WiFiClient improperly treats zero data available for read as an error

[MHotchin]

In "arduino-esp32/libraries/WiFi/src/WiFiClient.cpp".

If the client has run out of data that has been received, it will return an error, instead of returning 0 (indicating no data available right now).

Starting line 106:

    int read(uint8_t * dst, size_t len){
        if(!dst || !len || (_pos == _fill && !fillBuffer())){
            return -1;

Change to:

    int read(uint8_t * dst, size_t len){
        if(!dst || !len || (_pos == _fill && !fillBuffer())){
            return _failed ? -1: 0;

[MHotchin]

This appears to be the cause of #4390 as well.

[MHotchin]

Add to fix - function 'read() needs to handle the zero data available case as well. Found by using readBytes(), which calls the single byte version of read().

int WiFiClient::read()
{
    uint8_t data = 0;
    int res = read(&data, 1);
    if(res < 0) {
        return res;
    }
    if (res == 0) {
        return -1;
    }
    return data;
}

[MarkusAD]

@MHotchin you should submit your changes as a pull request so that they can be reviewed and merged into the current code base.

[MHotchin]

PR #4448 submitted.