Keep the real data-path error, reject an empty data path, and report a crypto/rand failure instead of seeding zero

data.go

@@ -10,11 +10,12 @@ import (
 )
 
 // dataSource is one tree to load: an fs.FS and the directory in it to start from.
-// label prefixes file names in errors; path, when set, is a directory on disk that
-// must exist.
+// label prefixes file names in errors; onDisk marks path as a directory that must
+// exist.
 type dataSource struct {
 	fsys   fs.FS
 	label  string
+	onDisk bool
 	path   string
 	root   string
 }
@@ -36,10 +37,13 @@ func (s dataSource) name(p string) string {
 func loadData(sources []dataSource) (map[string]node, error) {
 	root := map[string]node{}
 	for _, src := range sources {
-		if src.path != "" {
+		if src.onDisk {
+			if src.path == "" {
+				return nil, fmt.Errorf("a data path is empty")
+			}
 			info, err := os.Stat(src.path)
 			if err != nil {
-				return nil, fmt.Errorf("%s: no such directory", src.path)
+				return nil, err
 			}
 			if !info.IsDir() {
 				return nil, fmt.Errorf("%s is not a directory", src.path)

fejkdata.go

@@ -75,7 +75,7 @@ func WithSeed(seed uint64) Option {
 // layer several; the last wins a name clash.
 func WithDataPath(dir string) Option {
 	return func(c *config) {
-		c.sources = append(c.sources, dataSource{fsys: os.DirFS(dir), label: dir, path: dir})
+		c.sources = append(c.sources, dataSource{fsys: os.DirFS(dir), label: dir, onDisk: true, path: dir})
 	}
 }
 
@@ -112,7 +112,11 @@ func New(opts ...Option) (*Generator, error) {
 	if err != nil {
 		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
-	return &Generator{rand: newRand(c.seed, c.seeded), categories: cats}, nil
+	rng, err := newRand(c.seed, c.seeded)
+	if err != nil {
+		return nil, fmt.Errorf("fejkdata: %w", err)
+	}
+	return &Generator{rand: rng, categories: cats}, nil
 }
 
 // List returns the sorted dotted paths Fake can render: every category, the dotted
@@ -203,12 +207,19 @@ func join(prefix, name string) string {
 	return prefix + "." + name
 }
 
-func newRand(seed uint64, seeded bool) *session {
-	r := rand.New(rand.NewPCG(seed, seed^0x9e3779b97f4a7c15))
-	if !seeded {
+// randomBytes seeds an unseeded generator; a failure is an error, never a fixed seed.
+var randomBytes = crand.Read
+
+func newRand(seed uint64, seeded bool) (*session, error) {
+	var r *rand.Rand
+	if seeded {
+		r = rand.New(rand.NewPCG(seed, seed^0x9e3779b97f4a7c15))
+	} else {
 		var b [16]byte
-		_, _ = crand.Read(b[:])
+		if _, err := randomBytes(b[:]); err != nil {
+			return nil, fmt.Errorf("seeding from crypto/rand: %w", err)
+		}
 		r = rand.New(rand.NewPCG(binary.LittleEndian.Uint64(b[:8]), binary.LittleEndian.Uint64(b[8:])))
 	}
-	return &session{Rand: r, counters: map[string]uint64{}}
+	return &session{Rand: r, counters: map[string]uint64{}}, nil
 }