FreeTDS Development Group

[Joshua Lang <joshua.h.lang AT gmail.com>]

That seems reasonable. Would you like a pull request?

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
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