Place the change's own repository beside it at its trunk, so a shrink-only list test compares with the trunk's copy at $MESH_CHECK_BESIDE/<name> (issue 1887 A12, ADR 0529 §2)

modules/mesh-delivery-checker/internal/check/beside.go

@@ -35,13 +35,18 @@ var besideRules = map[string][]string{
 // besideFor are the siblings a pull request's tests read (ADR 0484 clause 4): the catalogue at its trunk, always, even in a
 // group holding a catalogue pull request; the node-engine at the commit the mesh runs (the commit of the indexer's module
 // mesh-host, its trunk head when none is known); mesh-sdk, for the controller's tests, at the commit the controller's go.mod
-// pins; any other sibling at the head of the group's pull request for it when the group holds one, else at its trunk.
+// pins; any other sibling at the head of the group's pull request for it when the group holds one, else at its trunk. And
+// first, the change's own repository at the trunk it is merged onto, so a test comparing a shrink-only list with the
+// trunk's copy reads it at $MESH_CHECK_BESIDE/<name> (issue 1887 A12, ADR 0529 §2).
 func (j *Examiner) besideFor(ctx context.Context, c *Case, p Pull) ([]BesideRef, error) {
 	var heads map[string]string
 	if j.Trunks != nil {
 		heads = j.Trunks.Heads()
 	}
 	var out []BesideRef
+	if p.Trunk != "" {
+		out = append(out, BesideRef{Repository: p.Repository, Commit: p.Trunk})
+	}
 	for _, repo := range besideRules[p.Repository] {
 		switch {
 		case repo == catalogueRepository:

modules/mesh-delivery-checker/internal/check/examiner_test.go

@@ -137,7 +137,7 @@ func newBench(t testing.TB) *bench {
 	b.steps.handlers = map[string]func(stepCall) (any, error){
 		"build_go_fetch":   func(stepCall) (any, error) { return b.fetch, nil },
 		"build_node_fetch": func(stepCall) (any, error) { return b.fetch, nil },
-		"build_go_digest":  func(c stepCall) (any, error) { return b.digest[c.args["level"].(string)], nil },
+		"build_go_digest":  func(c stepCall) (any, error) { return echoBeside(b, b.digest[c.args["level"].(string)]), nil },
 		"build_go_test":    func(c stepCall) (any, error) { return b.test(c) },
 		"build_node_test":  func(c stepCall) (any, error) { return b.test(c) },
 	}
@@ -571,6 +571,8 @@ func trunksOf(t *testing.T, heads map[string]string) *Trunks {
 
 func TestNothingIsKeptForAWorkspaceGivenSiblingsWhenTheDigestDoesNotSayItCoversThem(t *testing.T) { // review D1, ADR 0444, issue 1287
 	b := newBench(t)
+	// A build module whose digest does not echo the siblings (before DigestVersion 3).
+	b.steps.handlers["build_go_digest"] = func(c stepCall) (any, error) { return b.digest[c.args["level"].(string)], nil }
 	b.ex.Trunks = trunksOf(t, map[string]string{repoSDK: trunkS, "novox/mesh-catalog": trunkA})
 	b.judge(t, caseOf(t, pullOf(repoA, 7, head1)))
 	if got := len(b.steps.tool("fetch")); got != 1 || b.steps.tool("fetch")[0].args["beside"] == nil {
@@ -592,6 +594,21 @@ func TestNothingIsKeptForAWorkspaceGivenSiblingsWhenTheDigestDoesNotSayItCoversT
 
 // besideDigest stands for the Go build module's digest over a workspace with siblings (DigestVersion 3, lead decision L5):
 // each package's digest folds the siblings' commits in, and the answer echoes the lines it folded.
+// echoBeside is d with the siblings the fetches placed, as the Go build module's digest echoes them (its DigestResult's
+// Beside): every check now places the change's own repository at its trunk (issue 1887 A12).
+func echoBeside(b *bench, d Digested) Digested {
+	var lines []string
+	for _, f := range b.steps.tool("fetch") {
+		refs, _ := f.args["beside"].([]BesideRef)
+		for _, r := range refs {
+			lines = append(lines, r.Repository+" "+r.Commit)
+		}
+	}
+	sort.Strings(lines)
+	d.Beside = lines
+	return d
+}
+
 func besideDigest(b *bench) func(c stepCall) (any, error) {
 	return func(c stepCall) (any, error) {
 		level := c.args["level"].(string)

modules/mesh-delivery-checker/internal/check/judged_test.go

@@ -30,7 +30,7 @@ func candidateBench(t *testing.T, takesRunner bool) *bench {
 		return "installed"
 	}
 	b.steps.handlers["build_go_digest"] = func(c stepCall) (any, error) {
-		d := b.digest[c.args["level"].(string)]
+		d := echoBeside(b, b.digest[c.args["level"].(string)])
 		d.JudgedBy = judged(c)
 		return d, nil
 	}

modules/mesh-delivery-checker/internal/check/pins_test.go

@@ -265,3 +265,29 @@ func TestAForgeThatIsNotHeardFailsAtTheForgeAndIsJudgedAgainWhenTheNodesChange(t
 		t.Errorf("a forge that was not heard is not the change's failure: %+v is not judged again when the nodes change", v.Failed)
 	}
 }
+
+// 1887/A12: the change's own repository is placed beside it at the trunk it is merged onto, for every repository, so a
+// shrink-only list test reads the trunk's copy at $MESH_CHECK_BESIDE/<name>.
+func TestTheChangesOwnRepositoryIsBesideItAtItsTrunk(t *testing.T) {
+	b := newBench(t)
+	b.judge(t, caseOf(t, pullOf(repoA, 7, head1)))
+	got := besideOf(t, b, repoA)
+	if own := got[repoA]; own.Commit != trunkA || own.Ref != "" {
+		t.Fatalf("the repository beside its own change = %+v, want its trunk %s", own, trunkA)
+	}
+}
+
+// 1887/A12: a catalogue change's siblings hold the catalogue itself at the trunk it is merged onto, beside mesh-sdk and
+// mesh-controller, so the catalogue's shrink-only list tests read $MESH_CHECK_BESIDE/mesh-catalog.
+func TestACatalogueChangeHasTheCatalogueBesideItAtItsTrunk(t *testing.T) {
+	b := newBench(t)
+	b.ex.Trunks = trunksOf(t, map[string]string{catalogueRepository: trunkA, SDKRepository: trunkS, controllerRepo: trunkA2})
+	b.judge(t, caseOf(t, pullOf(catalogueRepository, 12, head1)))
+	got := besideOf(t, b, catalogueRepository)
+	if c := got[catalogueRepository]; c.Commit != trunkA || c.Ref != "" {
+		t.Fatalf("the catalogue beside its own change = %+v, want its trunk %s", c, trunkA)
+	}
+	if _, ok := got[SDKRepository]; !ok {
+		t.Errorf("mesh-sdk is no longer beside a catalogue change: %v", got)
+	}
+}