fix: return PortError for read timeouts in Unix and Windows serial ports

serial.go

@@ -165,6 +165,8 @@ const (
 	PortClosed
 	// FunctionNotImplemented the requested function is not implemented
 	FunctionNotImplemented
+	// ReadTimeout the read operation timed out
+	ReadTimeout
 )
 
 // EncodedErrorString returns a string explaining the error code
@@ -194,6 +196,8 @@ func (e PortError) EncodedErrorString() string {
 		return "Port has been closed"
 	case FunctionNotImplemented:
 		return "Function not implemented"
+	case ReadTimeout:
+		return "Read operation timed out"
 	default:
 		return "Other error"
 	}

serial_unix.go

@@ -90,7 +90,7 @@ func (port *unixPort) Read(p []byte) (int, error) {
 		}
 		if !res.IsReadable(port.handle) {
 			// Timeout happened
-			return 0, nil
+			return 0, &PortError{code: ReadTimeout}
 		}
 		n, err := unix.Read(port.handle, p)
 		if err == unix.EINTR {

serial_windows.go

@@ -108,7 +108,7 @@ func (port *windowsPort) Read(p []byte) (int, error) {
 		hasTimeout := port.hasTimeout
 		port.mu.Unlock()
 		if hasTimeout {
-			return 0, nil
+			return 0, &PortError{code: ReadTimeout}
 		}
 	}
 }