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schema.go
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schema.go
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package schema
import (
"bytes"
"encoding/json"
"errors"
"fmt"
"io"
"io/ioutil"
"reflect"
"github.com/dennwc/cas/types"
)
var ErrNotSchema = errors.New("not a schema file")
const (
typeField = "@type"
casNS = "cas:"
tab = " "
magic = "{\n" + tab + "\"" + typeField + `":`
maxSize = 16 * 1024 * 1024
)
const (
// MagicSize is a maximal size of a magic prefix of schema blobs.
MagicSize = len(magic)
)
func init() {
registerCAS(&types.SizedRef{})
registerCAS(&types.SchemaRef{})
registerCAS(&types.Pin{})
}
var _ BlobWrapper = (*types.SizedRef)(nil)
const (
StatDataSize = "size" // size of all raw blobs (excluding the schema)
StatDataCount = "cnt" // number of full objects (logical trees)
)
// Stats is a collection of stat values.
type Stats map[string]uint64
func (s Stats) Size() uint64 {
return s[StatDataSize]
}
// Object is a generic schema object for CAS.
type Object interface {
// TODO: split into DependsOn and Describes
References() []types.Ref
}
type BlobWrapper interface {
DataBlob() types.Ref
}
var (
typesMap = make(map[string]reflect.Type)
typeToName = make(map[reflect.Type]string)
)
func registerCAS(o Object) {
rt := reflect.TypeOf(o)
if rt.Kind() == reflect.Ptr {
rt = rt.Elem()
}
name := casNS + rt.Name()
typesMap[name] = rt
typeToName[rt] = name
}
// RegisterName associates a schema object with a given type name.
func RegisterName(name string, o Object) {
rt := reflect.TypeOf(o)
if rt.Kind() == reflect.Ptr {
rt = rt.Elem()
}
typesMap[name] = rt
typeToName[rt] = name
}
// TypeOf returns the type of an object.
func TypeOf(o Object) (string, error) {
rt := reflect.TypeOf(o)
if rt.Kind() == reflect.Ptr {
rt = rt.Elem()
}
name, ok := typeToName[rt]
if !ok {
return "", fmt.Errorf("unsupported schema type: %T", o)
}
return name, nil
}
// MustTypeOf is the same as TypeOf but panics on error.
func MustTypeOf(o Object) string {
typ, err := TypeOf(o)
if err != nil {
panic(err)
}
return typ
}
// NewType creates a schema object with a specified type.
// The type should be registered with RegisterName.
func NewType(typ string) (Object, error) {
rt, ok := typesMap[typ]
if !ok {
return nil, fmt.Errorf("unsupported schema type: %q", typ)
}
return reflect.New(rt).Interface().(Object), nil
}
// Encode writes a schema blob to w.
func Encode(w io.Writer, o Object) error {
typ, err := TypeOf(o)
if err != nil {
return err
}
buf := new(bytes.Buffer)
buf.WriteString(magic + ` "`)
buf.WriteString(typ)
buf.WriteString(`"`)
i := buf.Len()
enc := json.NewEncoder(buf)
enc.SetIndent("", tab)
enc.SetEscapeHTML(false)
if err := enc.Encode(o); err != nil {
return fmt.Errorf("failed to encode %T: %v", o, err)
}
p := buf.Bytes()
p[i] = ','
_, err = w.Write(p)
return err
}
// IsSchema checks if the buffer is likely to contain an object with a CAS schema.
// The buffer should be at least of MagicSize.
func IsSchema(p []byte) bool {
if len(p) < MagicSize {
return false
}
return string(p[:MagicSize]) == magic
}
func checkSchema(r io.Reader) (io.Reader, error) {
m := make([]byte, MagicSize)
_, err := io.ReadFull(r, m)
if err == io.EOF || err == io.ErrUnexpectedEOF {
return nil, ErrNotSchema
} else if err != nil {
return nil, fmt.Errorf("cannot check schema object: %v", err)
}
if !IsSchema(m) {
return nil, ErrNotSchema
}
return io.MultiReader(bytes.NewReader(m), r), nil
}
// Decode decodes a schema object from a stream. It will strictly validate formatting of the content.
func Decode(r io.Reader) (Object, error) {
var err error
r, err = checkSchema(r)
if err != nil {
return nil, err
}
return DecodeJSON(r)
}
// DecodeJSON decodes a JSON config. It will not validate JSON formatting as Decode does.
func DecodeJSON(r io.Reader) (Object, error) {
obj, err := decode(r)
if err != nil {
return nil, fmt.Errorf("cannot decode schema object: %v", err)
}
return obj, nil
}
func decode(r io.Reader) (Object, error) {
typ, data, err := decodeType(r)
if err != nil {
return nil, err
}
obj, err := NewType(typ)
if err != nil {
return nil, err
}
if err := json.Unmarshal(data, obj); err != nil {
return nil, err
}
return obj, nil
}
func decodeType(r io.Reader) (string, []byte, error) {
r = io.LimitReader(r, maxSize)
data, err := ioutil.ReadAll(r)
if err != nil {
return "", nil, err
} else if len(data) == maxSize {
return "", nil, fmt.Errorf("schema object is too large")
}
var h struct {
Type string `json:"@type"`
}
if err := json.Unmarshal(data, &h); err != nil {
return "", nil, err
}
return h.Type, data, nil
}
// DecodeType decodes the type of an object from the reader. Reader will not be usable after the call.
// See PeekType to reserve a reader in a usable state.
func DecodeType(r io.Reader) (string, error) {
var err error
r, err = checkSchema(r)
if err != nil {
return "", err
}
typ, _, err := decodeType(r)
return typ, err
}
// PeekType reads a portion of data from r, reports the type of the schema blob that it describes,
// and returns a new restored reader.
func PeekType(r io.Reader) (io.Reader, string, error) {
var err error
r, err = checkSchema(r)
if err != nil {
return r, "", err
}
typ, data, err := decodeType(r)
return io.MultiReader(bytes.NewReader(data), r), typ, err
}