Re: FreeTDS and buffering on client side

Hello *,

thanks very much for all answers.

I can confirm that:

1. when I set query timeout by call to dbsettime() then after specified 
number of seconds dbnextrow() returns FAIL and error handler is called 
with 20004 "Read from server failed" (any subsequent call to db-lib 
function returns 20047 "DB-process is dead or not enabled"

2. when I do not set query timeout and I break TCP connection using 
"Close connection" in TCPView same as in 1 happens

3. when I set in registry in 
HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters "KeepAliveTime" 
(and restart PC) this value is taken as timeout

4. when I do not set registry value and wait 2 hours on Windows same as 
in 1 happens

Thanks again

-Laco.


> 2017-10-29 16:38 GMT+00:00 Peter Deacon<[email protected]>:
>> On Fri, 27 Oct 2017, LacaK wrote:
>>
>>> Same behavior also on Windows (current master) and on Linux (Ubuntu with
>>> installed libsybdb5 package) Once network cable is unpluged loop freezes and
>>> FAIL is not returned from dbnextrow()
>>> So I think, that it is reproducible case ...
>> Some systems depending on configuration when physical link is lost actively
>> close logical interface and associated sockets.  These systems should return
>> an error right away.  Some do nothing.
>>
>> The difference you are seeing in your testing after waiting longer than a
>> certain period is a result of server side of the session giving up and
>> shutting down the TCP session.  Under your test condition the client is
>> likely to have no idea this has occurred.  All it knows is more data may or
>> may not arrive at some undetermined point in the future.  With TCP a client
>> can find out in one of three ways:
>>
>> 1. Something is transmitted over TCP session such as request for more data
>> or app layer keep-alive - at which point lack of connectivity is detected
>> when transmission timeout expires.
>>
>> 2. A TCP layer keep-alive fires off and time outs.  On windows the default
>> keep-alive interval is 2 hours.
>>
>> 3. Local application timeout such as a configured query timeout?
>>
>> Otherwise I would expect the application to sit and wait forever for data
>> that may never arrive over a server/network/socket that may no longer exist.
>> On Windows you are likely looking at 2 hours maximum wait before anything
>> times out.
>>
>> On Windows try waiting 2 hours, using a query timeout or sysinternals
>> tcpview/process explorer to force close socket of database session from
>> client process and see if you get an error.
>>
>> regards,
>> Peter
>>
> You are right.
>
> Linux should have a total keep alive or 1 minute. Weird that is not
> set that way.
> I think for Windows is not set so yes, default is 2 hours.
> You can set query timeout of use interrupt.
>
> Frediano
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