[#122] Add versioning cache

Signed-off-by: Denis Kirillov <denis@nspcc.ru>

api/layer/detector.go

@@ -3,24 +3,56 @@ package layer
 import (
 	"io"
 	"net/http"
-	"sync"
 )
 
-type detector struct {
-	io.Reader
-	sync.Once
+type (
+	detector struct {
+		io.Reader
+		err  error
+		data []byte
+	}
+	errReader struct {
+		data   []byte
+		err    error
+		offset int
+	}
+)
 
-	contentType string
+const contentTypeDetectSize = 512
+
+func newReader(data []byte, err error) *errReader {
+	return &errReader{data: data, err: err}
 }
 
-func newDetector(r io.Reader) *detector {
-	return &detector{Reader: r}
+func (r *errReader) Read(b []byte) (int, error) {
+	if r.offset >= len(r.data) {
+		return 0, io.EOF
+	}
+	n := copy(b, r.data[r.offset:])
+	r.offset += n
+	if r.offset >= len(r.data) {
+		return n, r.err
+	}
+	return n, nil
 }
 
-func (d *detector) Read(data []byte) (int, error) {
-	d.Do(func() {
-		d.contentType = http.DetectContentType(data)
-	})
-
-	return d.Reader.Read(data)
+func newDetector(reader io.Reader) *detector {
+	return &detector{
+		data:   make([]byte, contentTypeDetectSize),
+		Reader: reader,
+	}
+}
+
+func (d *detector) Detect() (string, error) {
+	n, err := d.Reader.Read(d.data)
+	if err != nil && err != io.EOF {
+		d.err = err
+		return "", err
+	}
+	d.data = d.data[:n]
+	return http.DetectContentType(d.data), nil
+}
+
+func (d *detector) MultiReader() io.Reader {
+	return io.MultiReader(newReader(d.data, d.err), d.Reader)
 }