loop, store: read a fact family by prefix (V-444)

A rule over a key set that only exists at read time cannot declare its keys
at wiring time. Kuma has one monitor per service and the names live in the
gauge, so the rule declares a prefix and the gatherer resolves the family per
tick.

ServiceDownRule now fires on any monitor reading down, names it through
DownServices, and is edge-triggered: a service that stays down is one nudge,
not one per tick with cooldown as the only brake.

internal/loop/gather.go

@@ -91,6 +91,20 @@ func (g *Gatherer) GatherState(ctx context.Context, now time.Time) (State, []sto
 		return State{}, nil, err
 	}
 
+	// prefix families — the keys a rule cannot name at wiring time (one fact
+	// per kuma monitor). Loaded into the same map; State.FactsUnder reads them.
+	for _, r := range g.rules {
+		for _, p := range r.WantPrefixes {
+			fam, err := g.store.LatestFactsByPrefix(ctx, p)
+			if err != nil {
+				return State{}, nil, err
+			}
+			for _, f := range fam {
+				facts[f.Key] = f
+			}
+		}
+	}
+
 	// last nudge per rule + cooldown-until derived from the active cooldown.
 	// "active" = the feedback tuner's persisted base if one exists, else the
 	// rule's static Base. LatestFactBySource is the trust-by-provenance read

internal/loop/rules.go

@@ -28,6 +28,13 @@ type Rule struct {
 	// that check itself, leave this empty. Otherwise set to the key(s) the rule
 	// needs and the gate will skip the rule when any are missing.
 	InertWhenNoData []string
+
+	// WantPrefixes — key prefixes whose whole family the gatherer must load.
+	// InertWhenNoData names keys that exist at wiring time; a rule over a key
+	// set that is only known at read time (one fact per kuma monitor) declares
+	// the prefix here instead. Prefixes never make a rule inert: an empty
+	// family is the predicate's own "no data" case.
+	WantPrefixes []string
 }
 
 // Cooldown — tunable bounded by the envelope so a weird week (auto-tuned) can't
@@ -98,23 +105,58 @@ func BreakRule() Rule {
 	}
 }
 
-// ServiceDownRule — sev4 ops hard: the `service_down` aggregate fact reads
-// "down". Source must be poll:uptimekuma — kuma is the source of truth for
-// service up/down (mavpoll writes this key). The predicate is provenance-scoped:
-// a compromised poller writing under a different source can't forge the trigger.
+// ServiceDownPrefix — mavpoll writes one fact per kuma monitor under this
+// prefix, `service_down:<monitor name>`. The suffix is the name he hears.
+const ServiceDownPrefix = "service_down:"
+
+// ServiceDownSource — kuma is the source of truth for service up/down. The
+// rule is provenance-scoped: a poller writing under a different source cannot
+// forge the trigger.
+const ServiceDownSource = "poll:uptimekuma"
+
+// DownServices — the monitors currently reading "down", by name, in key order.
+//
+// Pure, and the rule and the phraser both call it, so the message can never
+// name a service the predicate did not fire on.
+func DownServices(s State) []string {
+	var out []string
+	for _, f := range s.FactsUnder(ServiceDownPrefix) {
+		if f.Source == ServiceDownSource && f.Value == `"down"` {
+			out = append(out, strings.TrimPrefix(f.Key, ServiceDownPrefix))
+		}
+	}
+	return out
+}
+
+// ServiceDownRule — sev4 ops hard: at least one kuma monitor reads "down".
+//
+// It used to read one aggregate `service_down` fact, which is why it was
+// disabled in deploy: the nudge could say that something on homesrv was down
+// but never which thing. Per-monitor facts fix that, and pausing a monitor in
+// kuma now silences that monitor rather than nothing.
+//
+// Edge-triggered — see State.NudgedSince. Without it a service that stays down
+// for a day qualifies on every tick and cooldown alone is the only brake.
 func ServiceDownRule() Rule {
 	return Rule{
-		Name:            "service_down",
-		Severity:        Sev4,
-		Cooldown:        Cooldown{Base: 15 * time.Minute, Min: 5 * time.Minute, Max: 1 * time.Hour},
-		InertWhenNoData: []string{"service_down"},
+		Name:         "service_down",
+		Severity:     Sev4,
+		Cooldown:     Cooldown{Base: 15 * time.Minute, Min: 5 * time.Minute, Max: 1 * time.Hour},
+		WantPrefixes: []string{ServiceDownPrefix},
 		Predicate: func(s State) bool {
-			f, ok := s.Fact("service_down")
-			if !ok || f.Ts.IsZero() {
-				return false
+			var newest time.Time
+			for _, f := range s.FactsUnder(ServiceDownPrefix) {
+				if f.Source != ServiceDownSource || f.Value != `"down"` {
+					continue
+				}
+				if f.Ts.After(newest) {
+					newest = f.Ts
+				}
 			}
-			// value is json `"down"`; trivial check keyed off source provenance.
-			return f.Source == "poll:uptimekuma" && f.Value == `"down"`
+			if newest.IsZero() {
+				return false // nothing down, or no data at all → shut up
+			}
+			return !s.NudgedSince("service_down", newest)
 		},
 	}
 }

internal/loop/state.go

@@ -23,6 +23,8 @@
 package loop
 
 import (
+	"sort"
+	"strings"
 	"time"
 
 	"github.com/kami/maven/internal/store"
@@ -95,6 +97,32 @@ func (s State) Fact(key string) (store.Fact, bool) {
 	return f, true
 }
 
+// FactsUnder returns every gathered fact whose key starts with prefix, ordered
+// by key so a caller that names them speaks them in a stable order. Facts with
+// a zero Ts are skipped, the same "no data" rule Fact applies.
+func (s State) FactsUnder(prefix string) []store.Fact {
+	var out []store.Fact
+	for k, f := range s.Facts {
+		if strings.HasPrefix(k, prefix) && !f.Ts.IsZero() {
+			out = append(out, f)
+		}
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Key < out[j].Key })
+	return out
+}
+
+// NudgedSince reports whether rule already sent a nudge at or after ts.
+//
+// It is what makes a rule edge-triggered. A polled fact is written only when
+// the value changes, so its Ts is the moment the service went down — but the
+// predicate reads the current value, so a service that stays down keeps
+// qualifying forever and cooldown alone only slows the repetition. Asking
+// whether he was already told about THIS transition stops it.
+func (s State) NudgedSince(rule string, ts time.Time) bool {
+	n, ok := s.LastNudge[rule]
+	return ok && !n.Ts.Before(ts)
+}
+
 // Since returns the duration since the latest fact for key, or (0,false).
 // "false" ⇒ no data ⇒ shuts up when uncertain.
 func (s State) Since(key string) (time.Duration, bool) {