A4: a candidate's added modules take the branch the want's code takes

Builds: design 47 G A-316

modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/candidateadded.go

@@ -9,18 +9,26 @@ import (
 
 // addedOf is the `added` of node's candidate (design 47 G A-316, §379, §421): by module, each module the candidate adds to node
 // (laid over current) with no want there, the version and parts the want it would write carries, taken from the same code that
-// writes a want's (namedBuild, the want's build at the module's trunk fingerprint). A module whose build names none is not listed.
+// writes a want's (namedBuild, the want's build at the fingerprint of the branch the want's code takes). A module whose build names none is not listed.
 func (p *passer) addedOf(node string, laid *mma.Assignments, current map[string]mma.Assignments) map[string]mma.AddedModule {
 	if laid == nil {
 		return nil
 	}
-	trunk := p.w.trunk()
+	placed := map[string]string{}
+	for _, m := range laid.Modules {
+		placed[m.Module] = m.Branch
+	}
 	var out map[string]mma.AddedModule
 	for _, module := range plc.ChangeOf(current[node], *laid).Added {
 		if key, err := mwc.WantKey(node, module); err != nil || p.w.has(buckets.Wants(), key) {
 			continue
 		}
-		fp := trunk[module].Fp2
+		// The branch the want's code takes: the one the candidate's placement names, else the module's default (groups, wants).
+		branch := p.w.trunkName()
+		if p.ends.following() {
+			branch = p.ends.branchOf(module, placed[module])
+		}
+		fp := p.w.trunkOf(branch)[module].Fp2
 		if fp == "" {
 			continue
 		}

modules/mesh-delivery-assigner/cmd/mesh-delivery-assigner/candidateadded_test.go

@@ -43,4 +43,17 @@ func TestACandidateThatAddsAModuleCarriesItsParts(t *testing.T) {
 	if got := p.candidateOf("p1", "ace", nil, current); got.Added != nil {
 		t.Errorf("a candidate taking the key away adds %+v", got.Added)
 	}
+	// A placement on a watched branch is added at that branch's fingerprint, as the want's code takes it, not the trunk's.
+	w.putModule("feat", "mail", meshcatalog.ModulesByModuleValue{Fp2: "fp-feat"}, graph.Node{}, nil)
+	feat, _ := json.Marshal(builds.BuildEnded{Module: "mail", Fingerprint: "fp-feat", Made: map[string]string{"program": "sha256:4"}, KeptOn: "g14"})
+	if err := w.ends.put(feat); err != nil {
+		t.Fatal(err)
+	}
+	p.ends = newPipelineEnds(newMemEnds(), func() {}, t.Logf)
+	p.ends.trunk = w.trunkName
+	p.ends.on.Store(true)
+	onFeat := &mma.Assignments{Node: "g14", Modules: []mma.PlacedModule{{Module: "mail", Branch: "feat"}}}
+	if got, want := p.candidateOf("p2", "g14", onFeat, nil).Added, map[string]mma.AddedModule{"mail": {Parts: map[string]string{"mail:program": "sha256:4"}}}; !reflect.DeepEqual(got, want) {
+		t.Errorf("a placement on branch feat adds %+v, want %+v", got, want)
+	}
 }