Re: timeout of pg_receivexlog --status-interval

On Wed, Jul 16, 2014 at 2:29 AM, Sawada Masahiko <sawada(dot)mshk(at)gmail(dot)com> wrote:
> On Tue, Jul 15, 2014 at 7:38 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>> On Thu, Jul 10, 2014 at 11:10 PM, Sawada Masahiko <sawada(dot)mshk(at)gmail(dot)com> wrote:
>>> Hi,
>>>
>>> At 1047 line of receivelog.c:CopyStreamPoll(), we set NULL to
>>> timeoutptr variable.
>>> if the value of timeoutprt is set NULL then the process will wait
>>> until can read socket using by select() function as following.
>>>
>>> if (timeout_ms < 0)
>>> timeoutptr = NULL;
>>> else
>>> {
>>> timeout.tv_sec = timeout_ms / 1000L;
>>> timeout.tv_usec = (timeout_ms % 1000L) * 1000L;
>>> timeoutptr = &timeout;
>>> }
>>>
>>> ret = select(PQsocket(conn) + 1, &input_mask, NULL, NULL, timeoutptr);
>>>
>>> But the 1047 line of receivelog.c is never executed because the value
>>> of timeout_ms is NOT allowed less than 0 at CopyStreamReceive which is
>>> only one function calls CopyStreamPoll().
>>> The currently code, if we specify -s to 0 then CopyStreamPoll()
>>> function is never called.
>>> And the pg_receivexlog will be execute PQgetCopyData() and failed, in
>>> succession.
>>
>> Thanks for reporting this! Yep, this is a problem.
>>
>>> I think that it is contradiction, and should execute select() function
>>> with NULL of fourth argument.
>>> the attached patch allows to execute select() with NULL, i.g.,
>>> pg_receivexlog.c will wait until can read socket without timeout, if
>>> -s is specified to 0.
>>
>> Your patch changed the code so that CopyStreamPoll is called even
>> when the timeout is 0. I don't agree with this change because the
>> timeout=0 basically means that the caller doesn't request to block and
>> there is no need to call CopyStreamPoll in this case. So I'm thinking to
>> apply the attached patch. Thought?
>>
>
> Thank you for the response.
> I think this is better.
>
> One another point about select() function, I think that they are same
> behavior between the fifth argument is NULL and 0(i.g. 0 sec).

No, per manpage of select(2), select(2) with timeout=0 behaves differently
from select(2) with timeout=NULL. So I don't think your suggestion is
acceptable...

Regards,

--
Fujii Masao