restic: Make JSON unmarshal for ID more efficient

This commit reduces several allocations in UnmarshalJSON() by decoding
the hex string directly in a single step.

internal/restic/id.go

@@ -5,6 +5,7 @@ import (
 	"crypto/sha256"
 	"encoding/hex"
 	"encoding/json"
+	"fmt"
 	"io"
 
 	"github.com/restic/restic/internal/errors"
@@ -101,13 +102,33 @@ 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 {
-	var s string
-	err := json.Unmarshal(b, &s)
-	if err != nil {
-		return errors.Wrap(err, "Unmarshal")
+	// check string length
+	if len(b) < 2 {
+		return fmt.Errorf("invalid ID: %q", b)
 	}
 
-	_, err = hex.Decode(id[:], []byte(s))
+	if len(b)%2 != 0 {
+		return fmt.Errorf("invalid ID length: %q", b)
+	}
+
+	// check string delimiters
+	if b[0] != '"' && 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")
+	}
+
+	_, err := hex.Decode(id[:], b)
 	if err != nil {
 		return errors.Wrap(err, "hex.Decode")
 	}

internal/restic/id_test.go

@@ -51,10 +51,47 @@ func TestID(t *testing.T) {
 		var id3 ID
 		err = id3.UnmarshalJSON(buf)
 		if err != nil {
-			t.Fatal(err)
+			t.Fatalf("error for %q: %v", buf, err)
 		}
 		if !reflect.DeepEqual(id, id3) {
 			t.Error("ids are not equal")
 		}
 	}
 }
+
+func TestIDUnmarshal(t *testing.T) {
+	var tests = []struct {
+		s     string
+		valid bool
+	}{
+		{`"`, false},
+		{`""`, false},
+		{`'`, false},
+		{`"`, false},
+		{`"c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4"`, false},
+		{`"c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f"`, false},
+		{`"c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2"`, true},
+	}
+
+	wantID, err := ParseID("c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, test := range tests {
+		t.Run("", func(t *testing.T) {
+			id := &ID{}
+			err := id.UnmarshalJSON([]byte(test.s))
+			if test.valid && err != nil {
+				t.Fatal(err)
+			}
+			if !test.valid && err == nil {
+				t.Fatalf("want error for invalid value, got nil")
+			}
+
+			if test.valid && !id.Equal(wantID) {
+				t.Fatalf("wrong ID returned, want %s, got %s", wantID, id)
+			}
+		})
+	}
+}