When call(UART, read, 2000) times out, subsequent reads will always be {error, ealready}

[robinchew]

port:call(UART, read, 2000) finishes timing out, then a subsequent valid write expecting a response will result with a read that always return a {error, ealready} .

Example:

Length= 21,

UART = open_port(
    {spawn, "uart"},
    [
        {peripheral, <<"UART2">>},
        {rx, 16},
        {tx, 17},
        %   {controlling_process, self()},
        %   {stop_bits, 2},
        %   {active, false},
        %   {flow_control, hardware},
        %   {timeout, 4000}
        {speed, 115200}
    ]),

Req3 = uart_sensor_request(3, 0, Length),
io:format("Req3 = ~p~n", [Req3]), 
uart:write(UART, Req3),                            % Writing a valid request
io:format("read3 ~p~n", [call(UART, read, 2000)]), % Expecting a response

Req4 = uart_sensor_request(4, 0, Length),
io:format("Req4 = ~p~n", [Req4]),
uart:write(UART, Req4),                            % Writing an INVALID request
io:format("read4 ~p~n", [call(UART, read, 2000)]), % Expecting NO response and expecting read to block until timeout

uart:write(UART, Req3),                                  % Writing a valid request again
io:format("read3 again ~p~n", [call(UART, read, 2000)]), % Expect a response again BUT errors

Output:

Req3 = [3,4,0,0,0,21,48,39]
read3 {ok,<<3,4,42,0,58,0,9,0,7,0,23,0,5,7,231,8,155,117,175,71,98,136,33,54,176,11,173,11,176,2,117,13,221,1,145,25,66,1,244,0,20,190,31,0,1,127,58>>}
Req4 = [4,4,0,0,0,21,49,144]
read4 {error,timeout}
read3 again {error,ealready} <- Not expecting this!