mapper: retry slow sessions from current state

A full session channel after the send window evicted live but busy clients.
Requeue and regenerate; evict after no progress for connectionStallTimeout.
Retire blocked reconnect siblings once the newest stream accepts the frame.

Updates #3129.

hscontrol/mapper/batcher.go

@@ -318,11 +318,13 @@ func (b *Batcher) AddNode(
 	// Create new connection entry
 	now := time.Now()
 	newEntry := &connectionEntry{
+		done:        make(chan struct{}),
 		id:          connID,
 		c:           c,
 		version:     version,
 		created:     now,
 		stop:        stop,
+		batcherDone: b.done,
 	}
 	// Block broadcast sends to this connection until its initial map
 	// is delivered below, so a delta cannot become the stream's first
@@ -632,22 +634,18 @@ func (b *Batcher) worker(workerID int) {
 			// 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
+				// Requeue unready or backpressured work so the next tick
-				// their initial map are retried next tick: the
+				// regenerates it from current state.
-				// 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 {
+				for i, ch := range w.changes {
 					err := nc.change(ch)
-					if errors.Is(err, errNoReadyConnections) {
+					if errors.Is(err, errNoReadyConnections) || errors.Is(err, errConnectionBackpressure) {
-						retry = append(retry, ch)
+						// Keep the unsent suffix ahead of newer pending changes.
-						continue
+						// Older reconnect siblings can receive idempotent duplicates on retry.
+						retry = w.changes[i:]
+						break
 					}
 
 					if err != nil {
@@ -664,7 +662,7 @@ func (b *Batcher) worker(workerID int) {
 					nc.prependPending(retry...)
 				}
 
-				// Bundle delivered; allow the next tick's bundle to queue.
+				// Bundle delivered or requeued; allow the next tick's bundle to queue.
 				nc.inFlight.Store(false)
 			}
 		case <-b.done:

hscontrol/mapper/batcher_bench_test.go

@@ -185,6 +185,7 @@ func benchBatcher(nodeCount, bufferSize int) (*Batcher, map[types.NodeID]chan *t
 		mc := newMultiChannelNodeConn(id, nil)
 		ch := make(chan *tailcfg.MapResponse, bufferSize)
 		entry := &connectionEntry{
+			done:    make(chan struct{}),
 			id:      fmt.Sprintf("conn-%d", i),
 			c:       ch,
 			version: tailcfg.CapabilityVersion(100),
@@ -362,6 +363,7 @@ func BenchmarkMultiChannelBroadcast(b *testing.B) {
 						for j := range 2 {
 							ch := make(chan *tailcfg.MapResponse, b.N+1)
 							entry := &connectionEntry{
+								done:    make(chan struct{}),
 								id:      fmt.Sprintf("extra-%d-%d", i, j),
 								c:       ch,
 								version: tailcfg.CapabilityVersion(100),
@@ -520,6 +522,7 @@ func BenchmarkConnectionChurn(b *testing.B) {
 				// Add new connection
 				newCh := make(chan *tailcfg.MapResponse, 10)
 				entry := &connectionEntry{
+					done:    make(chan struct{}),
 					id:      fmt.Sprintf("churn-%d", i),
 					c:       newCh,
 					version: tailcfg.CapabilityVersion(100),
@@ -566,6 +569,7 @@ func BenchmarkConcurrentSendAndChurn(b *testing.B) {
 
 								newCh := make(chan *tailcfg.MapResponse, 100)
 								entry := &connectionEntry{
+									done:    make(chan struct{}),
 									id:      fmt.Sprintf("churn-%d", i),
 									c:       newCh,
 									version: tailcfg.CapabilityVersion(100),

hscontrol/mapper/batcher_concurrency_test.go

@@ -59,6 +59,7 @@ func setupLightweightBatcher(t *testing.T, nodeCount, bufferSize int) *lightweig
 		mc := newMultiChannelNodeConn(id, nil) // nil mapper is fine for channel tests
 		ch := make(chan *tailcfg.MapResponse, bufferSize)
 		entry := &connectionEntry{
+			done:    make(chan struct{}),
 			id:      fmt.Sprintf("conn-%d", i),
 			c:       ch,
 			version: tailcfg.CapabilityVersion(100),
@@ -579,9 +580,7 @@ func TestProcessBatchedChanges_ConcurrentAdd_NoDataLoss(t *testing.T) {
 			lb.b.addToBatch(change.Change{
 				Reason:      fmt.Sprintf("add-%d", i),
 				TargetNode:  types.NodeID(1),
-				PeerPatches: []*tailcfg.PeerChange{
+				PingRequest: &tailcfg.PingRequest{URL: fmt.Sprintf("https://ping/%d", i)},
-					{NodeID: tailcfg.NodeID(i + 100)}, //nolint:gosec // test
-				},
 			})
 			addedCount.Add(1)
 		}
@@ -600,18 +599,27 @@ func TestProcessBatchedChanges_ConcurrentAdd_NoDataLoss(t *testing.T) {
 	// One final process to flush any remaining
 	lb.b.processBatchedChanges()
 
-	// Count the patches across all bundled work items in the channel. Each
+	// Count the pings across all bundled work items in the channel: pending
-	// work item bundles a node's pending changes, coalescing peer deltas, so
+	// may fold or collapse state changes, but every ping must arrive.
-	// the patches, not the changes, are what must all arrive.
 	queuedChanges := 0
 
 	workItems := len(lb.b.workCh)
 	for range workItems {
 		w := <-lb.b.workCh
-		queuedChanges += countPatches(w.changes)
+		for _, ch := range w.changes {
+			if ch.PingRequest != nil {
+				queuedChanges++
+			}
+		}
+	}
+	// State can coalesce to a full update, but every ping must survive.
+	remaining := 0
+
+	for _, ch := range getPendingForNode(lb.b, types.NodeID(1)) {
+		if ch.PingRequest != nil {
+			remaining++
+		}
 	}
-	// Also count any still-pending
-	remaining := countPatches(getPendingForNode(lb.b, types.NodeID(1)))
 
 	total := queuedChanges + remaining
 	added := int(addedCount.Load())
@@ -691,6 +699,7 @@ func TestWorkMu_PreventsInterTickRace(t *testing.T) {
 	mc := newMultiChannelNodeConn(1, nil)
 	ch := make(chan *tailcfg.MapResponse, 100)
 	entry := &connectionEntry{
+		done:    make(chan struct{}),
 		id:      "test",
 		c:       ch,
 		version: tailcfg.CapabilityVersion(100),
@@ -990,6 +999,7 @@ func TestBug3_CleanupOfflineNodes_TOCTOU(t *testing.T) {
 
 	newCh := make(chan *tailcfg.MapResponse, 10)
 	entry := &connectionEntry{
+		done:    make(chan struct{}),
 		id:      "reconnected",
 		c:       newCh,
 		version: tailcfg.CapabilityVersion(100),
@@ -1033,6 +1043,7 @@ func TestBug3_CleanupOfflineNodes_TOCTOU(t *testing.T) {
 			if mc, ok := lb.b.nodes.Load(targetNode); ok {
 				reconnCh := make(chan *tailcfg.MapResponse, 10)
 				reconnEntry := &connectionEntry{
+					done:    make(chan struct{}),
 					id:      "race-reconn",
 					c:       reconnCh,
 					version: tailcfg.CapabilityVersion(100),
@@ -1469,6 +1480,7 @@ func TestScale1000_MultiChannelBroadcast(t *testing.T) {
 			ch := make(chan *tailcfg.MapResponse, bufferSize)
 			nc.channels[j] = ch
 			entry := &connectionEntry{
+				done:    make(chan struct{}),
 				id:      fmt.Sprintf("conn-%d-%d", i, j),
 				c:       ch,
 				version: tailcfg.CapabilityVersion(100),
@@ -1564,6 +1576,7 @@ func TestScale1000_ConnectionChurn(t *testing.T) {
 				// Add new connection
 				newCh := make(chan *tailcfg.MapResponse, 20)
 				entry := &connectionEntry{
+					done:    make(chan struct{}),
 					id:      fmt.Sprintf("churn-%d-%d", i, cycle),
 					c:       newCh,
 					version: tailcfg.CapabilityVersion(100),
@@ -1649,6 +1662,7 @@ func TestScale1000_ConcurrentAddRemove(t *testing.T) {
 			// Add a new connection
 			ch := make(chan *tailcfg.MapResponse, 10)
 			entry := &connectionEntry{
+				done:    make(chan struct{}),
 				id:      fmt.Sprintf("concurrent-%d", i),
 				c:       ch,
 				version: tailcfg.CapabilityVersion(100),
@@ -1768,6 +1782,7 @@ func TestScale1000_BroadcastDuringNodeChurn(t *testing.T) {
 						mc := newMultiChannelNodeConn(id, nil)
 						ch := make(chan *tailcfg.MapResponse, 10)
 						entry := &connectionEntry{
+							done:    make(chan struct{}),
 							id:      fmt.Sprintf("rechurn-%d-%d", i, cycle),
 							c:       ch,
 							version: tailcfg.CapabilityVersion(100),
@@ -1843,6 +1858,7 @@ func TestScale1000_WorkChannelSaturation(t *testing.T) {
 		mc := newMultiChannelNodeConn(id, nil)
 		ch := make(chan *tailcfg.MapResponse, 1)
 		entry := &connectionEntry{
+			done:    make(chan struct{}),
 			id:      fmt.Sprintf("conn-%d", i),
 			c:       ch,
 			version: tailcfg.CapabilityVersion(100),

hscontrol/mapper/batcher_scale_bench_test.go

@@ -121,6 +121,7 @@ func BenchmarkScale_ConnectionChurn(b *testing.B) {
 
 				newCh := make(chan *tailcfg.MapResponse, 10)
 				entry := &connectionEntry{
+					done:    make(chan struct{}),
 					id:      fmt.Sprintf("sc-%d", i),
 					c:       newCh,
 					version: tailcfg.CapabilityVersion(100),
@@ -408,6 +409,7 @@ func BenchmarkScale_MultiChannelBroadcast(b *testing.B) {
 						for j := range 2 {
 							ch := make(chan *tailcfg.MapResponse, b.N+1)
 							entry := &connectionEntry{
+								done:    make(chan struct{}),
 								id:      fmt.Sprintf("extra-%d-%d", i, j),
 								c:       ch,
 								version: tailcfg.CapabilityVersion(100),
@@ -513,6 +515,7 @@ func BenchmarkScale_ConcurrentSendAndChurn(b *testing.B) {
 
 								newCh := make(chan *tailcfg.MapResponse, b.N+1)
 								entry := &connectionEntry{
+									done:    make(chan struct{}),
 									id:      fmt.Sprintf("sc-churn-%d", i),
 									c:       newCh,
 									version: tailcfg.CapabilityVersion(100),
@@ -585,6 +588,7 @@ func BenchmarkScale_MixedWorkload(b *testing.B) {
 
 								newCh := make(chan *tailcfg.MapResponse, 10)
 								entry := &connectionEntry{
+									done:    make(chan struct{}),
 									id:      fmt.Sprintf("mix-churn-%d", i),
 									c:       newCh,
 									version: tailcfg.CapabilityVersion(100),

hscontrol/mapper/batcher_test.go

@@ -1177,8 +1177,8 @@ func TestBatcherCoalescesPolicyRecomputesPerTick(t *testing.T) {
 
 // TestBatcherCoalescesPeerDeltasPerTick lands a join storm, every other node
 // added and coming online, in one tick. Rendered one frame per change, a
-// storm outgrows the session channel within a single tick and the batcher
+// storm outgrows the session channel within a single tick and the rest backs
-// drops the connection as stale; the run must reach the node as one frame.
+// off to later ticks; the run must reach the node as one frame.
 func TestBatcherCoalescesPeerDeltasPerTick(t *testing.T) {
 	for _, bf := range allBatcherFunctions {
 		t.Run(bf.name, func(t *testing.T) {

hscontrol/mapper/batcher_unit_test.go

@@ -1,8 +1,8 @@
 package mapper
 
-// Unit tests for batcher components that do NOT require database setup.
+// Unit tests for batcher components: connectionEntry, multiChannelNodeConn,
-// These tests exercise connectionEntry, multiChannelNodeConn, computePeerDiff,
+// computePeerDiff, updateSentPeers, generateMapResponse branching and
-// updateSentPeers, generateMapResponse branching, and handleNodeChange in isolation.
+// handleNodeChange.
 
 import (
 	"errors"
@@ -11,6 +11,7 @@ import (
 	"sync"
 	"sync/atomic"
 	"testing"
+	"testing/synctest"
 	"time"
 
 	"github.com/juanfont/headscale/hscontrol/types"
@@ -181,6 +182,7 @@ func expectNoReceive(t *testing.T, ch <-chan *tailcfg.MapResponse, timeout time.
 // makeConnectionEntry creates a connectionEntry with the given channel.
 func makeConnectionEntry(id string, ch chan<- *tailcfg.MapResponse) *connectionEntry {
 	entry := &connectionEntry{
+		done:    make(chan struct{}),
 		id:      id,
 		c:       ch,
 		version: tailcfg.CapabilityVersion(100),
@@ -199,6 +201,8 @@ func TestConnectionEntry_SendSuccess(t *testing.T) {
 	ch := make(chan *tailcfg.MapResponse, 1)
 	entry := makeConnectionEntry("test-conn", ch)
 	data := testMapResponse()
+	stalled := time.Now().Add(-connectionStallTimeout)
+	entry.blockedSince.Store(&stalled)
 
 	beforeSend := time.Now().Unix()
 	err := entry.send(data)
@@ -206,6 +210,8 @@ func TestConnectionEntry_SendSuccess(t *testing.T) {
 	require.NoError(t, err)
 	assert.GreaterOrEqual(t, entry.lastUsed.Load(), beforeSend,
 		"lastUsed should be updated after successful send")
+	require.ErrorIs(t, entry.send(data), errConnectionBackpressure,
+		"progress must reset the stall deadline before the next full queue")
 
 	// Verify data was actually sent
 	received := expectReceive(t, ch, "data should be on channel")
@@ -223,18 +229,21 @@ func TestConnectionEntry_SendNilData(t *testing.T) {
 }
 
 func TestConnectionEntry_SendTimeout(t *testing.T) {
+	// Fake time keeps the bound independent of scheduler load.
+	synctest.Test(t, func(t *testing.T) {
 		// Unbuffered channel with no reader = always blocks
 		ch := make(chan *tailcfg.MapResponse)
 		entry := makeConnectionEntry("test-conn", ch)
-	data := testMapResponse()
+		stalled := time.Now().Add(-connectionStallTimeout)
+		entry.blockedSince.Store(&stalled)
 
 		start := time.Now()
-	err := entry.send(data)
+		err := entry.send(testMapResponse())
-	elapsed := time.Since(start)
 
 		require.ErrorIs(t, err, ErrConnectionSendTimeout)
-	assert.GreaterOrEqual(t, elapsed, 40*time.Millisecond,
+		assert.Less(t, time.Since(start), 100*time.Millisecond,
-		"should wait approximately 50ms before timeout")
+			"stale detection must not block the worker")
+	})
 }
 
 func TestConnectionEntry_SendClosed(t *testing.T) {
@@ -266,6 +275,423 @@ func TestConnectionEntry_SendUpdatesLastUsed(t *testing.T) {
 		"lastUsed should be updated to current time after send")
 }
 
+func TestConnectionEntry_CloseInterruptsWait(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		mc := newMultiChannelNodeConn(1, nil)
+		entry := makeConnectionEntry("blocked", make(chan *tailcfg.MapResponse))
+		mc.addConnection(entry)
+
+		result := make(chan error, 1)
+		go func() { result <- entry.send(testMapResponse()) }()
+
+		synctest.Wait()
+		mc.close()
+		synctest.Wait()
+
+		select {
+		case err := <-result:
+			require.ErrorIs(t, err, errConnectionClosed)
+		default:
+			t.Fatal("closing a connection must interrupt its send wait")
+		}
+	})
+}
+
+func TestConnectionEntry_WaitsForDrainingConsumer(t *testing.T) {
+	// Fake time: the drain always lands inside the send window.
+	synctest.Test(t, func(t *testing.T) {
+		ch := make(chan *tailcfg.MapResponse, 1)
+		ch <- testMapResponse()
+
+		entry := makeConnectionEntry("draining", ch)
+		data := testMapResponse()
+
+		drained := make(chan struct{})
+		go func() {
+			defer close(drained)
+
+			<-time.After(10 * time.Millisecond)
+			<-ch
+		}()
+
+		err := entry.send(data)
+
+		// Join the drainer first: fake time stops once the test body returns.
+		<-drained
+		require.NoError(t, err, "space freed within the send window must not cost another batch tick")
+		require.Same(t, data, expectReceive(t, ch, "send must use the freed slot"))
+	})
+}
+
+func TestBatcherRetryUsesCurrentPolicyAfterReconnect(t *testing.T) {
+	testData, cleanup := setupBatcherWithTestData(t, NewBatcherAndMapper, 1, 2, 1)
+	defer cleanup()
+
+	_, err := testData.State.SetPolicy([]byte(`{
+		"acls": [{"action":"accept","src":["*"],"dst":["*:*"]}],
+		"ssh": [{"action":"accept","src":["autogroup:member"],
+			"dst":["autogroup:self"],"users":["root"]}]
+	}`))
+	require.NoError(t, err)
+
+	b := NewBatcher(time.Hour, 1, testData.Batcher.mapper)
+
+	b.Start()
+	defer b.Close()
+
+	id := testData.Nodes[0].n.ID
+	slow := make(chan *tailcfg.MapResponse, 1)
+	require.NoError(t, b.AddNode(id, slow, tailcfg.CurrentCapabilityVersion, nil))
+	initial := expectReceive(t, slow, "permissive initial map")
+	require.NotEmpty(t, initial.PacketFilters["base"])
+	require.NotEmpty(t, initial.SSHPolicy.Rules)
+
+	slow <- testMapResponse()
+
+	nc, ok := b.nodes.Load(id)
+	require.True(t, ok)
+	b.AddWork(change.PolicyOnly())
+	b.processBatchedChanges()
+	require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+
+	_, err = testData.State.SetPolicy([]byte(`{"acls":[],"ssh":[]}`))
+	require.NoError(t, err)
+
+	fresh := make(chan *tailcfg.MapResponse, 10)
+	require.NoError(t, b.AddNode(id, fresh, tailcfg.CurrentCapabilityVersion, nil))
+	restrictive := expectReceive(t, fresh, "restrictive initial map")
+	require.Empty(t, restrictive.PacketFilters["base"])
+	require.Empty(t, restrictive.SSHPolicy.Rules)
+	require.True(t, b.RemoveNode(id, slow))
+
+	b.processBatchedChanges()
+	require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+	require.NotEmpty(t, fresh, "the blocked change must be retried")
+
+	for len(fresh) > 0 {
+		resp := <-fresh
+		require.Empty(t, resp.PacketFilters["base"], "retry must never restore revoked packet access")
+
+		if resp.SSHPolicy != nil {
+			require.Empty(t, resp.SSHPolicy.Rules, "retry must never restore revoked SSH access")
+		}
+	}
+
+	require.Zero(t, b.workErrors.Load())
+}
+
+func TestBatcherUnreadyRetryRegeneratesRemovals(t *testing.T) {
+	testData, cleanup := setupBatcherWithTestData(t, NewBatcherAndMapper, 1, 2, 1)
+	defer cleanup()
+
+	b := NewBatcher(time.Hour, 1, testData.Batcher.mapper)
+
+	b.Start()
+	defer b.Close()
+
+	id := testData.Nodes[0].n.ID
+	nc := newMultiChannelNodeConn(id, b.mapper)
+	ch := make(chan *tailcfg.MapResponse, 10)
+	entry := makeConnectionEntry("initial-in-flight", ch)
+	entry.pendingInitial.Store(true)
+	nc.addConnection(entry)
+	b.nodes.Store(id, nc)
+
+	initial, err := b.MapResponseFromChange(id, change.FullSelf(id))
+	require.NoError(t, err)
+	require.Len(t, initial.Peers, 1)
+	peerID := initial.Peers[0].ID
+
+	_, err = testData.State.SetPolicy([]byte(`{"acls":[]}`))
+	require.NoError(t, err)
+	b.AddWork(change.FullUpdate())
+	b.processBatchedChanges()
+	require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+	require.Empty(t, ch, "broadcast must wait for initial delivery")
+
+	// Finish the initial delivery after the worker saw empty sent-peer tracking.
+	nc.workMu.Lock()
+	ch <- initial
+
+	nc.updateSentPeers(initial)
+	entry.pendingInitial.Store(false)
+	nc.workMu.Unlock()
+	require.Same(t, initial, expectReceive(t, ch, "initial delivery"))
+	b.processBatchedChanges()
+	require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+	require.Equal(t, []tailcfg.NodeID{peerID}, expectReceive(t, ch, "regenerated removal").PeersRemoved)
+	require.Zero(t, b.workErrors.Load())
+}
+
+func TestPendingBacklogCollapsesToCurrentState(t *testing.T) {
+	testData, cleanup := setupBatcherWithTestData(t, NewBatcherAndMapper, 1, 1, 1)
+	defer cleanup()
+
+	b := NewBatcher(time.Hour, 1, testData.Batcher.mapper)
+
+	b.Start()
+	defer b.Close()
+
+	id := testData.Nodes[0].n.ID
+	nc := newMultiChannelNodeConn(id, b.mapper)
+	ch := make(chan *tailcfg.MapResponse, 10)
+	nc.addConnection(makeConnectionEntry("slow", ch))
+	nc.lastSentPeers.Store(999, struct{}{})
+	b.nodes.Store(id, nc)
+
+	for i := range 3 {
+		nc.appendPending(change.PingNode(id, &tailcfg.PingRequest{URL: fmt.Sprintf("https://ping/%d", i)}))
+	}
+
+	for range 2000 {
+		nc.appendPending(change.NodeOnline(id))
+	}
+
+	pending := nc.drainPending()
+	require.LessOrEqual(t, len(pending), 256, "state backlog must collapse while preserving pings")
+
+	// A retry must also collapse against a newer full update and keep older pings first.
+	nc.collapsePendingToFull([]change.Change{change.PingNode(id, &tailcfg.PingRequest{URL: "https://new"})})
+	nc.prependPending(pending...)
+	pending = nc.drainPending()
+	require.Len(t, pending, 5)
+	require.True(t, pending[0].IsFull())
+	nc.prependPending(pending...)
+	b.processBatchedChanges()
+	require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+	require.Equal(t, []tailcfg.NodeID{999}, expectReceive(t, ch, "full must retain needed removals").PeersRemoved)
+	require.NotNil(t, expectReceive(t, ch, "collapsed state must be a full map").Node)
+
+	for _, url := range []string{"https://ping/0", "https://ping/1", "https://ping/2", "https://new"} {
+		require.Equal(t, url, expectReceive(t, ch, "pings must survive in order").PingRequest.URL)
+	}
+
+	require.Empty(t, ch)
+	require.Zero(t, b.workErrors.Load())
+}
+
+func TestBatcherSlowConsumerRetriesInOrder(t *testing.T) {
+	testData, cleanup := setupBatcherWithTestData(t, NewBatcherAndMapper, 1, 2, 1)
+	defer cleanup()
+
+	b := NewBatcher(time.Hour, 1, testData.Batcher.mapper)
+
+	b.Start()
+	defer b.Close()
+
+	id := testData.Nodes[0].n.ID
+	nc := newMultiChannelNodeConn(id, b.mapper)
+
+	slow := make(chan *tailcfg.MapResponse, 1)
+	slow <- testMapResponse()
+
+	fast := make(chan *tailcfg.MapResponse, 100)
+
+	nc.addConnection(makeConnectionEntry("fast", fast))
+	nc.addConnection(makeConnectionEntry("slow", slow))
+	nc.lastSentPeers.Store(999, struct{}{})
+	b.nodes.Store(id, nc)
+
+	otherID := testData.Nodes[1].n.ID
+	other := newMultiChannelNodeConn(otherID, b.mapper)
+	otherCh := make(chan *tailcfg.MapResponse, 1)
+	other.addConnection(makeConnectionEntry("other", otherCh))
+	b.nodes.Store(otherID, other)
+
+	removal := change.PeersRemoved(999)
+	removal.TargetNode = id
+	b.AddWork(removal)
+
+	for i := range 8 {
+		b.AddWork(change.PingNode(id, &tailcfg.PingRequest{URL: fmt.Sprintf("https://ping/%d", i)}))
+	}
+
+	b.AddWork(change.PingNode(otherID, &tailcfg.PingRequest{URL: "https://other"}))
+	b.processBatchedChanges()
+
+	// Requeuing at the first blocked change costs one send window; waiting on
+	// each of the nine changes would take nine.
+	require.Eventually(t, func() bool { return !nc.inFlight.Load() && !other.inFlight.Load() },
+		250*time.Millisecond, time.Millisecond)
+	require.Equal(t, 2, nc.getActiveConnectionCount(), "a full queue is not a dead session")
+	_, tracked := nc.lastSentPeers.Load(999)
+	require.True(t, tracked, "partial delivery must not commit shared peer tracking")
+	require.Equal(t, "https://other", expectReceive(t, otherCh, "other node must progress").PingRequest.URL)
+	<-slow
+
+	for i := range 9 {
+		// Drain slower than the old send timeout, including between retries.
+		<-time.After(70 * time.Millisecond)
+		b.processBatchedChanges()
+		require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+		require.Equal(t, 2, nc.getActiveConnectionCount())
+
+		resp := expectReceive(t, slow, "every queued change must reach the slow session")
+		if i == 0 {
+			require.Equal(t, []tailcfg.NodeID{999}, resp.PeersRemoved)
+		} else {
+			require.NotNil(t, resp.PingRequest)
+			require.Equal(t, fmt.Sprintf("https://ping/%d", i-1), resp.PingRequest.URL)
+		}
+	}
+
+	_, tracked = nc.lastSentPeers.Load(999)
+	require.False(t, tracked, "delivered removal must update peer tracking")
+	require.Empty(t, nc.drainPending())
+	require.Zero(t, b.workErrors.Load())
+}
+
+func TestBatcherStalledConsumerEvicted(t *testing.T) {
+	testData, cleanup := setupBatcherWithTestData(t, NewBatcherAndMapper, 1, 1, 1)
+	defer cleanup()
+
+	b := NewBatcher(5*time.Millisecond, 1, testData.Batcher.mapper)
+	id := testData.Nodes[0].n.ID
+	nc := newMultiChannelNodeConn(id, b.mapper)
+
+	ch := make(chan *tailcfg.MapResponse, 1)
+	ch <- testMapResponse()
+
+	stopped := make(chan struct{})
+	entry := makeConnectionEntry("stalled", ch)
+	entry.stop = func() { close(stopped) }
+	nc.addConnection(entry)
+	b.nodes.Store(id, nc)
+
+	b.Start()
+	defer b.Close()
+
+	b.AddWork(change.PingNode(id, &tailcfg.PingRequest{URL: "https://ping"}))
+
+	select {
+	case <-stopped:
+		t.Fatal("transient backpressure evicted the session")
+	case <-time.After(100 * time.Millisecond):
+	}
+
+	select {
+	case <-stopped:
+	case <-time.After(6 * time.Second):
+		t.Fatal("a session that never drains must be stopped")
+	}
+
+	require.Eventually(t, func() bool { return nc.getActiveConnectionCount() == 0 }, time.Second, time.Millisecond)
+}
+
+func TestBatcherCloseWithBlockedConsumers(t *testing.T) {
+	testData, cleanup := setupBatcherWithTestData(t, NewBatcherAndMapper, 1, 1, 1)
+	defer cleanup()
+
+	b := NewBatcher(time.Hour, 1, testData.Batcher.mapper)
+
+	b.Start()
+	defer b.Close()
+
+	id := testData.Nodes[0].n.ID
+
+	var stopped atomic.Int64
+
+	for range 20 {
+		ch := make(chan *tailcfg.MapResponse, 1)
+		require.NoError(t, b.AddNode(id, ch, tailcfg.CurrentCapabilityVersion, func() { stopped.Add(1) }))
+	}
+
+	nc, ok := b.nodes.Load(id)
+	require.True(t, ok)
+
+	first := nc.connections[0]
+
+	b.AddWork(change.PingNode(id, &tailcfg.PingRequest{URL: "https://ping"}))
+	b.processBatchedChanges()
+	require.Eventually(t, func() bool { return first.blockedSince.Load() != nil && nc.inFlight.Load() },
+		time.Second, time.Millisecond)
+
+	done := make(chan struct{})
+
+	go func() {
+		b.Close()
+		close(done)
+	}()
+
+	select {
+	case <-done:
+	case <-time.After(200 * time.Millisecond):
+		t.Fatal("blocked consumers held up batcher shutdown")
+	}
+
+	require.Equal(t, int64(20), stopped.Load())
+}
+
+func TestBatcherAlternatingDrainsRevokePolicy(t *testing.T) {
+	testData, cleanup := setupBatcherWithTestData(t, NewBatcherAndMapper, 1, 1, 1)
+	defer cleanup()
+
+	b := NewBatcher(time.Hour, 1, testData.Batcher.mapper)
+
+	b.Start()
+	defer b.Close()
+
+	id := testData.Nodes[0].n.ID
+
+	channels := []chan *tailcfg.MapResponse{
+		make(chan *tailcfg.MapResponse, 1),
+		make(chan *tailcfg.MapResponse, 1),
+	}
+	for _, ch := range channels {
+		require.NoError(t, b.AddNode(id, ch, tailcfg.CurrentCapabilityVersion, nil))
+		require.NotEmpty(t, expectReceive(t, ch, "permissive initial map").PacketFilters["base"])
+
+		ch <- testMapResponse()
+	}
+
+	nc, ok := b.nodes.Load(id)
+	require.True(t, ok)
+
+	older := nc.connections[0]
+	_, err := testData.State.SetPolicy([]byte(`{"acls":[]}`))
+	require.NoError(t, err)
+	b.AddWork(change.PingNode(id, &tailcfg.PingRequest{URL: "https://ping"}), change.PolicyOnly())
+
+	revoked := [2]bool{}
+
+	for tick := range 8 {
+		side := tick % 2
+		select {
+		case resp := <-channels[side]:
+			if resp.PacketFilters != nil {
+				require.Empty(t, resp.PacketFilters["base"], "revocation must deny access")
+
+				revoked[side] = true
+			}
+		default:
+		}
+
+		b.processBatchedChanges()
+		require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+
+		if (revoked[0] || older.closed.Load()) && revoked[1] {
+			break
+		}
+	}
+
+	require.True(t, revoked[0] || older.closed.Load(), "older stream must be revoked or evicted")
+	require.True(t, revoked[1], "alternating progress must not strand revocation behind the ping")
+	require.Zero(t, b.workErrors.Load())
+}
+
+func TestPendingFullAbsorbsRetriedPingPrefix(t *testing.T) {
+	nc := newMultiChannelNodeConn(1, nil)
+	nc.collapsePendingToFull([]change.Change{change.PingNode(1, &tailcfg.PingRequest{URL: "https://new"})})
+	nc.prependPending(change.PingNode(1, &tailcfg.PingRequest{URL: "https://old"}))
+	nc.appendPending(change.NodeOnline(2))
+
+	pending := nc.drainPending()
+	require.Len(t, pending, 3)
+	require.True(t, pending[0].IsFull(), "a newer full must absorb older retried state")
+	require.Equal(t, "https://old", pending[1].PingRequest.URL)
+	require.Equal(t, "https://new", pending[2].PingRequest.URL)
+}
+
 // ============================================================================
 // multiChannelNodeConn.send() Tests
 // ============================================================================
@@ -304,7 +730,10 @@ func TestMultiChannelSend_PartialFailure(t *testing.T) {
 	badCh := make(chan *tailcfg.MapResponse) // unbuffered, no reader
 
 	mc.addConnection(makeConnectionEntry("good-1", goodCh1))
-	mc.addConnection(makeConnectionEntry("bad", badCh))
+	bad := makeConnectionEntry("bad", badCh)
+	stalled := time.Now().Add(-connectionStallTimeout)
+	bad.blockedSince.Store(&stalled)
+	mc.addConnection(bad)
 	mc.addConnection(makeConnectionEntry("good-2", goodCh2))
 
 	err := mc.send(testMapResponse())
@@ -324,7 +753,10 @@ func TestMultiChannelSend_AllFail(t *testing.T) {
 	// All unbuffered channels with no readers
 	for i := range 3 {
 		ch := make(chan *tailcfg.MapResponse) // unbuffered
-		mc.addConnection(makeConnectionEntry(fmt.Sprintf("bad-%d", i), ch))
+		entry := makeConnectionEntry(fmt.Sprintf("bad-%d", i), ch)
+		stalled := time.Now().Add(-connectionStallTimeout)
+		entry.blockedSince.Store(&stalled)
+		mc.addConnection(entry)
 	}
 
 	err := mc.send(testMapResponse())
@@ -362,7 +794,10 @@ func TestMultiChannelSend_FailedConnectionRemoved(t *testing.T) {
 	badCh := make(chan *tailcfg.MapResponse)      // unbuffered, will timeout
 
 	mc.addConnection(makeConnectionEntry("good", goodCh))
-	mc.addConnection(makeConnectionEntry("bad", badCh))
+	bad := makeConnectionEntry("bad", badCh)
+	stalled := time.Now().Add(-connectionStallTimeout)
+	bad.blockedSince.Store(&stalled)
+	mc.addConnection(bad)
 
 	assert.Equal(t, 2, mc.getActiveConnectionCount())
 
@@ -1053,7 +1488,7 @@ func TestBatcher_CloseThenStartIsNoop(t *testing.T) {
 	b.Start()
 
 	// Allow a moment for any hypothetical goroutine to appear.
-	time.Sleep(10 * time.Millisecond) //nolint:forbidigo // test timing
+	<-time.After(10 * time.Millisecond) //nolint:forbidigo // test timing
 
 	goroutinesAfter := runtime.NumGoroutine()
 
@@ -1170,7 +1605,7 @@ func TestRemoveConnectionAtIndex_NilsTrailingSlot(t *testing.T) {
 	// Manually add three entries under the lock.
 	entries := make([]*connectionEntry, 3)
 	for i := range entries {
-		entries[i] = &connectionEntry{id: fmt.Sprintf("conn-%d", i), c: make(chan<- *tailcfg.MapResponse)}
+		entries[i] = &connectionEntry{done: make(chan struct{}), id: fmt.Sprintf("conn-%d", i), c: make(chan<- *tailcfg.MapResponse)}
 	}
 
 	mc.mutex.Lock()
@@ -1278,6 +1713,50 @@ func TestMultiChannelSend_SSHPolicyPerConnection(t *testing.T) {
 	assert.Same(t, data.SSHPolicy, expectReceive(t, chB, "b").SSHPolicy, "b holds an older policy")
 }
 
+func TestPartialPeerAdditionThenRevocation(t *testing.T) {
+	td, cleanup := setupBatcherWithTestData(t, NewBatcherAndMapper, 1, 2, 1)
+	defer cleanup()
+
+	_, err := td.State.SetPolicy([]byte(`{"acls":[]}`))
+	require.NoError(t, err)
+
+	b := NewBatcher(time.Hour, 1, td.Batcher.mapper)
+
+	b.Start()
+	defer b.Close()
+
+	id := td.Nodes[0].n.ID
+	fast := make(chan *tailcfg.MapResponse, 10)
+	slow := make(chan *tailcfg.MapResponse, 1)
+
+	require.NoError(t, b.AddNode(id, fast, tailcfg.CurrentCapabilityVersion, nil))
+	require.NoError(t, b.AddNode(id, slow, tailcfg.CurrentCapabilityVersion, nil))
+	require.Empty(t, (<-fast).Peers)
+	require.Empty(t, (<-slow).Peers)
+
+	slow <- testMapResponse()
+
+	nc, _ := b.nodes.Load(id)
+	_, err = td.State.SetPolicy([]byte(`{"acls":[{"action":"accept","src":["*"],"dst":["*:*"]}]}`))
+	require.NoError(t, err)
+	b.AddWork(change.PolicyChange())
+	b.processBatchedChanges()
+	require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+	accepted := expectReceive(t, fast, "partial addition")
+	require.Len(t, accepted.PeersChanged, 1)
+	peer := accepted.PeersChanged[0].ID
+	_, committed := nc.lastSentPeers.Load(peer)
+	require.False(t, committed, "backpressure must not advance confirmed peer tracking")
+
+	_, err = td.State.SetPolicy([]byte(`{"acls":[]}`))
+	require.NoError(t, err)
+	<-slow
+	b.AddWork(change.FullUpdate())
+	b.processBatchedChanges()
+	require.Eventually(t, func() bool { return !nc.inFlight.Load() }, time.Second, time.Millisecond)
+	require.Equal(t, []tailcfg.NodeID{peer}, expectReceive(t, fast, "revocation must remove partially delivered peer").PeersRemoved)
+}
+
 func TestEmptyMapResponse(t *testing.T) {
 	now := time.Now()
 

hscontrol/mapper/initialmap_phantom_test.go

@@ -29,6 +29,7 @@ func TestUnreadyConnectionDefersBroadcastsUntilInitialMap(t *testing.T) {
 	// in between registering the channel and delivering the initial map.
 	nc := newMultiChannelNodeConn(targetNode.n.ID, batcher.mapper)
 	entry := &connectionEntry{
+		done:    make(chan struct{}),
 		id:      "test-unready",
 		c:       targetNode.ch,
 		version: tailcfg.CapabilityVersion(100),

hscontrol/mapper/node_conn.go

@@ -39,6 +39,15 @@ var errNoActiveConnections = errors.New("no active connections")
 // the change, so dropping it would lose the update.
 var errNoReadyConnections = errors.New("no connections ready for updates")
 
+// errConnectionBackpressure keeps unsent changes queued without declaring a busy client dead.
+var errConnectionBackpressure = errors.New("connection queue full")
+
+// Match the initial-map timeout while staying below reconnect grace and client keepalive.
+const connectionStallTimeout = 5 * time.Second
+
+// A full snapshot supersedes state backlog; pings remain one-shot work.
+const maxPendingChanges = 256
+
 // connectionEntry represents a single connection to a node.
 type connectionEntry struct {
 	id       string // unique connection ID
@@ -49,6 +58,10 @@ type connectionEntry struct {
 	lastUsed atomic.Int64 // Unix timestamp of last successful send
 	closed   atomic.Bool  // Indicates if this connection has been closed
 
+	blockedSince atomic.Pointer[time.Time]
+	done         chan struct{}
+	batcherDone  <-chan struct{}
+
 	// pendingInitial is set by [Batcher.AddNode] while this
 	// connection's initial map is still in flight, and cleared once it
 	// is delivered. Broadcast sends skip such connections so a delta
@@ -142,6 +155,9 @@ type multiChannelNodeConn struct {
 	// full peer lists (which clients interpret as "no change" when empty).
 	// Using xsync.Map for lock-free concurrent access.
 	lastSentPeers *xsync.Map[tailcfg.NodeID, struct{}]
+
+	// Partial additions remain removal candidates when retries render newer state.
+	partialPeers *xsync.Map[tailcfg.NodeID, struct{}]
 }
 
 // connIDCounter is a monotonically increasing counter used to generate
@@ -160,6 +176,7 @@ func newMultiChannelNodeConn(id types.NodeID, mapper *mapper) *multiChannelNodeC
 		id:            id,
 		mapper:        mapper,
 		lastSentPeers: xsync.NewMap[tailcfg.NodeID, struct{}](),
+		partialPeers:  xsync.NewMap[tailcfg.NodeID, struct{}](),
 		log:           log.With().Uint64(zf.NodeID, id.Uint64()).Logger(),
 	}
 }
@@ -179,6 +196,8 @@ func (mc *multiChannelNodeConn) close() {
 // 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.done)
+
 		if conn.stop != nil {
 			conn.stop()
 		}
@@ -294,15 +313,22 @@ func (mc *multiChannelNodeConn) offlineDuration() time.Duration {
 // Thread-safe via pendingMu; does not contend with the connection mutex.
 func (mc *multiChannelNodeConn) appendPending(changes ...change.Change) {
 	mc.pendingMu.Lock()
+
 	mc.pending = append(mc.pending, changes...)
+	fullAtHead := len(mc.pending) > 0 && mc.pending[0].IsFull()
 
 	// Pending drains only once the bundle in flight is delivered, so behind
 	// workers it grew by every change; it renders folded anyway, and
-	// folding twice gives what folding once does.
+	// folding twice gives what folding once does. Folding first leaves the
-	if len(mc.pending) >= max(mc.pendingFoldAt, minPendingFold) {
+	// cap's collapse to a full map for backlogs that do not fold.
+	if !fullAtHead && len(mc.pending) >= max(mc.pendingFoldAt, minPendingFold) {
 		mc.pending = change.CoalescePeerDeltas(mc.id, change.DedupePolicyChanges(mc.pending))
 		mc.pendingFoldAt = 2 * len(mc.pending)
 	}
+
+	if fullAtHead || len(mc.pending) > maxPendingChanges {
+		mc.pending = change.CollapseToFull(mc.id, mc.pending)
+	}
 	mc.pendingMu.Unlock()
 }
 
@@ -313,7 +339,13 @@ func (mc *multiChannelNodeConn) appendPending(changes ...change.Change) {
 // online/offline.
 func (mc *multiChannelNodeConn) prependPending(changes ...change.Change) {
 	mc.pendingMu.Lock()
+	full := (len(mc.pending) > 0 && mc.pending[0].IsFull()) ||
+		(len(changes) > 0 && changes[0].IsFull())
+
 	mc.pending = append(changes, mc.pending...)
+	if len(mc.pending) > maxPendingChanges || full {
+		mc.pending = change.CollapseToFull(mc.id, mc.pending)
+	}
 	mc.pendingMu.Unlock()
 }
 
@@ -357,10 +389,10 @@ func (mc *multiChannelNodeConn) takePending() []change.Change {
 
 // send broadcasts data to all active connections for the node.
 //
-// To avoid holding the write lock during potentially slow sends (each stale
+// To avoid holding the write lock during bounded channel waits, the method
-// connection can block for up to 50ms), the method snapshots connections under
+// snapshots connections under a read lock, sends without any lock held, then
-// a read lock, sends without any lock held, then write-locks only to remove
+// write-locks only to remove failures. New connections added between the
-// failures. New connections added between the snapshot and cleanup are safe:
+// snapshot and cleanup are safe:
 // they receive a full initial map via [Batcher.AddNode], so missing this update
 // causes no data loss.
 func (mc *multiChannelNodeConn) send(data *tailcfg.MapResponse) error {
@@ -406,16 +438,22 @@ func (mc *multiChannelNodeConn) send(data *tailcfg.MapResponse) error {
 		Msg("send: broadcasting")
 
 	// Send to all connections without holding any lock.
-	// Stale connection timeouts (50ms each) happen here without blocking
+	// Bounded channel waits do not block other goroutines that need the mutex.
-	// other goroutines that need the mutex.
 	var (
 		lastErr        error
 		successCount   int
 		failed         []*connectionEntry
+		blocked        []*connectionEntry
+		newestAccepted bool
 	)
 
 	for _, conn := range snapshot {
 		err := conn.send(conn.withSSHPolicyDelta(conn.withSelfDelta(data)))
+		if errors.Is(err, errConnectionBackpressure) {
+			blocked = append(blocked, conn)
+			continue
+		}
+
 		if err != nil {
 			lastErr = err
 
@@ -426,6 +464,7 @@ func (mc *multiChannelNodeConn) send(data *tailcfg.MapResponse) error {
 				Msg("send: connection failed")
 		} else {
 			successCount++
+			newestAccepted = conn == snapshot[len(snapshot)-1]
 
 			if data.Node != nil {
 				conn.lastSelf.Store(data.Node)
@@ -437,6 +476,12 @@ func (mc *multiChannelNodeConn) send(data *tailcfg.MapResponse) error {
 		}
 	}
 
+	// The newest ready stream supersedes blocked siblings left by reconnect overlap.
+	if newestAccepted {
+		failed = append(failed, blocked...)
+		blocked = nil
+	}
+
 	// Write-lock only to remove failed connections.
 	if len(failed) > 0 {
 		mc.mutex.Lock()
@@ -469,6 +514,17 @@ func (mc *multiChannelNodeConn) send(data *tailcfg.MapResponse) error {
 		Int("failed_connections", len(failed)).
 		Msg("send: broadcast complete")
 
+	if len(blocked) > 0 {
+		if successCount > 0 {
+			// Partial additions still need removal if a retry renders newer state without them.
+			for _, peer := range slices.Concat(data.Peers, data.PeersChanged) {
+				mc.partialPeers.Store(peer.ID, struct{}{})
+			}
+		}
+
+		return errConnectionBackpressure
+	}
+
 	// Success if at least one send succeeded
 	if successCount > 0 {
 		return nil
@@ -477,7 +533,7 @@ func (mc *multiChannelNodeConn) send(data *tailcfg.MapResponse) error {
 	return fmt.Errorf("node %d: all connections failed, last error: %w", mc.id, lastErr)
 }
 
-// send sends data to a single connection entry with timeout-based stale connection detection.
+// send distinguishes temporary backpressure from a connection that has stopped making progress.
 func (entry *connectionEntry) send(data *tailcfg.MapResponse) error {
 	if data == nil {
 		return nil
@@ -489,9 +545,7 @@ func (entry *connectionEntry) send(data *tailcfg.MapResponse) error {
 		return fmt.Errorf("connection %s: %w", entry.id, errConnectionClosed)
 	}
 
-	// Use a short timeout to detect stale connections where the client isn't reading the channel.
+	// Let a draining client free a slot before yielding this worker to other nodes.
-	// This is critical for detecting Docker containers that are forcefully terminated
-	// but still have channels that appear open.
 	//
 	// Use a timer rather than time.After so the timeout is explicitly released
 	// on the fast path; both are GC-recoverable since Go 1.23.
@@ -502,12 +556,28 @@ func (entry *connectionEntry) send(data *tailcfg.MapResponse) error {
 	case entry.c <- data:
 		// Update last used timestamp on successful send
 		entry.lastUsed.Store(time.Now().Unix())
+		entry.blockedSince.Store(nil)
+
 		return nil
+	case <-entry.done:
+		return fmt.Errorf("connection %s: %w", entry.id, errConnectionClosed)
+	case <-entry.batcherDone:
+		return ErrBatcherShuttingDown
 	case <-timer.C:
+	}
+
+	// A short stall can be a busy reader; only evict after progress stops for the stall budget.
+	since := entry.blockedSince.Load()
+	if since == nil {
+		now := time.Now()
+		entry.blockedSince.CompareAndSwap(nil, &now)
+	} else if time.Since(*since) >= connectionStallTimeout {
 		// Connection is likely stale - client isn't reading from channel
 		// This catches the case where Docker containers are killed but channels remain open
 		return fmt.Errorf("connection %s: %w", entry.id, ErrConnectionSendTimeout)
 	}
+
+	return errConnectionBackpressure
 }
 
 // nodeID returns the node ID.
@@ -538,6 +608,7 @@ func (mc *multiChannelNodeConn) updateSentPeers(resp *tailcfg.MapResponse) {
 
 	// Full peer list replaces tracked state entirely
 	if resp.Peers != nil {
+		mc.partialPeers.Clear()
 		mc.lastSentPeers.Clear()
 
 		for _, peer := range resp.Peers {
@@ -547,11 +618,13 @@ func (mc *multiChannelNodeConn) updateSentPeers(resp *tailcfg.MapResponse) {
 
 	// Incremental additions
 	for _, peer := range resp.PeersChanged {
+		mc.partialPeers.Delete(peer.ID)
 		mc.lastSentPeers.Store(peer.ID, struct{}{})
 	}
 
 	// Incremental removals
 	for _, id := range resp.PeersRemoved {
+		mc.partialPeers.Delete(id)
 		mc.lastSentPeers.Delete(id)
 	}
 }
@@ -567,13 +640,16 @@ func (mc *multiChannelNodeConn) computePeerDiff(currentPeers []tailcfg.NodeID) [
 	var removed []tailcfg.NodeID
 
 	// Find removed: in lastSentPeers but not in current
-	mc.lastSentPeers.Range(func(id tailcfg.NodeID, _ struct{}) bool {
+	for _, sent := range []*xsync.Map[tailcfg.NodeID, struct{}]{mc.lastSentPeers, mc.partialPeers} {
+		sent.Range(func(id tailcfg.NodeID, _ struct{}) bool {
 			if _, exists := currentSet[id]; !exists {
 				removed = append(removed, id)
+				currentSet[id] = struct{}{}
 			}
 
 			return true
 		})
+	}
 
 	return removed
 }

hscontrol/poll_test.go

@@ -36,6 +36,7 @@ type delayedSuccessResponseWriter struct {
 
 	mu         sync.Mutex
 	writeCount int
+	body       []byte
 }
 
 // deadlineResponseWriter simulates an HTTP/2 response blocked on flow control.
@@ -123,9 +124,8 @@ func (w *delayedSuccessResponseWriter) Write(data []byte) (int, error) {
 	w.mu.Unlock()
 
 	if writeCount == 1 {
-		// Only the first write is delayed. This simulates a transiently wedged map response:
+		// Only the first write is delayed: once it returns the session drains its
-		// long enough to make the batcher time out future sends,
+		// backlog (transient) or observes the stop signal (evicted).
-		// but short enough that the old session can still recover if we leave it alive
 		w.firstWriteStartedOnce.Do(func() {
 			close(w.firstWriteStarted)
 		})
@@ -140,6 +140,10 @@ func (w *delayedSuccessResponseWriter) Write(data []byte) (int, error) {
 		})
 	}
 
+	w.mu.Lock()
+	w.body = append(w.body, data...)
+	w.mu.Unlock()
+
 	return len(data), nil
 }
 
@@ -160,6 +164,13 @@ func (w *delayedSuccessResponseWriter) WriteCount() int {
 	return w.writeCount
 }
 
+func (w *delayedSuccessResponseWriter) Body() string {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	return string(w.body)
+}
+
 // recordingResponseWriter records the status code and whether anything was
 // written, so a test can tell an explicit error response apart from a handler
 // that returned without writing (which net/http turns into an empty 200 the
@@ -467,7 +478,7 @@ func TestDeleteNodeCancelsEphemeralGC(t *testing.T) {
 		"deleting a node must cancel its ephemeral GC timer")
 }
 
-// TestGitHubIssue3129_TransientlyBlockedWriteDoesNotLeaveLiveStaleSession
+// TestGitHubIssue3129_BlockedWriteSessionLifecycle
 // tests the scenario reported in
 // https://github.com/juanfont/headscale/issues/3129.
 //
@@ -476,13 +487,25 @@ func TestDeleteNodeCancelsEphemeralGC(t *testing.T) {
 //  2. Block the first map write long enough for the session to stop draining
 //     its buffered map-response channel.
 //  3. While that write is blocked, queue enough updates to fill the buffered
-//     channel and make the next batcher send hit the stale-send timeout.
+//     channel and make the next batcher send encounter backpressure.
-//  4. That stale-send path removes the session from the batcher, so without an
+//  4. A transient stall must keep the session registered and deliver the queued
-//     explicit teardown hook the old [mapSession.serveLongPoll] goroutine would stay alive
+//     updates once the write completes. A stall past the batcher's stall budget
-//     but stop receiving future updates.
+//     must remove the connection and signal its owning session before shutdown.
-//  5. Release the blocked write and verify the batcher-side stop signal makes
+//  5. After the stalled write completes, verify that [mapSession.serveLongPoll]
-//     that stale session exit instead of lingering as an orphaned goroutine.
+//     exits instead of lingering alive without receiving future updates.
-func TestGitHubIssue3129_TransientlyBlockedWriteDoesNotLeaveLiveStaleSession(t *testing.T) {
+func TestGitHubIssue3129_BlockedWriteSessionLifecycle(t *testing.T) {
+	t.Parallel()
+
+	for _, tt := range []struct {
+		name       string
+		writeDelay time.Duration
+		evicted    bool
+	}{
+		{name: "transient", writeDelay: 250 * time.Millisecond},
+		// Well past the stall budget, so a starved runner still evicts before the write returns.
+		{name: "stalled", writeDelay: 10 * time.Second, evicted: true},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
 			app := createTestApp(t)
@@ -515,7 +538,7 @@ func TestGitHubIssue3129_TransientlyBlockedWriteDoesNotLeaveLiveStaleSession(t *
 			node := nodeView.AsStruct()
 
 			ctx, cancel := context.WithCancel(context.Background())
-	writer := newDelayedSuccessResponseWriter(250 * time.Millisecond)
+			writer := newDelayedSuccessResponseWriter(tt.writeDelay)
 			session := app.newMapSession(ctx, tailcfg.MapRequest{
 				Stream:  true,
 				Version: tailcfg.CapabilityVersion(100),
@@ -551,21 +574,45 @@ func TestGitHubIssue3129_TransientlyBlockedWriteDoesNotLeaveLiveStaleSession(t *
 				close(streamsClosed)
 			}()
 
-	// One update fills the buffered session channel while the first write is blocked.
+			// Distinct pings cannot collapse into a state refresh on retry. Both must
-	// The second update then hits the 50ms stale-send timeout, so the batcher prunes
+			// reach a recovering client, including the one that encountered a full channel.
-	// the stale connection and triggers its stop hook.
+			const (
-	app.mapBatcher.AddWork(change.SelfUpdate(node.ID), change.SelfUpdate(node.ID))
+				firstPing  = "https://example.com/queued-first"
+				secondPing = "https://example.com/queued-second"
+			)
+			app.mapBatcher.AddWork(
+				change.PingNode(node.ID, &tailcfg.PingRequest{URL: firstPing}),
+				change.PingNode(node.ID, &tailcfg.PingRequest{URL: secondPing}),
+			)
+
+			if tt.evicted {
+				require.Eventually(t, func() bool {
+					return app.mapBatcher.Debug()[node.ID].ActiveConnections == 0
+				}, 8*time.Second, 10*time.Millisecond, "a stalled session must be pruned from the batcher")
+				require.Eventually(t, session.cancelChClosed.Load, time.Second, 10*time.Millisecond,
+					"stale eviction must stop the session before batcher shutdown")
+			}
 
 			select {
 			case <-writer.FirstWriteFinished():
-	case <-time.After(2 * time.Second):
+			case <-time.After(tt.writeDelay + time.Second):
 				t.Fatal("expected the blocked write to eventually complete")
 			}
 
-	require.Eventually(t, session.cancelChClosed.Load, 2*time.Second, 10*time.Millisecond,
+			if !tt.evicted {
-		"stale eviction must stop the session before batcher shutdown")
+				require.EventuallyWithT(t, func(c *assert.CollectT) {
+					body := writer.Body()
+					assert.Contains(c, body, firstPing)
+					assert.Contains(c, body, secondPing)
+				}, 2*time.Second, 10*time.Millisecond, "a transiently blocked session must receive both queued pings")
+				assert.Equal(t, 1, app.mapBatcher.Debug()[node.ID].ActiveConnections,
+					"transient backpressure must not evict the session")
+				assert.False(t, session.cancelChClosed.Load(), "transient backpressure must not stop the session")
 
-	// Shutdown skips reconnect grace; the stream wait now includes disconnect cleanup.
+				return
+			}
+
+			// Shutdown skips reconnect grace; the stream wait includes disconnect cleanup.
 			app.mapBatcher.Close()
 
 			assert.Eventually(t, func() bool {
@@ -576,6 +623,8 @@ func TestGitHubIssue3129_TransientlyBlockedWriteDoesNotLeaveLiveStaleSession(t *
 					return false
 				}
 			}, time.Second, 20*time.Millisecond, "after stale-send cleanup, the stale session should exit")
+		})
+	}
 }
 
 // TestDeletedNodeEndsLongPoll proves that deleting a node ends its long-poll