Re: [tcf-dev] Problem with ZeroCopy and flushing in TCF agent

["Tarassov, Eugene" <eugene.tarassov@xxxxxxxxxxxxx>]

Hi Patrick,
 
You should not need to call flush_stream().
json_splice_binary_offset() calls tcp_splice_block_stream(), which contains a code to flush the channel before doing splice into the socket:
 
    /* We need to flush the buffer then send our data */
    tcp_flush_with_flags(c, MSG_MORE);
#if ENABLE_OutputQueue
    while (!output_queue_is_empty(&c->out_queue)) {
        cancel_event(done_write_request, &c->wr_req, 1);
        done_write_request(&c->wr_req);
    }
#endif

The code looks correct to me.
Are you suggesting that this code might not work as expected?

Regards,
Eugene


________________________________

From: tcf-dev-bounces@xxxxxxxxxxx [mailto:tcf-dev-bounces@xxxxxxxxxxx] On Behalf Of Patrick Tasse
Sent: Thursday, June 16, 2011 11:07 AM
To: tcf-dev@xxxxxxxxxxx
Subject: [tcf-dev] Problem with ZeroCopy and flushing in TCF agent


Hi,

I am working on an agent plugin that requires the ZeroCopy service to be enabled in the TCF agent.

It writes an Event message to the channel's output stream using write_stream() for some message parameters and json_splice_binary_offset() for the binary data in the message. This method then performs a zero-copy splice.

The calling thread must be assured that the message is sent and that the zero-copied binary data is no longer needed before continuing. In previous code (which was based on an older version of TCF), it was calling the flush_stream() method, but this method does not exist anymore (instead, if I understand correctly, Event messages are automatically flushed once we write MARKER_EOM to the output stream).

However, the flushing mechanism in channel_tcp.c is to post a tcp_flush_event to the channel's event thread. The write_stream(out, MARKER_EOM) method can return before the flushing is even started. Therefore, the zero-copied binary data can be overwritten before the message is sent, and the TCF channel on the other side receives a corrupt message.

Is there a mechanism that an agent plugin could use to block until, or be notified that the flushing has completed?

If there is none, I could help to write and test a patch for it. But I would need advice on how it should be designed.

Thanks,
Patrick

(by the way the link to the tcf-dev mailing list is not working on this page: http://www.eclipse.org/projects/project_summary.php?projectid=tools.cdt.tcf)