format: bound the encoded adapter name symmetrically

DecodeLayout rejected names over 256 bytes while Encode accepted them,
so an oversized name encoded fine and then failed every decode. Enforce
the bound in Validate, shared by both directions.

weed/format/layout.go

@@ -16,6 +16,8 @@ const (
 	MaxExtentCount = 1 << 20
 	// MaxPayloadBytes bounds the adapter payload carried in entry metadata.
 	MaxPayloadBytes = 16 << 20
+	// MaxFormatNameBytes bounds the encoded adapter name.
+	MaxFormatNameBytes = 256
 )
 
 // TotalSize returns the file size the layout describes.
@@ -44,6 +46,9 @@ func (l *Layout) Validate(fileSize int64) error {
 	if l.Format == "" {
 		return fmt.Errorf("layout has no format name")
 	}
+	if len(l.Format) > MaxFormatNameBytes {
+		return fmt.Errorf("layout format name is too long: %d bytes", len(l.Format))
+	}
 	if l.Align < 1 {
 		return fmt.Errorf("layout align %d is invalid", l.Align)
 	}
@@ -97,7 +102,7 @@ func DecodeLayout(data []byte) (*Layout, error) {
 	if err != nil || version != layoutVersion {
 		return nil, fmt.Errorf("unsupported layout version")
 	}
-	name, err := readUvarintBytes(reader, 256)
+	name, err := readUvarintBytes(reader, MaxFormatNameBytes)
 	if err != nil {
 		return nil, fmt.Errorf("read layout format: %w", err)
 	}

weed/format/layout_test.go

@@ -66,6 +66,7 @@ func TestLayoutValidate(t *testing.T) {
 		{"no extents", Layout{Format: "x", Align: 1}, -1, "no extents"},
 		{"bad align", Layout{Format: "x", ExtentSizes: []int64{5}, Align: 0}, -1, "align"},
 		{"no name", Layout{ExtentSizes: []int64{5}, Align: 1}, -1, "format name"},
+		{"name too long", Layout{Format: strings.Repeat("x", MaxFormatNameBytes+1), ExtentSizes: []int64{5}, Align: 1}, -1, "too long"},
 	}
 	for _, test := range tests {
 		err := test.layout.Validate(test.fileSize)