WTSVirtualChannelRead sometimes drops packets when using timeouts

Question

WTSVirtualChannelRead sometimes drops packets when using timeouts

Michael Lombardi 1

I am using WTSVirtualChannelRead and have noticed some incorrect behavior that occurs when using timeouts. I believe that when a packet is sent from the client just as the function is timing out, GetLastError returns ERROR_IO_INCOMPLETE and pBytesRead is 0 as usual during a timeout, however the packet is dropped as calling WTSVirtualChannelRead again will read the next packet instead.

Using WTSVirtualChannelQuery to replace the calls to WTSVirtualChannelRead and WTSVirtualChannelWrite with ReadFile and WriteFile seems to fix the issue, but is a much more complicated process. Is there any way to get WTSVirtualChannelRead to work correctly instead?

Drake Wu - MSFT 996 Reputation points

2021-02-22T08:55:23.133+00:00

Hi @Michael Lombardi Are you using WTSVirtualChannelRead in multiple threads? Could you please show the code snippets you are using?
Michael Lombardi 1 Reputation point

2021-02-26T15:11:32.13+00:00

Example code:

void RunTest(HANDLE hChannel, PCHAR header)
{
DWORD bytesWritten = 0;
WTSVirtualChannelWrite(hChannel, header, 16, &bytesWritten);
BYTE pbHeader = new (std::nothrow) BYTE[1608];
Receive(hChannel, pbHeader, 16, 100);
BYTE pbFollowData = new (std::nothrow) BYTE[1608];
Receive(hChannel, pbFollowData, 700, 5000);
delete[] pbFollowData;
delete[] pbHeader;
}

void Receive(HANDLE hChannel, BYTE pData, DWORD dwDataLength, DWORD dwTimeout)
{
BYTE buffer[CHANNEL_PDU_LENGTH];
DWORD dwTotalBytesRead = 0;
while ((dwTotalBytesRead < dwDataLength))
{
DWORD dwBytesRead = 0;
if (::WTSVirtualChannelRead(hChannel, dwTimeout, (PCHAR)&buffer, CHANNEL_PDU_LENGTH, &dwBytesRead))
{
DWORD dwLastTotal = dwTotalBytesRead;
dwBytesRead -= sizeof(CHANNEL_PDU_HEADER);
dwTotalBytesRead += dwBytesRead;
BYTE pStartOfData = (BYTE*)&buffer + sizeof(CHANNEL_PDU_HEADER);
memcpy_s(pData+dwLastTotal, dwDataLength, pStartOfData, dwBytesRead);
}
}
}
Michael Lombardi 1 Reputation point

2021-02-26T15:18:23.08+00:00

I am not using WTSVirtualChannelRead in multiple threads. I've added some code snippets as a subcomment, though the formatting seems to have been altered. In my code snippet, I send a 16 byte header to the client, which sends a 16 byte header back, then a 700 byte payload. About once every 5000-20000 times, I never get the 16 byte header from the client and only get the 700 byte payload. It seems tied to the timeout, as it does not have the issue if the timeout is infinite. Also completely changing the code to use WTSVirtualChannelQuery, WriteFile, and ReadFile fixes it as well.

Your answer

Drake Wu - MSFT 996 Reputation points

2021-02-22T08:55:23.133+00:00

Hi @Michael Lombardi Are you using WTSVirtualChannelRead in multiple threads? Could you please show the code snippets you are using?
Michael Lombardi 1 Reputation point

2021-02-26T15:11:32.13+00:00

Example code:

void RunTest(HANDLE hChannel, PCHAR header)
{
DWORD bytesWritten = 0;
WTSVirtualChannelWrite(hChannel, header, 16, &bytesWritten);
BYTE pbHeader = new (std::nothrow) BYTE[1608];
Receive(hChannel, pbHeader, 16, 100);
BYTE pbFollowData = new (std::nothrow) BYTE[1608];
Receive(hChannel, pbFollowData, 700, 5000);
delete[] pbFollowData;
delete[] pbHeader;
}

void Receive(HANDLE hChannel, BYTE pData, DWORD dwDataLength, DWORD dwTimeout)
{
BYTE buffer[CHANNEL_PDU_LENGTH];
DWORD dwTotalBytesRead = 0;
while ((dwTotalBytesRead < dwDataLength))
{
DWORD dwBytesRead = 0;
if (::WTSVirtualChannelRead(hChannel, dwTimeout, (PCHAR)&buffer, CHANNEL_PDU_LENGTH, &dwBytesRead))
{
DWORD dwLastTotal = dwTotalBytesRead;
dwBytesRead -= sizeof(CHANNEL_PDU_HEADER);
dwTotalBytesRead += dwBytesRead;
BYTE pStartOfData = (BYTE*)&buffer + sizeof(CHANNEL_PDU_HEADER);
memcpy_s(pData+dwLastTotal, dwDataLength, pStartOfData, dwBytesRead);
}
}
}
Michael Lombardi 1 Reputation point

2021-02-26T15:18:23.08+00:00

I am not using WTSVirtualChannelRead in multiple threads. I've added some code snippets as a subcomment, though the formatting seems to have been altered. In my code snippet, I send a 16 byte header to the client, which sends a 16 byte header back, then a 700 byte payload. About once every 5000-20000 times, I never get the 16 byte header from the client and only get the 700 byte payload. It seems tied to the timeout, as it does not have the issue if the timeout is infinite. Also completely changing the code to use WTSVirtualChannelQuery, WriteFile, and ReadFile fixes it as well.

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WTSVirtualChannelRead sometimes drops packets when using timeouts

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