logtail: reduce PublicID.UnmarshalText from 2 allocs to 0

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/id.go

@@ -121,14 +121,17 @@ func (id PublicID) MarshalText() ([]byte, error) {
 }
 
 func (id *PublicID) UnmarshalText(s []byte) error {
-	b, err := hex.DecodeString(string(s))
-	if err != nil {
-		return fmt.Errorf("logtail.PublicID.UnmarshalText: %v", err)
+	if len(s) != len(id)*2 {
+		return fmt.Errorf("logtail.PublicID.UnmarshalText: invalid hex length: %d", len(s))
 	}
-	if len(b) != len(id) {
-		return fmt.Errorf("logtail.PublicID.UnmarshalText: invalid hex length: %d", len(b))
+	for i := range id {
+		a, ok1 := fromHexChar(s[i*2+0])
+		b, ok2 := fromHexChar(s[i*2+1])
+		if !ok1 || !ok2 {
+			return errors.New("invalid hex character")
+		}
+		id[i] = (a << 4) | b
 	}
-	copy(id[:], b)
 	return nil
 }
 

logtail/logtail_test.go

@@ -303,3 +303,26 @@ func TestParseAndRemoveLogLevel(t *testing.T) {
 		}
 	}
 }
+
+func TestPublicIDUnmarshalText(t *testing.T) {
+	const hexStr = "6c60a9e0e7af57170bb1347b2d477e4cbc27d4571a4923b21651456f931e3d55"
+	x := []byte(hexStr)
+
+	var id PublicID
+	if err := id.UnmarshalText(x); err != nil {
+		t.Fatal(err)
+	}
+	if id.String() != hexStr {
+		t.Errorf("String = %q; want %q", id.String(), hexStr)
+	}
+
+	n := int(testing.AllocsPerRun(1000, func() {
+		var id PublicID
+		if err := id.UnmarshalText(x); err != nil {
+			t.Fatal(err)
+		}
+	}))
+	if n != 0 {
+		t.Errorf("allocs = %v; want 0", n)
+	}
+}