Workaround for Ublox G450 firmware issue

[NeilMacMullen]

Description

When using a TCP socket, the Ublox G450 appears to suffer from a firmware bug where, if more bytes are requested with USORD than are available in the internal modem buffer, and then more data is received over the air, an unsolicited response can be generated. Eg.

---> Client requests 20K bytes from remote server <--- +UUSORD 1500 ---> AT+USORD 1000 <--- +USORD 1000 ... <---- +UUSORD 500 ---> AT+USORD 1000 <--- +USORD 500 ... <---- more data received from network. <--- +USORD 1000.... //'unsolicited' response <--- ++UUSORD ...

The Ublox AT command manual says

If the number of data bytes requested to be read from the buffer is bigger than the number of bytes stored in the buffer only the available amount of data bytes will be read.

and the stack has been written with this assumption but the following code appears to be a safer implementation for UBLOX_AT_CellularStack::socket_recvfrom_impl in UBLOX_AT_CellularStack.cpp

 } else if (socket->proto == NSAPI_TCP) {
        while (success && (size > 0)) {

            read_blk = MIN(read_blk,UBLOX_MAX_PACKET_SIZE); //don't request more than +USORD can handle in a single chunk
            read_blk = MIN(read_blk,size); //don't request more than are remaining for socket-read
            read_blk = MIN(read_blk,socket->pending_bytes); //never request more than are actually in modem buffer
            if (socket->pending_bytes > 0) {
                _at.cmd_start("AT+USORD=");
                _at.write_int(socket->id);
                _at.write_int(read_blk);
                _at.cmd_stop();

Issue request type

[ ] Question
[ ] Enhancement
[X Bug

[0xc0170]

cc @ARMmbed/mbed-os-wan @ARMmbed/team-ublox

[0xc0170]

@ARMmbed/team-ublox Please review

[fahimalavi]

Looks fine to me

[0xc0170]

@NeilMacMullen Please send a pull request to fix the issue

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers. Internal Jira reference: https://jira.arm.com/browse/IOTOSM-2280

[adbridge]

@NeilMacMullen Please send a pull request to fix the issue

As there has been no response to this I am closing this issue.