Keep a build node's pause in a pauses state of its own, so the steps state can drop its old keys without ending a pause

The steps state of the build modules filled to the bus's 64 MiB cap (27,370 keys of mesh-build-go,
none ever removed) and refused a passed check's end. Its keys get an age in the catalogue; the
pause key lived among them and would then have ended by itself. Puller.PausePut writes it to
delivery.PausesState instead; Put stays the fallback for a module not yet moved.

go/buildstep/pull.go

@@ -65,10 +65,13 @@ type Puller struct {
 	PullWait  time.Duration
 	Heartbeat time.Duration
 	Logf      func(format string, args ...any)
-	// Load reads one key of the module's steps state (nil and no error when it is absent). When set, Pull reads the
-	// node's pause key (delivery.PauseKey) before it pulls, and Pause and Resume write it with Put, so a pause outlives
-	// a restart.
+	// Load reads one key of the state the node's pause is kept in (nil and no error when it is absent). When set, Pull
+	// reads the node's pause key (delivery.PauseKey) before it pulls, and Pause and Resume write it with PausePut, so a
+	// pause outlives a restart.
 	Load func(key string) ([]byte, error)
+	// PausePut writes the node's pause key: the module's pauses state (delivery.PausesState), which has no age, so a
+	// pause never ends by itself while the steps state drops its old keys. Put when nil.
+	PausePut Put
 	// WatchConfig, when set, follows the module's config value `capacity` (CapacitySetting) as mesh-config resolves it
 	// for this node, and wakes the puller on each change (watchCapacity): WatchConfigServed in a module, a test's own
 	// double. Until it hands a value the puller judges by Capacity.
@@ -121,7 +124,11 @@ func (p *Puller) setPaused(paused bool, by string) error {
 		if err != nil {
 			return err
 		}
-		if _, err := p.Put(key, json.RawMessage(raw)); err != nil {
+		put := p.PausePut
+		if put == nil {
+			put = p.Put
+		}
+		if _, err := put(key, json.RawMessage(raw)); err != nil {
 			return fmt.Errorf("buildstep: writing the pause of %s: %w", p.Node, err)
 		}
 	}

go/buildstep/pull_test.go

@@ -252,10 +252,10 @@ func TestPullAtCapacityZeroTakesNothing(t *testing.T) {
 // 1887/A2b: a paused puller takes nothing, its pause is kept on the pause key, and a puller started again on that state
 // stays paused until resumed.
 func TestPullPauseOutlivesARestart(t *testing.T) {
-	src, st := NewFakeAsks(), &kv{}
+	src, st, steps := NewFakeAsks(), &kv{}, &kv{}
 	run := func(ctx context.Context, a delivery.StepAsk) (any, error) { return nil, nil }
-	p := &Puller{Source: src, Language: delivery.AskGo, Node: "shanks", Put: st.put, Load: st.load, PullWait: 10 * time.Millisecond,
-		Capacity: func() int { return 2 }, Run: run}
+	p := &Puller{Source: src, Language: delivery.AskGo, Node: "shanks", Put: steps.put, PausePut: st.put, Load: st.load,
+		PullWait: 10 * time.Millisecond, Capacity: func() int { return 2 }, Run: run}
 	if err := p.Pause("the operator"); err != nil {
 		t.Fatal(err)
 	}
@@ -264,9 +264,12 @@ func TestPullPauseOutlivesARestart(t *testing.T) {
 	if v, err := delivery.DecodePause(raw); err != nil || !v.Paused || v.By != "the operator" {
 		t.Fatalf("the pause key holds %s (%v)", raw, err)
 	}
+	if raw, _ := steps.load(key); raw != nil {
+		t.Fatalf("the pause was written in the steps state, whose keys age out: %s", raw)
+	}
 	// The restart: a new puller on the same state.
-	again := &Puller{Source: src, Language: delivery.AskGo, Node: "shanks", Put: st.put, Load: st.load, PullWait: 10 * time.Millisecond,
-		Capacity: func() int { return 2 }, Run: run}
+	again := &Puller{Source: src, Language: delivery.AskGo, Node: "shanks", Put: steps.put, PausePut: st.put, Load: st.load,
+		PullWait: 10 * time.Millisecond, Capacity: func() int { return 2 }, Run: run}
 	_ = src.PublishAsk(testAsk(delivery.AskDelivery))
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()

go/delivery/stepasks.go

@@ -535,9 +535,13 @@ func ParseAskStepKey(key string) (node, askID string, err error) {
 	return node, askID, nil
 }
 
-// PauseKey is the key, in a build module's StepsBucket, that keeps whether the module on node takes asks:
+// PausesState is the state a build module keeps its nodes' pause keys in. It has no age, unlike StepsState, whose keys
+// are per step and dropped after their age: a pause kept among the steps would end by itself.
+const PausesState = "pauses"
+
+// PauseKey is the key, in a build module's PausesState, that keeps whether the module on node takes asks:
 // `paused.<node>` (issue 1887's design limit 2: every holder pulls from one shared worker, so a node's pause is its holder
-// not fetching, kept here so a restart keeps it). A pause key is never an ask's step key: its second part is a node.
+// not fetching, kept so a restart keeps it). A pause key is never an ask's step key: its second part is a node.
 func PauseKey(node string) (string, error) {
 	if err := checkName("node", node); err != nil {
 		return "", err