Ask git ls-tree for nested named paths in rounds: given go and go/seats/x together it lists go's children, so mesh-contracts' preview failed (issue 1966)

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

modules/mesh-build-go/buildstep/trees.go

@@ -82,18 +82,20 @@ func narrow(ctx context.Context, run forgekey.Runner, env []string, dir, commit,
 	for i, p := range paths {
 		full[i] = path.Join(mdir, p)
 	}
-	r := run(ctx, env, "git", append([]string{"-C", dir, "ls-tree", "-z", commit, "--"}, full...)...)
-	if r.Status != 0 || r.Err != "" {
-		return fmt.Errorf("git ls-tree %s of the module %s: %s", commit, mdir, forgekey.Said(r.Err+"\n"+r.Stderr))
-	}
 	objects := map[string]string{}
-	for _, l := range strings.Split(r.Stdout, "\x00") {
-		meta, name, ok := strings.Cut(l, "\t")
-		fields := strings.Fields(meta)
-		if !ok || len(fields) != 3 {
-			continue
+	for _, round := range unnested(full) {
+		r := run(ctx, env, "git", append([]string{"-C", dir, "ls-tree", "-z", commit, "--"}, round...)...)
+		if r.Status != 0 || r.Err != "" {
+			return fmt.Errorf("git ls-tree %s of the module %s: %s", commit, mdir, forgekey.Said(r.Err+"\n"+r.Stderr))
+		}
+		for _, l := range strings.Split(r.Stdout, "\x00") {
+			meta, name, ok := strings.Cut(l, "\t")
+			fields := strings.Fields(meta)
+			if !ok || len(fields) != 3 {
+				continue
+			}
+			objects[name] = fields[2]
 		}
-		objects[name] = fields[2]
 	}
 	mt.Paths = map[string]string{}
 	for i, p := range paths {
@@ -106,4 +108,35 @@ func narrow(ctx context.Context, run forgekey.Runner, env []string, dir, commit,
 	return nil
 }
 
+// unnested splits paths into rounds of one git ls-tree each, none of whose paths lies inside another of its round: given
+// "go" and "go/seats/x" together, git ls-tree descends into go and lists its children instead of go itself, so the
+// object of go is missing from its answer (issue 1966: mesh-contracts' manifest names both). Usually one round; the
+// order of paths is kept within a round.
+func unnested(paths []string) [][]string {
+	var rounds [][]string
+	for _, p := range paths {
+		placed := false
+		for i, round := range rounds {
+			if !nestsWithAny(p, round) {
+				rounds[i] = append(round, p)
+				placed = true
+				break
+			}
+		}
+		if !placed {
+			rounds = append(rounds, []string{p})
+		}
+	}
+	return rounds
+}
+
+func nestsWithAny(p string, round []string) bool {
+	for _, q := range round {
+		if p == q || strings.HasPrefix(p, q+"/") || strings.HasPrefix(q, p+"/") || p == "." || q == "." {
+			return true
+		}
+	}
+	return false
+}
+
 func sha256Hex(b []byte) string { h := sha256.Sum256(b); return hex.EncodeToString(h[:]) }

modules/mesh-build-go/buildstep/trees_integration_test.go

@@ -62,4 +62,16 @@ func TestAGoOnlyCommitLeavesTheNamedObjectsEqualOnRealGit(t *testing.T) {
 	if src["."].Paths["src"] != realGit(t, repo, "rev-parse", "HEAD:src") {
 		t.Fatalf("the object of src is %s, git says %s", src["."].Paths["src"], realGit(t, repo, "rev-parse", "HEAD:src"))
 	}
+
+	// A manifest naming a path and a path inside it (mesh-contracts' "go" and "go/seats/meshvault"; issue 1966): each
+	// gets its own object, not git ls-tree's listing of the outer one's children.
+	writeFile(t, repo, "nested/module.json", `{"module":"nested","version":"1","build":{"artifacts":[`+
+		`{"name":"go","kind":"package","language":"go","paths":["go"]},`+
+		`{"name":"go-seat","kind":"package","language":"go","paths":["go/seats/a"]}]}}`)
+	writeFile(t, repo, "nested/go/go.mod", "module x\n")
+	writeFile(t, repo, "nested/go/seats/a/a.go", "package a\n")
+	nested := trees(commit("nested"))["nested"]
+	if nested.Paths["go"] != realGit(t, repo, "rev-parse", "HEAD:nested/go") || nested.Paths["go/seats/a"] != realGit(t, repo, "rev-parse", "HEAD:nested/go/seats/a") {
+		t.Fatalf("a path and a path inside it: %v", nested.Paths)
+	}
 }