[libvirt] [PATCH 06/38] virfdstream: Use messages instead of pipe

[Daniel P. Berrange]

On Thu, Apr 13, 2017 at 06:52:31PM +0200, Michal Privoznik wrote:
> On 04/13/2017 03:55 PM, Daniel P. Berrange wrote:
> > On Thu, Apr 13, 2017 at 03:31:14PM +0200, Michal Privoznik wrote:
> >> One big downside of using the pipe to transfer the data is that
> >> we can really transfer just bare data. No metadata can be carried
> >> through unless some formatted messages are introduced. That would
> >> be quite painful to achieve so let's use a message queue. It's
> >> fairly easy to exchange info between threads now that iohelper is
> >> no longer used.
> > 
> > I'm not seeing how this works correctly with the event loop.
> > 
> >> @@ -752,8 +1014,6 @@ virFDStreamOpenFileInternal(virStreamPtr st,
> >>      if ((st->flags & VIR_STREAM_NONBLOCK) &&
> >>          ((!S_ISCHR(sb.st_mode) &&
> >>            !S_ISFIFO(sb.st_mode)) || forceIOHelper)) {
> >> -        int fds[2] = { -1, -1 };
> >> -
> >>          if ((oflags & O_ACCMODE) == O_RDWR) {
> >>              virReportError(VIR_ERR_INTERNAL_ERROR,
> >>                             _("%s: Cannot request read and write flags together"),
> >> @@ -761,12 +1021,6 @@ virFDStreamOpenFileInternal(virStreamPtr st,
> >>              goto error;
> >>          }
> >>  
> >> -        if (pipe(fds) < 0) {
> >> -            virReportSystemError(errno, "%s",
> >> -                                 _("Unable to create pipe"));
> >> -            goto error;
> >> -        }
> > 
> > Here we previously created the pipe....
> > 
> >> @@ -775,18 +1029,14 @@ virFDStreamOpenFileInternal(virStreamPtr st,
> >>  
> >>          if ((oflags & O_ACCMODE) == O_RDONLY) {
> >>              threadData->fdin = fd;
> >> -            threadData->fdout = fds[1];
> >> -            if (VIR_STRDUP(threadData->fdinname, path) < 0 ||
> >> -                VIR_STRDUP(threadData->fdoutname, "pipe") < 0)
> >> +            threadData->fdout = -1;
> >> +            if (VIR_STRDUP(threadData->fdinname, path) < 0)
> >>                  goto error;
> >> -            fd = fds[0];
> > 
> > And here we set 'fd' to be the pipe
> > 
> >>          } else {
> >> -            threadData->fdin = fds[0];
> >> +            threadData->fdin = -1;
> >>              threadData->fdout = fd;
> >> -            if (VIR_STRDUP(threadData->fdinname, "pipe") < 0 ||
> >> -                VIR_STRDUP(threadData->fdoutname, path) < 0)
> >> +            if (VIR_STRDUP(threadData->fdoutname, path) < 0)
> >>                  goto error;
> >> -            fd = fds[1];
> > 
> > Likewise here
> > 
> >>          }
> >>      }
> > 
> > ...now here 'fd' is passed to virFDStreamOpenInternal() and is the thing
> > that the event loop watches are registered against by virFDStreamAddCallback
> > 
> > 
> > With this change 'fd' is the actual plain file the thread is reading to/from,
> > so the callbacks are being registered against the plain file, not the pipe.
> > 
> > poll/select on POSIX always reports plain files as readable/writable even
> > when they would block.  So with this change we're just going to busy loop
> > in the main event thread even when we'll block, which defeats the whole
> > purpose of having a iohelper and/or thread.
> 
> Oh, I've misunderstood what we've discussed on IRC then. The way I've
> understood it was that if an FD is set to nonblock mode and poll()
> claims there are some data available, subsequent read() might block. If
> that was the case we would be safe with this code. However, I didn't
> expect poll() to lie.

This code wouldn't be safe - anytime poll claims data available, we *must*
be able to read without blocking.

> Any link for further reading on this? I guess it's not only us who has
> to deal with this problem. Basically any application with poll() and
> disk read()/write() has to suffer from this.

Yes, that's correct - QEMU has the same issue for example - it is why there
is no 'file:' protocol for migration for example - it would block the QEMU
main loop.

> So what are our options here? Because I don't see any right now.

IIUC, you didn't want to use a pipe because you want to send structured
messages, not just plain data. If we just have a linked list of messages
there's nothing we can poll on, so we need to keep the pipe in use, but
find a way to get the special messages in the flow.

I think we could do a trick where we have two pipes in use, one for
monitoring the readability, and one for monitoring writability.


When the I/O thread has data on the queue ready for read by the main
thread, it can write a single byte to the read-monitor pipe.

When the I/O thread is ready to accept more data to write from the
main thread, it can write a single byte to the write-monitor pipe.

The main thread would monitor for POLLIN condition on both the
read-monitor pipe and write-monitor pipe.

BTW, we also need to make sure the I/O thread doesn't proactively
queue too much data on the message queue when reading it, in case
the main thread is being slow at consuming this read data and
sending it to the TCP client.

Regards,
Daniel
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