Do not process intermediate one if new ones are fresher just use the freshest

client/internal/mapsync.go

@@ -17,33 +17,29 @@ import (
 // desired state. A single background goroutine (run) applies it to the engine in
 // bounded passes via apply() until converged, releasing syncMsgMux between passes
 // so other subsystems interleave. If a newer update arrives mid-flight, the loop
-// coalesces: it keeps converging toward the latest target rather than replaying
-// the intermediate ones.
+// coalesces: it keeps converging toward the latest target and the intermediate one
+// is SKIPPED — never applied on its own (logged, no onConverged).
 //
 // Convergence is a single comparison: appliedGen == targetGen. targetGen
 // increments on every SetTarget (an internal generation counter, so it also covers
 // config-only updates that carry no network-map serial).
 //
-// onConverged still fires once per MAP, not once per convergence: every update's
-// receive time is recorded in `pending` and the whole list is drained on each
-// settle. So a map that was superseded mid-flight (its apply coalesced into a newer
-// target) still gets its finish-sync signal when the client reaches a converged
-// state covering it — the signal is delayed at worst, never lost. That log/metric
-// is a strong problem indicator and must not silently disappear.
+// onConverged fires once for each — and only each — map that is actually processed
+// (i.e. converged as the target). Skipped/superseded maps and dropped-on-error maps
+// do NOT fire it. So "sync finished in X" / RecordSyncDuration always corresponds
+// to a real, completed alignment.
 type mapStateManager struct {
 	// apply performs one bounded apply pass and reports whether more passes are needed.
 	apply func(*mgmProto.SyncResponse) (bool, error)
-	// onConverged is called once per applied map, with the elapsed time since that
+	// onConverged is called once per processed map, with the elapsed time since that
 	// map was received (for the sync-duration metric / "sync finished" log).
 	onConverged func(time.Duration)
 
-	mu         sync.Mutex
-	target     *mgmProto.SyncResponse
-	targetGen  uint64
-	appliedGen uint64
-	// pending holds the receive time of every update accepted since the last settle,
-	// oldest first. Drained on convergence so onConverged fires once per map.
-	pending []time.Time
+	mu          sync.Mutex
+	target      *mgmProto.SyncResponse
+	targetGen   uint64
+	appliedGen  uint64
+	targetSetAt time.Time
 
 	wake chan struct{}
 }
@@ -61,6 +57,12 @@ func newMapStateManager(apply func(*mgmProto.SyncResponse) (bool, error), onConv
 // staleness of the network map is still enforced inside apply (updateNetworkMap).
 func (m *mapStateManager) SetTarget(update *mgmProto.SyncResponse) error {
 	m.mu.Lock()
+	// A target that has not settled yet (targetGen > appliedGen) is being superseded
+	// before it converged: we coalesce to the latest map and never apply this one on
+	// its own. It is SKIPPED — logged here, and it will not fire onConverged.
+	if m.target != nil && m.targetGen > m.appliedGen {
+		log.Debugf("sync map (gen %d) superseded before convergence, skipping", m.targetGen)
+	}
 	m.target = update
 	// Bump an internal generation counter, NOT the map serial: config-only updates
 	// (relay token rotation, STUN/TURN) arrive with NetworkMap == nil and carry no
@@ -68,7 +70,7 @@ func (m *mapStateManager) SetTarget(update *mgmProto.SyncResponse) error {
 	// target regardless of payload. Map-serial staleness is enforced separately
 	// inside apply (updateNetworkMap).
 	m.targetGen++
-	m.pending = append(m.pending, time.Now())
+	m.targetSetAt = time.Now()
 	m.mu.Unlock()
 
 	select {
@@ -119,21 +121,21 @@ func (m *mapStateManager) run(ctx context.Context) {
 			continue
 		}
 
-		// This pass converged. Mark applied + signal every pending map.
+		// This pass converged. Mark applied and signal this one map.
 		m.settle(tg, true)
 		// if a newer target arrived mid-pass, settle is a no-op (targetGen != tg) and
-		// ag<tg next iteration -> apply it; pending carries over until the next settle.
+		// ag<tg next iteration -> apply it; this generation was skipped (logged in
+		// SetTarget) and is not signaled.
 	}
 }
 
 // settle marks generation tg as processed so the loop goes idle instead of
 // re-applying the same target. It is a no-op when a newer target arrived during the
-// pass (targetGen != tg), leaving appliedGen + pending behind so that target
-// re-applies and its maps are signaled at the next settle.
+// pass (targetGen != tg), leaving appliedGen behind so that target re-applies — the
+// just-finished generation was already counted as skipped.
 //
-// When signal is true (the pass converged) it drains pending and fires onConverged
-// once per map. When false (the target was dropped on error) it discards pending
-// without signaling — those maps did not converge; management re-delivers them.
+// When signal is true (the pass converged) it fires onConverged once for this map;
+// when false (the target was dropped on error) it does not — the map did not converge.
 func (m *mapStateManager) settle(tg uint64, signal bool) {
 	m.mu.Lock()
 	if m.targetGen != tg {
@@ -141,13 +143,10 @@ func (m *mapStateManager) settle(tg uint64, signal bool) {
 		return
 	}
 	m.appliedGen = tg
-	toSignal := m.pending
-	m.pending = nil
+	setAt := m.targetSetAt
 	m.mu.Unlock()
 
 	if signal && m.onConverged != nil {
-		for _, setAt := range toSignal {
-			m.onConverged(time.Since(setAt))
-		}
+		m.onConverged(time.Since(setAt))
 	}
 }