FreeTDS Development Group

[Frediano Ziglio <freddy77 AT gmail.com>]

There are multiple reasons why read our other functions could return EAGAIN
or similar. The issue here it that poll returns 1 so it does not set errno
but tds_select returns - 1 as there is an error in the socket. If errno was
already EAGAIN loop can happen.
On 18 Mar 2016 06:38, "Joshua Lang" <joshua.h.lang AT gmail.com> wrote:

> Ugh. Well I don't have a Windows host setup currently for testing...
> Also, it does seem strange that the code loops if the returned error is
> EWOULDBLOCK or EAGAIN. That should never happen in a read after the select
> (poll) has successfully returned the file descriptor is ready for reading.
>
> On Thu, Mar 17, 2016 at 2:24 PM, Frediano Ziglio <freddy77 AT gmail.com>
> wrote:
>
> > On 17 Mar 2016 21:21, "Joshua Lang" <joshua.h.lang AT gmail.com> wrote:
> > >
> > > That seems reasonable. Would you like a pull request?
> > >
> >
> > You can. However I think on Windows you have to call a function to set
> the
> > error and I don't know which constant would be better. On Windows these
> > kind of error (socket ones) have no relation with c errno
> >
> > Frediano
> >
> > > On Thu, Mar 17, 2016 at 12:55 PM, Frediano Ziglio <freddy77 AT gmail.com>
> > > wrote:
> > >
> > > > 2016-03-16 19:23 GMT+00:00 Joshua Lang <joshua.h.lang AT gmail.com>:
> > > > > FreeTDS version: 0.95.79
> > > > > OS: Debian 7.9 (wheezy)
> > > > >
> > > > > Here's the scenario:
> > > > >
> > > > > Application has a long-running (pooled) connection to a database.
> > > > > The database is restarted.
> > > > > The application then consumes as much of one CPU as it can.
> > > > >
> > > > > Unfortunately I have yet to find a simple repro for this issue
> > locally,
> > > > but
> > > > > here is what I've found debugging the live application:
> > > > >
> > > > > strace shows a constant stream of the following call:
> > > > >
> > > > > poll([{fd=150, events=POLLOUT}, {fd=98, events=POLLIN}], 2, 1000)
> = 1
> > > > > ([{fd=150, revents=POLLOUT|POLLERR|POLLHUP}])
> > > > >
> > > > > The (relevant) stack trace from gdb:
> > > > >
> > > > > (gdb) bt
> > > > > #0  0x00007f1fae4d16b3 in *__GI___poll (fds=<optimized out>,
> > > > > nfds=<optimized out>, timeout=1000)
> > > > >     at ../sysdeps/unix/sysv/linux/poll.c:87
> > > > > #1  0x00007f1fa50ff883 in tds_select () from
> > > > > /opt/zillow/services/mortgage-api-external/zpr/lib/libsybdb.so.5
> > > > > #2  0x00007f1fa50ffe70 in tds_goodwrite () from
> > > > > /opt/zillow/services/mortgage-api-external/zpr/lib/libsybdb.so.5
> > > > > #3  0x00007f1fa5100062 in tds_connection_write () from
> > > > > /opt/zillow/services/mortgage-api-external/zpr/lib/libsybdb.so.5
> > > > > #4  0x00007f1fa51046f7 in tds_put_cancel () from
> > > > > /opt/zillow/services/mortgage-api-external/zpr/lib/libsybdb.so.5
> > > > > #5  0x00007f1fa50f574b in tds_send_cancel () from
> > > > > /opt/zillow/services/mortgage-api-external/zpr/lib/libsybdb.so.5
> > > > > #6  0x00007f1fa50c3560 in dbcancel () from
> > > > > /opt/zillow/services/mortgage-api-external/zpr/lib/libsybdb.so.5
> > > > >
> > > > >
> > > > > I've stepped through this with gdb and here are the results:
> > > > > 1. tds_select is called, returning -1 (verified in gdb)
> > > > > 2. errno returns to 11 (EAGAIN) (verified in gdb, by stepping
> through
> > the
> > > > > c-lib function call to return it)
> > > > > 3. TDSSOCK_WOULDBLOCK therefore causes the loop to repeat:
> > > > > https://github.com/FreeTDS/freetds/blob/master/src/tds/net.c#L803
> > and
> > > > we're
> > > > > back to step 1 and an infinite loop
> > > > >
> > > > > The TDSSOCK_WOULDBLOCK seems strange to me, and the comment after
> it
> > > > > agrees. I wouldn't expect the socket read to block if the poll call
> > just
> > > > > indicated it was ready for read.
> > > >
> > > >
> > > > I think the problem is in tds_select. After poll returns 1 we check
> > > > for error and return correctly -1 as failure however errno is not set
> > > > so it's still the old value. I would try to set to something like
> > > > EPIPE before returning -1.
> > > >
> > > > Frediano
> > > > _______________________________________________
> > > > FreeTDS mailing list
> > > > FreeTDS AT lists.ibiblio.org
> > > > http://lists.ibiblio.org/mailman/listinfo/freetds
> > > >
> > > _______________________________________________
> > > FreeTDS mailing list
> > > FreeTDS AT lists.ibiblio.org
> > > http://lists.ibiblio.org/mailman/listinfo/freetds
> > _______________________________________________
> > FreeTDS mailing list
> > FreeTDS AT lists.ibiblio.org
> > http://lists.ibiblio.org/mailman/listinfo/freetds
> >
> _______________________________________________
> FreeTDS mailing list
> FreeTDS AT lists.ibiblio.org
> http://lists.ibiblio.org/mailman/listinfo/freetds
>