FreeTDS Development Group

[Frediano Ziglio <freddy77 AT gmail.com>]

2014-10-30 17:56 GMT+00:00 Tolga Ceylan <tolga.ceylan AT gmail.com>:

> I mostly agree with this. I think app level timeouts is easy to add
> since callbacks are firing
> and the app code can keep track of the overall time there.
>
> However, 'query_timeout' is misleading. This really is a
> 'socket_activity_timeout'
> since it's based on arriving bits of data from the socket.
>
> Let say 'query_timeout' is 300 secs. If the TDS packet can be
> assembled after 5 calls to socket
> read, then this could get blown up to 5*300 secs. This means even if
> we rename 'query_timeout', it's not
> really meaningful.
>
>
yes, probably query_timeout is quite misleading.


> I also see that the TDS header (8 bytes) is expected to arrive in one
> read. If the header
> arrives in more than one read, then the connection gets closed. This
> is a bug. TCP code
> needs to handle tiny read cases even if this is rare.
>
>
I don't think so. The only case tds_goodread returns < 8 is when user
requested to close connection or on a socket error. Never for less bytes
readed. And if crypted all packets are crypted so is impossible it does not
return at least 8 bytes.


> I think 'query_timeout' can be renamed to 'tds packet timeout.'
> Instead of tds_select(), the higher
> level goodread() function can keep track of this timer and decide if
> there is a packet timeout or not.
>
> I can put together a patch if you like.
>
> Cheers,
> Tolga
>
>
Would be useful. I didn't actually understand what you want to achieve
however. A kind of full packet timeout? Server tends to send packet
together so you would catch some network delay or problem. If you remove
the timeout from tds_select how can it return if server does not send data?

Frediano


> On Thu, Oct 30, 2014 at 3:02 AM, Frediano Ziglio <freddy77 AT gmail.com>
> wrote:
> > 2014-10-30 4:10 GMT+00:00 Tolga Ceylan <tolga.ceylan AT gmail.com>:
> >
> >> Hi All,
> >>
> >> Recently, I've stumbled on a potential issue with the timeout handling
> >> in ct_lib.
> >>
> >> Given these two cases of SQL executing;
> >>
> >> DECLARE @counter int
> >> WHILE (@counter < 99999999)
> >> BEGIN
> >>     PRINT @counter
> >>     SET @counter = @counter + 1
> >> END
> >>
> >> versus;
> >>
> >> DECLARE @counter int
> >> WHILE (@counter < 99999999)
> >> BEGIN
> >>     SET @counter = @counter + 1
> >> END
> >>
> >> (only difference is print or no-print basically)
> >>
> >> seems to affect timeout handling since this trips the ::select() (aka
> >> tds_select) based query timeout.
> >>
> >> In other words, continuously feeding such server messages back to the
> >> client bypasses
> >> the query timeout. The issue is tricky because there has to be enough
> >> 'print' executions
> >> to actually trigger this. (Probably due to Nagle or other buffering.)
> >>
> >> Has anybody seen this before? Any work around for it(*) ?
> >>
> >> (*) the fix probably is to consider beginning/end of the actual
> >> queries as opposed to blocking on select
> >> with query timeout.
> >>
> >> I'm using 0.82 on linux centos 6.5 although I couldn't see changes
> >> related to this in 0.91.
> >>
> >> Cheers,
> >> Tolga
> >>
> >
> >
> > Hi,
> >   It's not a bug, it's a feature :)
> >
> > Really, this is the intention. Also considering that you should handle
> > results (print IS a result) the library should return so your timeout
> > should be managed by application.
> >
> > Frediano
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