Resolve selected lazy modules by exact native cache identity

Authored-By: Codex (GPT-6) <noreply@openai.com>

tools/private-modules/main.go

@@ -99,13 +99,9 @@ func generate() error {
 			return errors.New("private module must have exact unreplaced version")
 		}
 		mod := module{Path: src.Path, Version: src.Version, Files: []artifact{}}
-		rel, err := filepath.Rel(cacheRoot, src.GoMod)
-		if err != nil || !safePath(rel) || !strings.HasSuffix(rel, ".mod") || !strings.HasSuffix(filepath.Dir(rel), "/@v") {
-			return errors.New("unexpected native module cache layout")
-		}
-		stem, err := moduleStem(src.Path, src.Version)
-		if err != nil || filepath.ToSlash(rel) != stem+".mod" {
-			return errors.New("native artifact path differs from module identity")
+		stem, err := cacheStem(cacheRoot, src.Path, src.Version, src.GoMod)
+		if err != nil {
+			return err
 		}
 		for _, ext := range []string{".info", ".mod", ".zip"} {
 			name := stem + ext
@@ -185,6 +181,22 @@ func moduleStem(path, version string) (string, error) {
 	return escape(path) + "/@v/" + escape(version), nil
 }
 
+func cacheStem(cacheRoot, path, version, goMod string) (string, error) {
+	stem, err := moduleStem(path, version)
+	if err != nil {
+		return "", err
+	}
+	// Lazy module graphs omit GoMod for selected but unexpanded modules.
+	// Their canonical cache path is still determined by exact identity.
+	if goMod != "" {
+		rel, err := filepath.Rel(cacheRoot, goMod)
+		if err != nil || filepath.ToSlash(rel) != stem+".mod" {
+			return "", errors.New("native artifact path differs from module identity")
+		}
+	}
+	return stem, nil
+}
+
 func safePath(p string) bool {
 	return p != "" && filepath.Clean(p) == p && !filepath.IsAbs(p) && p != ".." && !strings.HasPrefix(p, "../") && !strings.ContainsAny(p, "\\\x00")
 }

tools/private-modules/main_test.go

@@ -164,3 +164,17 @@ func TestCanonicalModuleArtifactPathBinding(t *testing.T) {
 		}
 	}
 }
+
+func TestLazyGraphModuleCacheIdentity(t *testing.T) {
+	cache := t.TempDir()
+	path, version := "code.fritzlab.net/fixture/module", "v0.1.0"
+	stem, err := cacheStem(cache, path, version, "")
+	if err != nil || stem != path+"/@v/"+version {
+		t.Fatalf("selected lazy module rejected: %q %v", stem, err)
+	}
+	for _, metadata := range []string{filepath.Join(cache, "foreign/@v/v0.1.0.mod"), filepath.Join(cache, "../escaped.mod"), filepath.Join(cache, path, "@v/v0.2.0.mod")} {
+		if _, err := cacheStem(cache, path, version, metadata); err == nil {
+			t.Fatal("contradictory cache identity accepted")
+		}
+	}
+}