Restart-neutral VPP LB sync; deterministic AS ordering; maglevt cadence; v0.9.5

Three reliability fixes bundled with docs updates.

Restart-neutral VPP LB sync via a startup warmup window
(internal/vpp/warmup.go). Before this, a maglevd restart would
immediately issue SyncLBStateAll with every backend still in
StateUnknown — mapped through BackendEffectiveWeight to weight
0 — and VPP would black-hole all new flows until the checker's
rise counters caught up, several seconds later. The new warmup
tracker owns a process-wide state machine gated by two config
knobs: vpp.lb.startup-min-delay (default 5s) is an absolute
hands-off window during which neither the periodic sync loop
nor the per-transition reconciler touches VPP; vpp.lb.
startup-max-delay (default 30s) is the watchdog for a per-VIP
release phase that runs between the two, releasing each frontend
as soon as every backend it references reaches a non-Unknown
state. At max-delay a final SyncLBStateAll runs for any stragglers
still in Unknown. Config reload does not reset the clock. Both
delays can be set to 0 to disable the warmup entirely. The
reconciler's suppressed-during-warmup events log at DEBUG so
operators can still see them with --log-level debug. Unit tests
cover the tracker state machine, allBackendsKnown precondition,
and the zero-delay escape hatch.

Deterministic AS iteration in VPP LB sync. reconcileVIP and
recreateVIP now issue their lb_as_add_del / lb_as_set_weight
calls in numeric IP order (IPv4 before IPv6, ascending within
each family) via a new sortedIPKeys helper, instead of Go map
iteration order. VPP's LB plugin breaks per-bucket ties in the
Maglev lookup table by insertion position in its internal AS
vec, so without a stable call order two maglevd instances on
the same config could push identical AS sets into VPP in
different orders and produce divergent new-flow tables. Numeric
sort is used in preference to lexicographic so the sync log
stays human-readable: string order would place 10.0.0.10 before
10.0.0.2, and the same problem in v6. Unit tests cover empty,
single, v4/v6 numeric vs lexicographic, v4-before-v6 grouping,
a 1000-iteration stability loop against Go's randomised map
iteration, insertion-order invariance, and the desiredAS
call-site type.

maglevt interval fix. runProbeLoop used to sleep the full
jittered interval after every probe, so a 100ms --interval
with a 30ms probe actually produced a 130ms period. The sleep
now subtracts result.Duration so cadence matches the flag.
Probes that overrun clamp sleep to zero and fire the next
probe immediately without trying to catch up on missed cycles
— a slow backend doesn't get flooded with back-to-back probes
at the moment it's already struggling.

Docs. config-guide now documents flush-on-down and the new
startup-min-delay / startup-max-delay knobs; user-guide's
maglevd section explains the restart-neutrality property, the
three warmup phases, and the relevant slog lines operators
should watch for during a bounce.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/vpp/reconciler.go

@@ -70,6 +70,15 @@ func (r *Reconciler) Run(ctx context.Context) {
 // Frontend-transition events are observational only — the dataplane work
 // they would imply has already been done by the backend-transition event
 // that triggered them.
+//
+// The handler consults the VPP client's warmup tracker before doing any
+// dataplane work. During the startup warmup window the reconciler is
+// either fully suppressed (inside min-delay) or per-VIP gated (the
+// frontend must have been released before events for it pass through).
+// When a transition fires for a VIP that isn't yet released but whose
+// backends have now all settled, the handler opportunistically releases
+// it here so the per-VIP release fires on the event rather than waiting
+// for the next warmup poll tick.
 func (r *Reconciler) handle(ev checker.Event) {
 	if ev.FrontendTransition != nil {
 		return // frontend-only event; no dataplane work
@@ -81,20 +90,66 @@ func (r *Reconciler) handle(ev checker.Event) {
 	if cfg == nil {
 		return
 	}
+
+	w := r.client.warmup
+	feName := ev.FrontendName
+	if !w.isReleased(feName) {
+		// Warmup is still gating this frontend. Decide whether to
+		// release it now, or defer until a later event / the warmup
+		// poll / the final max-delay SyncLBStateAll.
+		if w.inMinDelay() {
+			slog.Debug("vpp-reconciler-suppressed-min-delay",
+				"frontend", feName,
+				"backend", ev.BackendName,
+				"from", ev.Transition.From.String(),
+				"to", ev.Transition.To.String(),
+				"elapsed", w.elapsed(),
+				"reason", "inside vpp.lb.startup-min-delay window")
+			return
+		}
+		fe, ok := cfg.Frontends[feName]
+		if !ok {
+			return
+		}
+		if !allBackendsKnown(fe, r.stateSrc) {
+			slog.Debug("vpp-reconciler-suppressed-warmup",
+				"frontend", feName,
+				"backend", ev.BackendName,
+				"from", ev.Transition.From.String(),
+				"to", ev.Transition.To.String(),
+				"elapsed", w.elapsed(),
+				"reason", "frontend has backends still in StateUnknown; "+
+					"waiting for all to settle or for max-delay watchdog")
+			return
+		}
+		if !w.tryRelease(feName) {
+			// Lost a race with finishAll or another release caller.
+			// Either way the next isReleased call will return true, but
+			// for this event we've already done the right thing by
+			// letting the next few lines re-check and proceed.
+			return
+		}
+		slog.Info("vpp-lb-warmup-release",
+			"frontend", feName,
+			"trigger", "reconciler-event",
+			"backend", ev.BackendName,
+			"elapsed", w.elapsed())
+	}
+
 	slog.Debug("vpp-reconciler-event",
-		"frontend", ev.FrontendName,
+		"frontend", feName,
 		"backend", ev.BackendName,
 		"from", ev.Transition.From.String(),
 		"to", ev.Transition.To.String())
 
-	if err := r.client.SyncLBStateVIP(cfg, ev.FrontendName, ""); err != nil {
+	if err := r.client.SyncLBStateVIP(cfg, feName, ""); err != nil {
 		if errors.Is(err, ErrFrontendNotFound) {
 			// Frontend was removed between the event being emitted and
 			// us handling it; a periodic SyncLBStateAll will clean it up.
 			return
 		}
 		slog.Warn("vpp-reconciler-error",
-			"frontend", ev.FrontendName,
+			"frontend", feName,
 			"backend", ev.BackendName,
 			"from", ev.Transition.From.String(),
 			"to", ev.Transition.To.String(),