tailcfg.MapResponse, error) {
	resultCh := make(chan workResult, 1)

	// Queue the work with a result channel using the safe queueing method
	b.queueWork(work{changes: []change.Change{ch}, nodeID: id, resultCh: resultCh})

	// Wait for the result
	select {
	case result := <-resultCh:
		return result.mapResponse, result.err
	case <-b.done:
		return nil, fmt.Errorf("%w while generating map response for node %d", ErrBatcherShuttingDown, id)
	}
}

// DebugNodeInfo contains debug information about a node's connections.
type DebugNodeInfo struct {
	Connected         bool `json:"connected"`
	ActiveConnections int  `json:"active_connections"`
}

// Debug returns a pre-baked map of node debug information for the debug interface.
func (b *Batcher) Debug() map[types.NodeID]DebugNodeInfo {
	result := make(map[types.NodeID]DebugNodeInfo)

	b.nodes.Range(func(id types.NodeID, nc *multiChannelNodeConn) bool {
		if nc == nil {
			return true
		}

		result[id] = DebugNodeInfo{
			Connected:         nc.isConnected(),
			ActiveConnections: nc.getActiveConnectionCount(),
		}

		return true
	})

	return result
}

func (b *Batcher) DebugMapResponses() (map[types.NodeID][]tailcfg.MapResponse, error) {
	return b.mapper.debugMapResponses()
}

// WorkErrors returns the count of work errors encountered.
// This is primarily useful for testing and debugging.
func (b *Batcher) WorkErrors() int64 {
	return b.workErrors.Load()
}