Judge a node's own composition in its candidate answer only where it runs one, as its agent applies it only on a want (issue 1126)

go/compose/frombus/answer.go

@@ -16,7 +16,7 @@ import (
 // for its own composition), compared with what it runs (delivery.CompareComposed). It applies nothing and writes
 // nothing; head and revision are the candidate's, said back in the answer.
 //
-// fps gives the build each module is composed at: what the node runs, and for a module the candidate places that the
+// The node's own composition is compared only where the node runs one. fps gives the build each module is composed at: what the node runs, and for a module the candidate places that the
 // node does not run, the build the trunk holds; a placed module with none is added, with no build of it known. What the
 // composition stands in for is judged: a value of a module the group adds (on this node or another: its credential, own
 // secrets and boxes are made only once it is placed, ADR 0549 §5) is not yet issued, which is no refusal; any other
@@ -49,6 +49,19 @@ func Answer(ctx context.Context, r Reader, mesh Mesh, s Scope, fps map[string]st
 	if err != nil {
 		return refuse(err)
 	}
+	// The node's own composition is judged only where the node runs one: one that runs none applies it only once a want
+	// names it (the node agent's own composer hands it on that want alone), so a candidate does not add it.
+	runsOwn := false
+	for _, c := range running {
+		runsOwn = runsOwn || c.Module == ""
+	}
+	if !runsOwn {
+		for key, c := range composed {
+			if c.Module == "" {
+				delete(composed, key)
+			}
+		}
+	}
 	cmp := delivery.CompareComposed(composed, running)
 	a.Same, a.Differs, a.Removed = cmp.Same, cmp.Differs, cmp.Removed
 

go/compose/frombus/frombus_test.go

@@ -232,6 +232,15 @@ func TestInput(t *testing.T) {
 	for _, c := range runs {
 		runsNow = append(runsNow, c)
 	}
+	withoutOwn := func(cs []delivery.Composed) []delivery.Composed {
+		var out []delivery.Composed
+		for _, c := range cs {
+			if c.Module != "" {
+				out = append(out, c)
+			}
+		}
+		return out
+	}
 	placedAs := func(modules ...string) map[string]*delivery.Placement {
 		p := one(modules...).placements["n1"]
 		return map[string]*delivery.Placement{"n1": &p}
@@ -245,6 +254,8 @@ func TestInput(t *testing.T) {
 	}{
 		"the same": {one("keeper", "lemurs"), placedAs("keeper", "lemurs"), runsNow, delivery.CandidateSame,
 			func(a delivery.CandidateAnswer) bool { return len(a.Same) == 3 && a.Head == "h1" && a.Revision == 7 }},
+		"the node runs no own composition": {one("keeper", "lemurs"), placedAs("keeper", "lemurs"), withoutOwn(runsNow), delivery.CandidateSame,
+			func(a delivery.CandidateAnswer) bool { return len(a.Same) == 2 && len(a.Added) == 0 }},
 		"one removed": {one("keeper", "lemurs"), placedAs("keeper"), runsNow, delivery.CandidateDiffers,
 			func(a delivery.CandidateAnswer) bool { return reflect.DeepEqual(a.Removed, []string{"lemurs"}) }},
 		"one added, its secret not yet issued": {func() *bus { b := one("lemurs"); b.own = nil; return b }(), placedAs("keeper", "lemurs"),