cmd/mesh-controller/acting.go

@@ -13,6 +13,7 @@ import (
 	"github.com/nats-io/nats.go/jetstream"
 
 	"github.com/novox/mesh-controller/internal/broker"
+	"github.com/novox/mesh-controller/internal/conditions"
 	"github.com/novox/mesh-controller/internal/inventory"
 	"github.com/novox/mesh-controller/internal/lease"
 	"github.com/novox/mesh-controller/internal/link"
@@ -72,6 +73,14 @@ type actor struct {
 	// lease over in a second rather than in a renewal of five (novox/hq issue 591). A renewal is given
 	// the lease's own RenewEvery to answer however often it is made, so a slow bus does not lose it.
 	renewEvery, poll time.Duration
+	// releasing is held for the whole of a release, so a second caller — the stop signal's and serve's own
+	// deferred one — waits until the lease is given back and its epoch recorded, never past it to the store's
+	// close (novox/hq issue 906).
+	releasing sync.Mutex
+	// keeper and inv are where a takeover is said (sayTakeover): set once the serving controller's condition
+	// store opens; nil before, and in a command.
+	keeper *conditions.Keeper
+	inv    *inventory.Inventory
 }
 
 // theLease is this process's standing to the lease.
@@ -174,8 +183,11 @@ func (a *actor) forACommand(ctx context.Context) (uint64, error) {
 	return holder.Epoch, nil
 }
 
-// release gives back what this process holds, at its end.
+// release gives back what this process holds, at its end — or at the stop signal, before the rest of the
+// process is taken down (serveUnderTheLease). A second call waits for the first to finish.
 func (a *actor) release() {
+	a.releasing.Lock()
+	defer a.releasing.Unlock()
 	a.mu.Lock()
 	closing := a.close
 	a.close = nil
@@ -266,20 +278,40 @@ func (a *actor) serveUnderTheLease(ctx context.Context, inv *inventory.Inventory
 	}
 	a.mu.Lock()
 	a.close = func() {
-		if held, err := l.Epoch(); err == nil {
+		// Given back first, then recorded: an epoch is recorded released only when its key is gone, and
+		// whatever the gate says by then — a shutdown longer than half the key's age closed it, and the epoch
+		// was left open for the next holder to find "expired" (novox/hq issue 906, 2026-10-26).
+		if given := l.Release(context.Background()); given != 0 {
 			ending, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-			if err := inv.EndEpoch(ending, held, inventory.EpochReleased); err != nil {
-				say("how epoch %d ended could not be recorded: %v", held, err)
+			if err := inv.EndEpoch(ending, given, inventory.EpochReleased); err != nil {
+				say("how epoch %d ended could not be recorded: %v", given, err)
 			}
 			cancel()
 		}
-		l.Release(context.Background())
 		js.Close()
 	}
 	a.mu.Unlock()
+	// **The stop signal gives the lease back first** (novox/hq issue 906): the moment ctx ends, before the rest
+	// of the process is taken down, the gate closes and the key goes, so the next controller takes the lease
+	// at once and the epoch is recorded released. Waiting for the process's own end left the key unrenewed for
+	// as long as the shutdown took — ten seconds on 2026-10-26, past the gate's half-age.
+	go func() {
+		<-ctx.Done()
+		if a.holds(l) {
+			say("stopping: the controller lease is given back first")
+		}
+		a.release()
+	}()
 	return gone, nil
 }
 
+// holds is whether this process holds l now.
+func (a *actor) holds(l *lease.Lease) bool {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	return a.held == l
+}
+
 // took is the lease taken: recorded, earlier epochs nobody gave back ended as expired, kept.
 func (a *actor) took(ctx context.Context, l *lease.Lease, inv *inventory.Inventory, epoch uint64, gone chan struct{}) {
 	a.mu.Lock()
@@ -288,15 +320,19 @@ func (a *actor) took(ctx context.Context, l *lease.Lease, inv *inventory.Invento
 	h := holderOf(instance)
 	recording, cancel := context.WithTimeout(ctx, 10*time.Second)
 	expired, err := inv.TookEpoch(recording, inventory.Epoch{Epoch: epoch, Instance: h.Instance, Host: h.Host,
-		Build: h.Build, Taken: time.Now()})
+		Build: h.Build, Taken: time.Now()}, l.GivenBack())
 	cancel()
 	if err != nil {
 		fmt.Printf("epoch %d could not be recorded as taken, so a stale refusal from it will not name it: %v\n", epoch, err)
 	}
+	if given := l.GivenBack(); given != 0 {
+		fmt.Printf("the lease was given back by the controller of epoch %d: taken at once, at epoch %d\n", given, epoch)
+	}
 	for _, e := range expired {
 		fmt.Printf("the controller of epoch %d (%s) stopped renewing the lease without giving it back: it is "+
 			"taken over at epoch %d\n", e.Epoch, e.Instance, epoch)
 	}
+	a.sayTakeover(ctx, epoch)
 	go l.Keep(ctx)
 	go func() {
 		<-l.Lost()

cmd/mesh-controller/lease_integration_test.go

@@ -89,15 +89,28 @@ func TestTwoControllersOneActs(t *testing.T) {
 		t.Fatalf("B, waiting, may act: %v", err)
 	}
 
-	// A hands over, as a controller being replaced does: B takes the lease at once, at a higher epoch.
+	// A is stopped, as its service manager stops a controller being replaced (novox/hq issue 906): the stop
+	// alone gives the lease back, before anything else of A is taken down, and B takes it within a second, at
+	// a higher epoch — no wait for the key's age, and no condition said.
 	stopA()
-	a.release()
+	stopped := time.Now()
 	var lostB <-chan struct{}
 	select {
 	case lostB = <-tookB:
 	case <-time.After(10 * time.Second):
 		t.Fatal("B did not take the lease A gave back")
 	}
+	if waited := time.Since(stopped); waited > time.Second {
+		t.Fatalf("B took the lease %s after A was stopped: the stop did not give it back first", waited)
+	}
+	a.release() // what serve's own end does after: waits for the release under way, gives nothing twice
+	conditionsOfB := conditions.NewInMemory()
+	keeperOfB := conditions.NewKeeper(ctx, conditions.Options{Store: conditionsOfB, History: conditionsOfB})
+	b.watchTakeovers(ctx, inv, keeperOfB)
+	keeperOfB.Close(ctx)
+	if said, err := conditionsOfB.Since(ctx, stopped.Add(-time.Minute)); err != nil || len(said) != 0 {
+		t.Fatalf("a lease given back is said as a condition: %+v, %v", said, err)
+	}
 	select {
 	case <-lostA:
 	default:

cmd/mesh-controller/push.go

@@ -180,6 +180,9 @@ func serve(ctx context.Context) (err error) {
 	if keeper != nil {
 		defer closeConditions(keeper)
 		defer stopRunning() // on an error return, the goroutines end before the keeper closes
+		// The takeover of the lease said as it happened, raised and cleared (novox/hq issue 906).
+		theLease.watchTakeovers(ctx, inv, keeper)
+		defer theLease.watchTakeovers(ctx, nil, nil) // the keeper closes after this: nothing said into it then
 	}
 
 	// The bus's own objects, asserted on every start. **Not created once at genesis**: a stream

cmd/mesh-controller/signals.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"sort"
 	"strings"
@@ -147,9 +148,10 @@ var signalsTable = []signalRow{
 			return newestOf(f.machines, func(m machineFacts) time.Time { return m.toolsHeard })
 		}},
 	{Row: "S12", Signal: "the controller lease renewed", Emitter: "controller", Trigger: "every 5 s",
-		Bound: "15 s (the key's age); a holder that lost the lease, or stopped renewing and was taken over, and a " +
-			"lease bucket found raised again from nothing, are said for an hour after; a controller serving without " +
-			"the lease, for as long as it does",
+		Bound: "15 s (the key's age); a holder that lost the lease or stopped renewing it is said once by the " +
+			"controller that took it over, raised and cleared as it takes it, and for an hour after while none " +
+			"holds it; a lease given back is never said; a lease bucket found raised again from nothing is said " +
+			"for an hour after; a controller serving without the lease, for as long as it does",
 		Kind: "lease-lost", Severity: conditions.Urgent, Phase: 2,
 		needs: func(f *signalFacts) error { return f.leaseErr }, watch: watchLease,
 		newest: func(f *signalFacts) time.Time { return f.lease.renewed }},
@@ -644,10 +646,29 @@ func watchLease(f *signalFacts) []conditions.Observation {
 				"does not know it lost it",
 			Said: l.resetSaid})
 	}
+	// An epoch below the one this controller holds was taken over by it: said once by the takeover itself,
+	// raised and cleared (sayTakeover), not observed for an hour after (novox/hq issue 906). One that ended
+	// while this controller does not hold the lease — serving without it — is still said for the hour.
+	var ended []inventory.Epoch
+	for _, e := range l.ended {
+		if l.held && e.Epoch < l.epoch {
+			continue
+		}
+		ended = append(ended, e)
+	}
+	if o, ok := lostObservation(ended, l.epoch, f.now); ok {
+		out = append(out, o)
+	}
+	return out
+}
+
+// lostObservation is the lease-lost condition of the epochs that lost the lease or let it expire within the
+// hour, named by the newest; false when none did. An epoch given back is no loss.
+func lostObservation(ended []inventory.Epoch, acting uint64, now time.Time) (conditions.Observation, bool) {
 	var lost []string
 	newest := inventory.Epoch{}
-	for _, e := range l.ended {
-		if e.Ended == nil || e.How == inventory.EpochReleased || f.now.Sub(*e.Ended) > advisoryQuiet {
+	for _, e := range ended {
+		if e.Ended == nil || e.How == inventory.EpochReleased || now.Sub(*e.Ended) > advisoryQuiet {
 			continue
 		}
 		lost = append(lost, fmt.Sprintf("epoch %d (%s) %s at %s", e.Epoch, e.Instance, e.How,
@@ -656,18 +677,95 @@ func watchLease(f *signalFacts) []conditions.Observation {
 			newest = e
 		}
 	}
-	if len(lost) > 0 {
-		how := "lost it: another holder wrote its key, or no renewal was made within the key's age"
-		if newest.How == inventory.EpochExpired {
-			how = "stopped renewing it without giving it back, and was taken over"
+	if len(lost) == 0 {
+		return conditions.Observation{}, false
+	}
+	how := "lost it: another holder wrote its key, or no renewal was made within the key's age"
+	if newest.How == inventory.EpochExpired {
+		how = "stopped renewing it without giving it back, and was taken over"
+	}
+	return conditions.Observation{Scope: conditions.ScopeCore, ID: "controller.lease", Token: "lost",
+		Kind: "lease-lost", Machine: newest.Host, Severity: conditions.Urgent, Source: "S12",
+		Summary: fmt.Sprintf("the controller of epoch %d (%s) %s; the controller of epoch %d acts now", newest.Epoch,
+			newest.Instance, how, acting),
+		Said: strings.Join(lost, "; ")}, true
+}
+
+// takeover is the condition a takeover says: the epoch just before the one taken, when it lost the lease or
+// let it expire; false when it gave it back, or none is recorded.
+func takeover(ended []inventory.Epoch, taken uint64, now time.Time) (conditions.Observation, bool) {
+	var before *inventory.Epoch
+	for i, e := range ended {
+		if e.Epoch < taken && (before == nil || e.Epoch > before.Epoch) {
+			before = &ended[i]
 		}
-		out = append(out, conditions.Observation{Scope: conditions.ScopeCore, ID: "controller.lease", Token: "lost",
-			Kind: "lease-lost", Machine: newest.Host, Severity: conditions.Urgent,
-			Summary: fmt.Sprintf("the controller of epoch %d (%s) %s; the controller of epoch %d acts now", newest.Epoch,
-				newest.Instance, how, l.epoch),
-			Said: strings.Join(lost, "; ")})
+	}
+	if before == nil {
+		return conditions.Observation{}, false
+	}
+	return lostObservation([]inventory.Epoch{*before}, taken, now)
+}
+
+// sayTakeover is the takeover said as an event (novox/hq issue 906): when the controller before this one lost
+// the lease or let it expire, the lease-lost condition is raised and cleared at once by this controller, which
+// holds the lease — the operator reads that it happened and that it is over, not an urgent alarm for an hour.
+// A lease given back says nothing. Said once the condition store is open; a take before that is said then.
+func (a *actor) sayTakeover(ctx context.Context, epoch uint64) {
+	a.mu.Lock()
+	keeper, inv := a.keeper, a.inv
+	a.mu.Unlock()
+	if keeper == nil || inv == nil || epoch == 0 {
+		return
+	}
+	now := time.Now()
+	reading, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+	ended, err := inv.EpochsSince(reading, now.Add(-advisoryQuiet))
+	if err != nil {
+		// Failure is loud: S12 no longer observes an epoch below the one held, so an unread takeover is said as
+		// one that was not a handover — raised, and left for S12 to clear (ADR 0558 §3).
+		fmt.Printf("how the epoch before %d ended cannot be read, so its takeover is said as an expiry: %v\n", epoch, err)
+		o := conditions.Observation{Scope: conditions.ScopeCore, ID: "controller.lease", Token: "lost",
+			Kind: "lease-lost", Severity: conditions.Urgent, Source: "S12",
+			Summary: fmt.Sprintf("the controller of epoch %d took the lease, and how the controller before it ended "+
+				"cannot be read: it is said as one that stopped renewing without giving it back", epoch),
+			Said: err.Error()}
+		if _, err := keeper.Observe(reading, o); err != nil {
+			fmt.Printf("the takeover at epoch %d could not be said: %v\n", epoch, err)
+		}
+		return
+	}
+	sayTakeoverOf(reading, keeper, ended, epoch, now)
+}
+
+// sayTakeoverOf raises and clears the takeover's condition over the epochs read.
+func sayTakeoverOf(ctx context.Context, keeper *conditions.Keeper, ended []inventory.Epoch, epoch uint64, now time.Time) {
+	o, ok := takeover(ended, epoch, now)
+	if !ok {
+		return
+	}
+	if _, err := keeper.Observe(ctx, o); err != nil {
+		fmt.Printf("the takeover at epoch %d could not be said: %v\n", epoch, err)
+		return
+	}
+	if _, err := keeper.Clear(ctx, o.Key(), fmt.Sprintf("taken over: the controller of epoch %d holds the lease", epoch)); err != nil {
+		fmt.Printf("the takeover at epoch %d was said and could not be cleared: %v\n", epoch, err)
+	}
+}
+
+// watchTakeovers gives the serving controller's condition store to the lease, and says the takeover of the
+// epoch it holds, if it took one over before the store opened.
+func (a *actor) watchTakeovers(ctx context.Context, inv *inventory.Inventory, keeper *conditions.Keeper) {
+	a.mu.Lock()
+	a.keeper, a.inv = keeper, inv
+	held := a.held
+	a.mu.Unlock()
+	if held == nil {
+		return
+	}
+	if epoch, err := held.Epoch(); err == nil {
+		a.sayTakeover(ctx, epoch)
 	}
-	return out
 }
 
 // handActsWithin is how far back a repeated cause counts (S15).

cmd/mesh-controller/signals_test.go

@@ -491,13 +491,41 @@ func TestAStaleWriterIsNamedByItsEpoch(t *testing.T) {
 	}
 }
 
-// **The lease lost is said for an hour, and serving without it for as long as it lasts** (S12).
+// **The lease lost is said for an hour while nobody took it over, a takeover is said once as it happens —
+// raised and cleared by the controller that took it — a lease given back is never said, and serving without
+// it for as long as it lasts** (S12, novox/hq issue 906).
 func TestALeaseLostOrMissingIsSaid(t *testing.T) {
 	now := time.Now()
 	f := calm(now)
 	expired := now.Add(-59 * time.Minute)
 	f.lease.ended = []inventory.Epoch{{Epoch: 41, Instance: "controller@anchor pid 7", Host: "anchor",
 		Ended: &expired, How: inventory.EpochExpired}}
+	// Held at epoch 57, above it: the takeover said it, and no hour of observation follows.
+	if got := watchLease(f); len(got) != 0 {
+		t.Fatalf("an epoch this controller took over is observed after the takeover: %+v", got)
+	}
+	store := conditions.NewInMemory()
+	k := conditions.NewKeeper(t.Context(), conditions.Options{Store: store, History: store, Now: func() time.Time { return now }})
+	sayTakeoverOf(t.Context(), k, f.lease.ended, 57, now)
+	if open, err := k.Open(t.Context()); err != nil || len(open) != 0 {
+		t.Fatalf("the takeover is left open: %+v, %v", open, err)
+	}
+	k.Close(t.Context()) // its transitions written to the history
+	history, err := store.Since(t.Context(), now.Add(-time.Minute))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var changes []string
+	for _, e := range history {
+		if e.Key == "core.controller.lease.lost" {
+			changes = append(changes, e.Change)
+		}
+	}
+	if !slices.Equal(changes, []string{"raised", "cleared"}) {
+		t.Fatalf("the takeover of an expired lease is not raised and cleared: %v", changes)
+	}
+	// Not holding the lease, the loss is said for the hour.
+	f.lease.held = false
 	got := watchLease(f)
 	if len(got) != 1 || got[0].Key() != "core.controller.lease.lost" || !strings.Contains(got[0].Summary, "epoch 41") ||
 		got[0].Severity != conditions.Urgent {
@@ -512,6 +540,9 @@ func TestALeaseLostOrMissingIsSaid(t *testing.T) {
 	if got := watchLease(f); len(got) != 0 {
 		t.Fatalf("a lease given back is said as lost: %+v", got)
 	}
+	if _, said := takeover(f.lease.ended, 57, now); said {
+		t.Fatal("the takeover of a lease given back is said")
+	}
 	f.lease = leaseFacts{held: true, epoch: 501, renewed: now, reset: now.Add(-time.Minute), resetSaid: "moved past 500"}
 	if got := watchLease(f); len(got) != 1 || got[0].Key() != "core.controller.lease.reset" {
 		t.Fatalf("a lease bucket raised again from nothing was not said: %+v", got)

internal/inventory/order.go

@@ -67,14 +67,23 @@ func (i *Inventory) HighestEpoch(ctx context.Context) (uint64, error) {
 }
 
 // TookEpoch records an epoch taken, and ends every earlier one still open as found expired: the lease
-// was free to take, so whoever held it last neither holds it nor said it let go. Answers the epochs it
-// ended that way, newest first.
-func (i *Inventory) TookEpoch(ctx context.Context, e Epoch) ([]Epoch, error) {
+// was free to take, so whoever held it last neither holds it nor said it let go. givenBack is the epoch the
+// bus says gave the key back (the lease's own delete marker names it, novox/hq issue 906, ADR 0558 §3): that
+// epoch, if still open, is ended as released — its holder gave the key back and had not yet recorded it — and
+// every other as expired; zero is none. Answers the epochs it ended as expired, newest first.
+func (i *Inventory) TookEpoch(ctx context.Context, e Epoch, givenBack uint64) ([]Epoch, error) {
 	tx, err := i.store.Pool().Begin(ctx)
 	if err != nil {
 		return nil, err
 	}
 	defer func() { _ = tx.Rollback(ctx) }()
+	if givenBack != 0 && givenBack < e.Epoch {
+		if _, err := tx.Exec(ctx,
+			`update controller_epoch set ended = now(), how = $2 where epoch = $1 and ended is null`,
+			int64(givenBack), EpochReleased); err != nil {
+			return nil, err
+		}
+	}
 	rows, err := tx.Query(ctx,
 		`update controller_epoch set ended = now(), how = $2
 		  where ended is null and epoch < $1

internal/lease/lease.go

@@ -34,9 +34,11 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"strconv"
 	"sync"
 	"time"
 
+	"github.com/nats-io/nats.go"
 	"github.com/nats-io/nats.go/jetstream"
 )
 
@@ -115,6 +117,7 @@ type Options struct {
 // Lease is one instance's hold, or its wait for one.
 type Lease struct {
 	kv     jetstream.KeyValue
+	js     jetstream.JetStream
 	stream jetstream.Stream
 	ttl    time.Duration
 	o      Options
@@ -139,6 +142,9 @@ type Lease struct {
 	lostWhy      error
 	lost         chan struct{}
 	taken        time.Time
+	// givenBack is the epoch whose holder gave back the key this instance took — the epoch its delete marker
+	// names on the bus — and zero when the key was left to expire, or freed by anything else (novox/hq issue 906).
+	givenBack uint64
 	// update writes the key at a revision: the bucket's own, or a test's bus that does not answer.
 	update func(ctx context.Context, key string, value []byte, last uint64) (uint64, error)
 }
@@ -184,7 +190,7 @@ func Open(ctx context.Context, js jetstream.JetStream, bucket string, o Options)
 		return nil, fmt.Errorf("a lease renewed every %s with a margin of %s does not fit a key that lives %s",
 			o.RenewEvery, o.Margin, status.TTL())
 	}
-	return &Lease{kv: kv, stream: stream, ttl: status.TTL(), o: o, lost: make(chan struct{}), update: kv.Update}, nil
+	return &Lease{kv: kv, js: js, stream: stream, ttl: status.TTL(), o: o, lost: make(chan struct{}), update: kv.Update}, nil
 }
 
 // TTL is how long the key lives unrenewed.
@@ -271,6 +277,7 @@ func (l *Lease) TryTake(ctx context.Context) (uint64, error) {
 	} else if found {
 		return 0, &heldBy{h}
 	}
+	givenBack := l.wasGivenBack(ctx)
 	if err := l.aboveTheFloor(ctx); err != nil {
 		return 0, err
 	}
@@ -307,13 +314,51 @@ func (l *Lease) TryTake(ctx context.Context) (uint64, error) {
 			"back to be taken again", revision, floor)
 	}
 	l.mu.Lock()
-	l.held, l.epoch, l.revision, l.taken = true, revision, revision, now
+	l.held, l.epoch, l.revision, l.taken, l.givenBack = true, revision, revision, now, givenBack
 	l.renewedAt(anchor)
 	l.mu.Unlock()
 	l.o.Say("took the controller lease at epoch %d (%s)", revision, h.Instance)
 	return revision, nil
 }
 
+// ReleasedHeader is the header of the delete marker a holder's Release writes, naming the epoch it gave back:
+// the one mark that says a handover. A delete marker without it — a command's own lease given back, a take
+// given back over the floor — names no epoch, so it says nothing about the holder before (novox/hq issue 906).
+const ReleasedHeader = "Mesh-Lease-Released"
+
+// wasGivenBack is the epoch the free key was given back by: the epoch its last message on the bus — the delete
+// marker a holder's Release writes — names. Zero when the key expired (the bucket's age removed every message),
+// when the marker names no epoch, and when it cannot be read: a takeover nobody can prove was a handover is
+// said as one that was not (novox/hq issue 906, ADR 0558 §3).
+func (l *Lease) wasGivenBack(ctx context.Context) uint64 {
+	if l.stream == nil {
+		return 0
+	}
+	last, err := l.stream.GetLastMsgForSubject(ctx, l.subject())
+	if err != nil || last == nil {
+		return 0
+	}
+	if op := last.Header.Get("KV-Operation"); op != "DEL" && op != "PURGE" {
+		return 0
+	}
+	epoch, err := strconv.ParseUint(last.Header.Get(ReleasedHeader), 10, 64)
+	if err != nil {
+		return 0
+	}
+	return epoch
+}
+
+// subject is the lease key's subject in its bucket's stream.
+func (l *Lease) subject() string { return "$KV." + l.kv.Bucket() + "." + Key }
+
+// GivenBack is the epoch whose holder gave back the key this instance took, zero when it was not given back:
+// what the takeover was, read from the bus (novox/hq issue 906).
+func (l *Lease) GivenBack() uint64 {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	return l.givenBack
+}
+
 // floor is the highest epoch issued, zero without a floor.
 func (l *Lease) floor(ctx context.Context) (uint64, error) {
 	if l.o.Floor == nil {
@@ -588,9 +633,11 @@ func (l *Lease) lose(why error) error {
 	return l.lostWhy
 }
 
-// Release gives the lease back, so the next candidate takes it at once rather than after its age.
-// Only the key this instance last wrote is deleted: one that moved is somebody else's.
-func (l *Lease) Release(ctx context.Context) {
+// Release gives the lease back, so the next candidate takes it at once rather than after its age, and answers
+// the epoch it gave back — zero when it gave nothing back: it held none, had lost it, or the bus would not
+// delete the key. Only the key this instance last wrote is deleted: one that moved is somebody else's. The
+// gate closes before the key goes, so nothing this instance starts after overlaps its successor.
+func (l *Lease) Release(ctx context.Context) uint64 {
 	l.mu.Lock()
 	held, revision, epoch := l.held && l.lostWhy == nil, l.revision, l.epoch
 	l.held = false
@@ -600,11 +647,11 @@ func (l *Lease) Release(ctx context.Context) {
 	}
 	l.mu.Unlock()
 	if !held {
-		return
+		return 0
 	}
 	releasing, cancel := context.WithTimeout(context.WithoutCancel(ctx), 5*time.Second)
 	defer cancel()
-	err := l.kv.Delete(releasing, Key, jetstream.LastRevision(revision))
+	err := l.giveBack(releasing, epoch, revision)
 	if err != nil && isWrongLast(err) {
 		// A renewal was in flight as this began and landed first: the key moved, and is still this
 		// instance's if it still names it at this epoch. Deleted at the revision it has now.
@@ -612,14 +659,29 @@ func (l *Lease) Release(ctx context.Context) {
 			var h Holder
 			if json.Unmarshal(entry.Value(), &h) == nil && h.Instance == l.o.Holder.Instance &&
 				(h.Epoch == epoch || h.Epoch == 0 && entry.Revision() == epoch) {
-				err = l.kv.Delete(releasing, Key, jetstream.LastRevision(entry.Revision()))
+				err = l.giveBack(releasing, epoch, entry.Revision())
 			}
 		}
 	}
 	if err != nil {
 		l.o.Say("the controller lease (epoch %d) could not be given back, so the next controller waits for "+
 			"it to expire: %v", epoch, err)
-		return
+		return 0
 	}
 	l.o.Say("gave the controller lease back (epoch %d)", epoch)
+	return epoch
+}
+
+// giveBack deletes the key at the revision this instance last wrote, as the bucket's own delete does — a
+// marker with KV-Operation DEL, compare-and-set on the key's last revision — and names the epoch given back in
+// the marker, so the next holder can tell this handover from any other delete (ReleasedHeader).
+func (l *Lease) giveBack(ctx context.Context, epoch, revision uint64) error {
+	if l.js == nil {
+		return l.kv.Delete(ctx, Key, jetstream.LastRevision(revision))
+	}
+	msg := nats.NewMsg(l.subject())
+	msg.Header.Set("KV-Operation", "DEL")
+	msg.Header.Set(ReleasedHeader, strconv.FormatUint(epoch, 10))
+	_, err := l.js.PublishMsg(ctx, msg, jetstream.WithExpectLastSequencePerSubject(revision))
+	return err
 }

internal/lease/lease_test.go

@@ -133,7 +133,12 @@ func TestTheSecondControllerWaitsAndTakesAHigherEpochOnHandover(t *testing.T) {
 	// a gives it back: b takes it at once, not after the key's age, at a higher epoch.
 	stop()
 	handedOver := time.Now()
-	a.Release(t.Context())
+	if given := a.Release(t.Context()); given != epochA {
+		t.Fatalf("a's release answers epoch %d given back, want %d", given, epochA)
+	}
+	if again := a.Release(t.Context()); again != 0 {
+		t.Fatalf("a second release gives epoch %d back again", again)
+	}
 	select {
 	case epochB := <-took:
 		if epochB <= epochA {
@@ -142,6 +147,9 @@ func TestTheSecondControllerWaitsAndTakesAHigherEpochOnHandover(t *testing.T) {
 		if waited := time.Since(handedOver); waited > testTTL {
 			t.Fatalf("b took the lease %s after a gave it back: it waited out the key's age", waited)
 		}
+		if given := b.GivenBack(); given != epochA {
+			t.Fatalf("b reads from the bus that epoch %d gave the key back, want a's %d (novox/hq issue 906)", given, epochA)
+		}
 	case <-time.After(2 * testTTL):
 		t.Fatal("b never took the lease a gave back")
 	}
@@ -182,6 +190,9 @@ func TestAHolderThatStopsRenewingStopsActingBeforeItIsTakenOver(t *testing.T) {
 		if epochB <= epochA {
 			t.Fatalf("b took epoch %d after a's %d", epochB, epochA)
 		}
+		if given := b.GivenBack(); given != 0 {
+			t.Fatalf("b reads a key that expired as given back by epoch %d (novox/hq issue 906)", given)
+		}
 		// And a, the moment it tries to renew, knows it lost: the key moved.
 		if err := a.Renew(t.Context()); err == nil {
 			t.Fatal("a renewed a lease b holds")
@@ -226,8 +237,11 @@ func TestARenewalRefusedIsALossAtOnce(t *testing.T) {
 	if _, err := a.Epoch(); !errors.Is(err, ErrNotHeld) {
 		t.Fatalf("a acts after its renewal was refused: %v", err)
 	}
-	// And giving back a lease it lost deletes nothing of the one who holds it now.
-	a.Release(t.Context())
+	// And giving back a lease it lost deletes nothing of the one who holds it now, and answers no epoch given
+	// back, so none is recorded released (novox/hq issue 906).
+	if given := a.Release(t.Context()); given != 0 {
+		t.Fatalf("a says it gave back epoch %d of a lease it lost", given)
+	}
 	if h, found, err := Current(t.Context(), kv); err != nil || !found || h.Instance != "intruder" {
 		t.Fatalf("after a gave back what it lost, the key holds %+v (%v, %v)", h, found, err)
 	}