chore: support ETag for providers

constant/provider/hash.go

@@ -0,0 +1,29 @@
+package provider
+
+import (
+	"bytes"
+	"crypto/md5"
+)
+
+type HashType [md5.Size]byte // MD5
+
+func MakeHash(data []byte) HashType {
+	return md5.Sum(data)
+}
+
+func (h HashType) Equal(hash HashType) bool {
+	return h == hash
+}
+
+func (h HashType) EqualBytes(hashBytes []byte) bool {
+	return bytes.Equal(hashBytes, h[:])
+}
+
+func (h HashType) Bytes() []byte {
+	return h[:]
+}
+
+func (h HashType) IsValid() bool {
+	var zero HashType
+	return h != zero
+}

constant/provider/interface.go

@@ -32,7 +32,7 @@ func (v VehicleType) String() string {
 }
 
 type Vehicle interface {
-	Read(ctx context.Context) ([]byte, error)
+	Read(ctx context.Context, oldHash HashType) (buf []byte, hash HashType, err error)
 	Path() string
 	Url() string
 	Proxy() string