Wait out a forge not served for a moment when registering a build (novox/hq issue 486)

A registration asked the forge while node-tools re-served after a tool-runner
rollout, heard "nothing serves gitea.gitea_api", and failed a good build's
plan. The bus's not-served answer is now link.ErrNothingServes (and a timeout
link.ErrNoAnswer), and the registration asks again for up to 90 s on those
alone, never on a refusal; past the bound it fails saying how long it waited.

internal/link/ask.go

@@ -16,6 +16,19 @@ type Answer struct {
 	Error  string          `json:"error,omitempty"`
 }
 
+// ErrNoAnswer is a tool asked that did not answer within the asker's wait: nothing refused, and asking again
+// may be answered. With ErrNothingServes, what an asker may wait out (novox/hq issue 486).
+var ErrNoAnswer = errors.New("no answer within the wait")
+
+// saidAs is an error said in its own words that is still the error it is a case of, for errors.Is.
+type saidAs struct {
+	text string
+	is   error
+}
+
+func (e saidAs) Error() string { return e.text }
+func (e saidAs) Unwrap() error { return e.is }
+
 // Ask calls one of a module's tools over the broker and waits for its answer.
 //
 // **The control plane is the way in** (novox/hq 04-ISSUES/049, ADR 0095). A module's broker
@@ -37,16 +50,16 @@ func Ask(ctx context.Context, bus Bus, module, tool string, args json.RawMessage
 	reply, err := bus.AskTool(ctx, module, tool, args, timeout)
 	if err != nil {
 		if errors.Is(err, nats.ErrNoResponders) {
-			return Answer{}, fmt.Errorf(
+			return Answer{}, saidAs{fmt.Sprintf(
 				"nothing serves %s: no runtime has bound %q on the bus. The module is not "+
 					"assigned, its runtime is not up, or it serves no such tool — `status` says "+
-					"whether the machine carrying it has applied", module, key)
+					"whether the machine carrying it has applied", module, key), ErrNothingServes}
 		}
 		if errors.Is(err, context.DeadlineExceeded) || errors.Is(err, nats.ErrTimeout) {
-			return Answer{}, fmt.Errorf(
+			return Answer{}, saidAs{fmt.Sprintf(
 				"%s did not answer within %s. Its runtime serves %q when it is up and has bound "+
 					"the bus — `status` says whether the machine carrying it has applied",
-				module, timeout, key)
+				module, timeout, key), ErrNoAnswer}
 		}
 		return Answer{}, fmt.Errorf("cannot ask %s: %w", key, err)
 	}

internal/link/bus.go

@@ -206,8 +206,9 @@ func (b OverNATS) AskTool(ctx context.Context, module, tool string, args []byte,
 	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)
+			// subject, which is a different fact from a tool being slow. Said as ErrNothingServes, so an
+			// asker can tell a tool not served now from one that refused (novox/hq issue 486).
+			return nil, saidAs{fmt.Sprintf("nothing serves %s.%s", module, tool), ErrNothingServes}
 		}
 		return nil, fmt.Errorf("asking %s.%s: %w", module, tool, err)
 	}

internal/link/bus_test.go

@@ -3,6 +3,7 @@ package link
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"strings"
 	"testing"
 	"time"
@@ -115,6 +116,15 @@ func TestAskingAToolNobodyServesFailsAtOnce(t *testing.T) {
 	if !strings.Contains(err.Error(), "nothing serves") {
 		t.Errorf("the refusal does not say nobody is there: %v", err)
 	}
+	// And it is ErrNothingServes, through Ask too, so an asker can wait out a tool runner re-serving and
+	// never a refusal (novox/hq issue 486).
+	if !errors.Is(err, ErrNothingServes) {
+		t.Errorf("nothing serving the tool is not ErrNothingServes: %v", err)
+	}
+	if _, err := Ask(context.Background(), bus, "nobody", "home", nil, 30*time.Second); !errors.Is(err, ErrNothingServes) ||
+		!strings.Contains(err.Error(), "nothing serves nobody.home") {
+		t.Errorf("Ask does not say ErrNothingServes in its words: %v", err)
+	}
 }
 
 // And a served tool answers, with no reply queue to declare and no correlation to check.