Asking a tool goes through the seam, and loses two problems

Step 3.4. On the new bus there is no reply queue to declare and no
correlation to check: each account is granted one inbox prefix and no
other, so an answer cannot reach the wrong asker. That settles a cost
build.go records having paid — on a shared reply exchange every asker saw
every result, which is why the correlation was checked rather than assumed.

And a tool nobody serves says so at once rather than after the whole wait.
The difference between "that module is down" and "that tool is slow" is the
first thing a person asking wants, and both tests are against a real server
because both are claims about what the server does, not about this code.

RequestBuild stays as it is, and is a different shape on the new bus rather
than the same one: a build takes minutes, so it is work submitted to a
queue with the outcome returning to a reply subject the request carries —
the pattern design 25 §2 already sets for anything crossing a stream. It
touches the builder too, so it goes with that conversion.

internal/link/bus.go

@@ -2,6 +2,7 @@ package link
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"time"
 
@@ -29,6 +30,11 @@ type Bus interface {
 	// declaration is not an event, and replaying yesterday's is actively harmful
 	// (design 29 §4, the *state* shape).
 	PublishDeclaration(ctx context.Context, node string, body []byte) error
+
+	// AskTool sends one question to a module's tool and awaits one answer. A tool nobody serves
+	// must say so **at once** rather than after the whole wait: the difference between "that
+	// module is down" and "that tool is slow" is the first thing a person asking wants.
+	AskTool(ctx context.Context, module, tool string, args []byte, timeout time.Duration) ([]byte, error)
 }
 
 // --- The bus the mesh runs on today -----------------------------------------------------
@@ -57,6 +63,16 @@ func (b OverCurrent) PublishEvent(ctx context.Context, key, source, node string,
 	})
 }
 
+// AskTool is implemented over the existing reply-queue machinery in ask.go; this seam does not
+// change how it works today.
+func (b OverCurrent) AskTool(ctx context.Context, module, tool string, args []byte, timeout time.Duration) ([]byte, error) {
+	answer, err := Ask(ctx, b.Channel, module, tool, args, timeout)
+	if err != nil {
+		return nil, err
+	}
+	return answer.Result, nil
+}
+
 func (b OverCurrent) PublishDeclaration(ctx context.Context, node string, body []byte) error {
 	// To the queue directly rather than through an exchange: a declaration is for one node, and
 	// routing it by name through a shared exchange would mean a binding per node that nothing
@@ -71,7 +87,10 @@ func (b OverCurrent) PublishDeclaration(ctx context.Context, node string, body [
 // --- NATS, the bus being built ----------------------------------------------------------------
 
 // OverNATS is the bus as a JetStream context.
-type OverNATS struct{ JS nats.JetStreamContext }
+type OverNATS struct {
+	Conn *nats.Conn
+	JS   nats.JetStreamContext
+}
 
 // EventSubject is where a module's event lands. Derived from the emitter, never taken from the
 // caller: a source that could differ from the subject is an envelope that can lie about its
@@ -118,3 +137,30 @@ func (b OverNATS) PublishDeclaration(ctx context.Context, node string, body []by
 	}
 	return nil
 }
+
+// ToolSubject is where a module answers. Derived from the module and the tool, so a caller names
+// what it wants rather than where it lives.
+func ToolSubject(module, tool string) string { return "mesh.mod." + module + ".tool." + tool }
+
+func (b OverNATS) AskTool(ctx context.Context, module, tool string, args []byte, timeout time.Duration) ([]byte, error) {
+	if len(args) == 0 {
+		args = []byte(`{}`)
+	}
+	ask, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
+
+	// **No reply queue, and no correlation to check.** The caller's inbox is its own — each
+	// account is granted one prefix and no other (design 25 §4) — so an answer cannot reach the
+	// wrong asker and there is nothing to correlate against. That also settles a cost recorded
+	// in build.go: on a shared reply exchange every asker saw every result.
+	msg, err := b.Conn.RequestWithContext(ask, ToolSubject(module, tool), args)
+	if err != nil {
+		if errors.Is(err, nats.ErrNoResponders) {
+			// Said at once rather than after the whole wait: nothing is subscribed to that
+			// subject, which is a different fact from a tool being slow.
+			return nil, fmt.Errorf("nothing serves %s.%s", module, tool)
+		}
+		return nil, fmt.Errorf("asking %s.%s: %w", module, tool, err)
+	}
+	return msg.Data, nil
+}

internal/link/bus_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"os"
+	"strings"
 	"testing"
 	"time"
 
@@ -93,3 +94,61 @@ func TestAnEventsSubjectIsDerivedFromItsSource(t *testing.T) {
 		t.Fatal("two modules share an event subject, so neither owns its own name")
 	}
 }
+
+// A tool nobody serves says so at once. The difference between "that module is down" and "that
+// tool is slow" is the first thing a person asking wants, and waiting out the timeout to say it
+// is how a fast answer becomes a slow non-answer.
+func TestAskingAToolNobodyServesFailsAtOnce(t *testing.T) {
+	url := os.Getenv("MESH_TEST_NATS")
+	if url == "" {
+		t.Skip("MESH_TEST_NATS unset")
+	}
+	conn, err := nats.Connect(url)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	bus := OverNATS{Conn: conn}
+	began := time.Now()
+	_, err = bus.AskTool(context.Background(), "nobody", "home", nil, 30*time.Second)
+	if err == nil {
+		t.Fatal("asking a tool nothing serves succeeded")
+	}
+	if took := time.Since(began); took > 2*time.Second {
+		t.Errorf("took %v to say nothing serves it; the caller waited out the timeout", took)
+	}
+	if !strings.Contains(err.Error(), "nothing serves") {
+		t.Errorf("the refusal does not say nobody is there: %v", err)
+	}
+}
+
+// And a served tool answers, with no reply queue to declare and no correlation to check.
+func TestAskingAServedToolAnswers(t *testing.T) {
+	url := os.Getenv("MESH_TEST_NATS")
+	if url == "" {
+		t.Skip("MESH_TEST_NATS unset")
+	}
+	conn, err := nats.Connect(url)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	sub, err := conn.Subscribe(ToolSubject("shop", "price"), func(m *nats.Msg) {
+		_ = m.Respond([]byte(`{"total":12}`))
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer sub.Unsubscribe()
+
+	got, err := OverNATS{Conn: conn}.AskTool(context.Background(), "shop", "price",
+		[]byte(`{"qty":4}`), 5*time.Second)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != `{"total":12}` {
+		t.Fatalf("the answer came back as %s", got)
+	}
+}