Measure a declaration against the bus's largest message, raise a condition near it, and send the vault's export compact (issue 1689)

A declaration is one bus message and the control node's passed the limit by 2 KB. The size is checked at every send
against the connection's own limit, with a condition from 90% and a message naming size and limit when over; the
export, 138 KB compact of 165 KB indented, is sent compact.

internal/link/declare_room_test.go

@@ -0,0 +1,57 @@
+package link
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+)
+
+type roomBus struct {
+	Bus
+	limit int64
+	sent  [][]byte
+}
+
+func (b *roomBus) MaxPayload() int64 { return b.limit }
+func (b *roomBus) PublishDeclaration(_ context.Context, _ string, body []byte) error {
+	b.sent = append(b.sent, body)
+	return nil
+}
+
+type fixedSigner struct{}
+
+func (fixedSigner) Sign(context.Context, []byte) ([]byte, error) { return []byte("sig"), nil }
+
+// A declaration is measured as it goes on the wire against the bus's own limit; one over it is refused saying size and
+// limit, and what the measure sees is told to whoever raises a condition (issue 1689).
+func TestADeclarationOverTheBusLimitIsRefusedSayingSizeAndLimit(t *testing.T) {
+	var seen []int64
+	was := DeclarationMeasured
+	DeclarationMeasured = func(_ context.Context, node string, size, limit int64) {
+		if node != "novox" || limit != 400 {
+			t.Errorf("measured %s against %d", node, limit)
+		}
+		seen = append(seen, size)
+	}
+	t.Cleanup(func() { DeclarationMeasured = was })
+
+	bus := &roomBus{limit: 400}
+	small := []byte(`{"a":"` + strings.Repeat("x", 100) + `"}`)
+	if err := Declare(t.Context(), bus, fixedSigner{}, "novox", small, time.Second); err != nil || len(bus.sent) != 1 {
+		t.Fatalf("a small declaration: %v, %d sent", err, len(bus.sent))
+	}
+	if int64(len(bus.sent[0])) != seen[0] {
+		t.Fatalf("measured %d bytes, sent %d", seen[0], len(bus.sent[0]))
+	}
+
+	big := []byte(`{"a":"` + strings.Repeat("x", 300) + `"}`)
+	err := Declare(t.Context(), bus, fixedSigner{}, "novox", big, time.Second)
+	if err == nil || !strings.Contains(err.Error(), "400 bytes") || !strings.Contains(err.Error(), "novox") ||
+		!strings.Contains(err.Error(), "over") || strings.Contains(err.Error(), "maximum payload exceeded") {
+		t.Fatalf("an oversize declaration: %v", err)
+	}
+	if len(bus.sent) != 1 || len(seen) != 2 || seen[1] <= 400 {
+		t.Fatalf("it was sent (%d) or not measured (%v)", len(bus.sent), seen)
+	}
+}