Bound the bodies an answer shows, naming the rest (issue 1126)

go/compose/frombus/answer.go

@@ -119,6 +119,22 @@ func Answer(ctx context.Context, r Reader, mesh Mesh, s Scope, fps map[string]st
 		running = judged
 	}
 	cmp := delivery.CompareComposedWith(composed, running, s.SameValue)
+	// The answer rides in the node's have key: its shown bodies are bounded, the rest named only.
+	budget := MaxShown
+	for i := range cmp.Differs {
+		kept := cmp.Differs[i].Bodies[:0]
+		for _, b := range cmp.Differs[i].Bodies {
+			if n := len(b.Composed) + len(b.Running); n <= budget {
+				budget -= n
+				kept = append(kept, b)
+			} else {
+				kept = append(kept, delivery.ResourceBodies{ID: b.ID, Why: b.Why})
+				a.Note = "some differing resources are named without their bodies: the answer holds at most " +
+					fmt.Sprint(MaxShown) + " bytes of them"
+			}
+		}
+		cmp.Differs[i].Bodies = kept
+	}
 	// A refused module was composed as a stub: what it composed is no difference; its refusal is said.
 	notFailed := func(m string) bool { _, ok := failed[m]; return !ok }
 	a.Same = slices.DeleteFunc(cmp.Same, func(m string) bool { return !notFailed(m) })
@@ -196,6 +212,9 @@ func Answer(ctx context.Context, r Reader, mesh Mesh, s Scope, fps map[string]st
 	return a
 }
 
+// MaxShown bounds the bytes of shown resource bodies in one answer (delivery.ModuleDiffers.Bodies).
+const MaxShown = 256 << 10
+
 var (
 	grantStandIn = regexp.MustCompile(`^the grant of \S+ for (\S+) on (\S+)$`)
 	needsStandIn = regexp.MustCompile(`^(\S+) on (\S+) needs `)