fix(s3/lifecycle): walker dispatcher nil-response guard + retention-comment

Two PR-review fixes on 9459:

1. WalkerDispatcher.Delete used to panic on a (nil, nil) RPC return —
   add a defensive nil-response check so the walk halts cleanly
   instead. Spotted by coderabbit.

2. The retentionWindow=maxTTL comment in runShard claimed PromotedHash
   "stays empty" in fallback mode, which gemini correctly pointed out
   is only true once rules are active. During bootstrap (rules
   compiled but IsActive=false) MaxEffectiveTTL is 0 while
   PromotedHash counts every non-disabled rule, so promoted becomes
   non-empty and the next post-activation run hits the recovery
   branch. That's the intended bootstrap walk — rewrite the comment
   to explain it rather than misstate the invariant.

Test: pins nil-response → error path on WalkerDispatcher.

weed/s3api/s3lifecycle/dailyrun/run.go

@@ -172,11 +172,16 @@ func runShard(ctx context.Context, cfg Config, snap *engine.Snapshot, runNow tim
 	// one-time rewind to runNow - maxTTL, self-healing on save.
 	rsh := engine.ReplayContentHash(snap)
 	maxTTL := engine.MaxEffectiveTTL(snap)
-	// Operator-supplied retention falls back to maxTTL, which forces
-	// promoted-empty (no rule's TTL can exceed the max). Once the
-	// walker handles walk-bound rules, the handler will pass the real
-	// meta-log retention here and PromotedHash starts catching
-	// partition flips.
+	// Operator-supplied retention falls back to maxTTL. In steady
+	// state every active replay rule has TTL <= maxTTL by construction,
+	// so promoted is empty and the partition-flip trigger is dormant.
+	// During bootstrap (rules compiled but not yet active) maxTTL is
+	// 0, retentionWindow is 0, and every rule with TTL > 0 lands in
+	// the walk partition; the resulting non-empty promoted forces a
+	// recovery walk on the first run after rules activate, which is
+	// the intended bootstrap behavior. Once the handler plumbs the
+	// real meta-log retention here, PromotedHash starts catching
+	// retention-driven partition flips in addition.
 	retentionWindow := cfg.RetentionWindow
 	if retentionWindow <= 0 {
 		retentionWindow = maxTTL

weed/s3api/s3lifecycle/dailyrun/walker_dispatcher.go

@@ -59,6 +59,12 @@ func (d *WalkerDispatcher) Delete(ctx context.Context, action *engine.CompiledAc
 	if err != nil {
 		return fmt.Errorf("walker dispatch %s/%s %s: %w", action.Bucket, objectPath, action.Key.ActionKind, err)
 	}
+	if resp == nil {
+		// A misbehaving server stub returning (nil, nil) would panic on
+		// the switch below. Surface as an error so the walk halts at
+		// this entry, preserving the in-flight cursor's correctness.
+		return fmt.Errorf("walker dispatch %s/%s %s: nil response", action.Bucket, objectPath, action.Key.ActionKind)
+	}
 	switch resp.Outcome {
 	case s3_lifecycle_pb.LifecycleDeleteOutcome_DONE,
 		s3_lifecycle_pb.LifecycleDeleteOutcome_NOOP_RESOLVED,

weed/s3api/s3lifecycle/dailyrun/walker_dispatcher_test.go

@@ -20,6 +20,7 @@ type walkerStubClient struct {
 	outcome s3_lifecycle_pb.LifecycleDeleteOutcome
 	err     error
 	reason  string
+	nilResp bool // return (nil, nil) — pin the dispatcher's defensive guard
 }
 
 func (c *walkerStubClient) LifecycleDelete(_ context.Context, req *s3_lifecycle_pb.LifecycleDeleteRequest) (*s3_lifecycle_pb.LifecycleDeleteResponse, error) {
@@ -27,6 +28,9 @@ func (c *walkerStubClient) LifecycleDelete(_ context.Context, req *s3_lifecycle_
 	if c.err != nil {
 		return nil, c.err
 	}
+	if c.nilResp {
+		return nil, nil
+	}
 	return &s3_lifecycle_pb.LifecycleDeleteResponse{Outcome: c.outcome, Reason: c.reason}, nil
 }
 
@@ -135,6 +139,16 @@ func TestWalkerDispatcher_TransportErrorReturnsWrappedError(t *testing.T) {
 	assert.Contains(t, err.Error(), "transport boom")
 }
 
+func TestWalkerDispatcher_NilResponseReturnsError(t *testing.T) {
+	// A server returning (nil, nil) would otherwise panic on the
+	// outcome switch.
+	c := &walkerStubClient{nilResp: true}
+	d := &WalkerDispatcher{Client: c}
+	err := d.Delete(context.Background(), sampleAction(t, s3lifecycle.ActionKindExpirationDays), &bootstrap.Entry{Path: "obj"})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "nil response")
+}
+
 func TestWalkerDispatcher_NilGuardsReturnError(t *testing.T) {
 	d := &WalkerDispatcher{Client: &walkerStubClient{}}
 	require.Error(t, d.Delete(context.Background(), nil, &bootstrap.Entry{Path: "obj"}))