Answer an ask the build module cannot read on its step key as refused (issue 1921, seams must 1)

Claude-Session: https://claude.ai/code/session_01NcbgfFSnn24M3GcqttoU7i

modules/mesh-build-go/buildstep/pull.go

@@ -561,6 +561,36 @@ func (p *Puller) declineRun(msg *Pulled, ask delivery.StepAsk, key string, step
 	p.mu.Unlock()
 }
 
+// refuseUnread ends an ask this module cannot read: when its data holds an id that keys a step, the step is written
+// refused with the reason, so the asker learns at once that no node began it; otherwise (no id, not JSON) the ask is only
+// dropped and logged. The message is dropped either way.
+func (p *Puller) refuseUnread(msg *Pulled, why error) {
+	var probe struct {
+		ID   string `json:"id"`
+		Kind string `json:"kind"`
+	}
+	if json.Unmarshal(msg.Data, &probe) == nil && probe.ID != "" {
+		if key, kerr := delivery.AskStepKey(p.Node, probe.ID); kerr == nil {
+			tool := "ask"
+			if probe.Kind != "" {
+				tool = probe.Kind + " ask"
+			}
+			at := now().UTC()
+			step := delivery.Step{ID: probe.ID, Tool: tool, Node: p.Node, State: delivery.StepRefused, Started: at, Ended: at,
+				Reason: fmt.Sprintf("this build module on %s cannot read the ask: %v", p.Node, why), Owner: sdkstep.Owner}
+			if werr := writeEnd(p.Put, key, step); werr != nil {
+				p.logf("the unread ask %s cannot be answered on %s: %v", probe.ID, p.Node, werr)
+			} else {
+				p.logf("an ask refused: %v", why)
+				_ = msg.Term()
+				return
+			}
+		}
+	}
+	p.logf("an ask dropped: %v", why)
+	_ = msg.Term()
+}
+
 func withKind(msg *Pulled, kind string) *Pulled {
 	if msg != nil {
 		msg.Kind = kind
@@ -576,9 +606,10 @@ func (p *Puller) take(ctx context.Context, msg *Pulled) {
 		err = fmt.Errorf("the ask %s is a %s ask of %s, pulled from the %s %s worker", ask.ID, ask.Kind, ask.Language, p.Language, kind)
 	}
 	if err != nil {
-		// An ask that is not one runs nowhere: delivering it again would only fail again.
-		p.logf("an ask dropped: %v", err)
-		_ = msg.Term()
+		// An ask that is not one runs nowhere: delivering it again would only fail again. But an ask that names an id
+		// is answered on its step key as refused, naming why (a job this build module does not know, a version it does
+		// not read): a dropped ask must always be answered, or its asker waits to its bound for nothing.
+		p.refuseUnread(msg, err)
 		return
 	}
 	if lacks := delivery.Lacks(ask.Needs, p.Has); len(lacks) > 0 {

modules/mesh-build-go/buildstep/pull_test.go

@@ -519,3 +519,36 @@ func TestAPoisonedAskIsSaidAndRunNoMore(t *testing.T) {
 		t.Errorf("acked %v naked %v, want it termed alone", acked, naked)
 	}
 }
+
+// An ask whose job this build module does not know is answered on its step key as refused, naming why, never only dropped
+// (a dropped ask leaves its asker waiting to its bound); one with no id is still dropped.
+func TestPullAnswersAnAskItCannotRead(t *testing.T) {
+	src, st := NewFakeAsks(), newSteps()
+	good := testAsk(delivery.AskCheck)
+	raw, err := delivery.EncodeStepAsk(good)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var m map[string]any
+	_ = json.Unmarshal(raw, &m)
+	delete(m, "check")
+	m["frobnicate"] = map[string]any{"program": "x"}
+	odd, _ := json.Marshal(m)
+	// The job is not one this reader holds, so the ask carries none it knows: decode refuses it.
+	if _, err := delivery.DecodeStepAsk(odd); err == nil {
+		t.Fatal("the fixture ask decodes")
+	}
+	src.Publish(delivery.AskCheck, odd)
+	src.Publish(delivery.AskCheck, []byte(`{"v":1}`))
+	p := &Puller{Source: src, Language: delivery.AskGo, Node: "ace", Put: st.put, Capacity: func() int { return 1 },
+		PullWait: 10 * time.Millisecond, Run: func(ctx context.Context, a delivery.StepAsk) (any, error) { t.Error("an unread ask ran"); return nil, nil }}
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	go p.Pull(ctx)
+	waitFor(t, "both termed", func() bool { _, tm, _ := src.Fates(); return len(tm) == 2 })
+	key, _ := delivery.AskStepKey("ace", good.ID)
+	s, ok := st.get(key)
+	if !ok || s.State != delivery.StepRefused || !strings.Contains(s.Reason, "cannot read the ask") || !strings.Contains(s.Reason, "exactly one") {
+		t.Fatalf("the unread ask's step = %+v (%v)", s, ok)
+	}
+}