internal/restic: Fix ID.UnmarshalJSON, ParseID

ID.UnmarshalJSON accepted non-JSON input with ' as the string delimiter.
Also, the error message for non-hex input was less informative than it
could be and it performed too many checks.

Changed ParseID to keep the error messages consistent.

internal/restic/id.go

@@ -6,8 +6,6 @@ import (
 	"fmt"
 	"io"
 
-	"github.com/restic/restic/internal/errors"
-
 	"github.com/minio/sha256-simd"
 )
 
@@ -24,14 +22,13 @@ type ID [idSize]byte
 
 // ParseID converts the given string to an ID.
 func ParseID(s string) (ID, error) {
-	b, err := hex.DecodeString(s)
-
-	if err != nil {
-		return ID{}, errors.Wrap(err, "hex.DecodeString")
+	if len(s) != hex.EncodedLen(idSize) {
+		return ID{}, fmt.Errorf("invalid length for ID: %q", s)
 	}
 
-	if len(b) != idSize {
-		return ID{}, errors.New("invalid length for hash")
+	b, err := hex.DecodeString(s)
+	if err != nil {
+		return ID{}, fmt.Errorf("invalid ID: %s", err)
 	}
 
 	id := ID{}
@@ -96,34 +93,21 @@ func (id ID) MarshalJSON() ([]byte, error) {
 // UnmarshalJSON parses the JSON-encoded data and stores the result in id.
 func (id *ID) UnmarshalJSON(b []byte) error {
 	// check string length
-	if len(b) < 2 {
-		return fmt.Errorf("invalid ID: %q", b)
+	if len(b) != len(`""`)+hex.EncodedLen(idSize) {
+		return fmt.Errorf("invalid length for ID: %q", b)
 	}
 
-	if len(b)%2 != 0 {
-		return fmt.Errorf("invalid ID length: %q", b)
-	}
-
-	// check string delimiters
-	if b[0] != '"' && b[0] != '\'' {
+	if b[0] != '"' {
 		return fmt.Errorf("invalid start of string: %q", b[0])
 	}
 
-	last := len(b) - 1
-	if b[0] != b[last] {
-		return fmt.Errorf("starting string delimiter (%q) does not match end (%q)", b[0], b[last])
-	}
-
-	// strip JSON string delimiters
-	b = b[1:last]
-
-	if len(b) != 2*len(id) {
-		return fmt.Errorf("invalid length for ID")
-	}
+	// Strip JSON string delimiters. The json.Unmarshaler contract says we get
+	// a valid JSON value, so we don't need to check that b[len(b)-1] == '"'.
+	b = b[1 : len(b)-1]
 
 	_, err := hex.Decode(id[:], b)
 	if err != nil {
-		return errors.Wrap(err, "hex.Decode")
+		return fmt.Errorf("invalid ID: %s", err)
 	}
 
 	return nil