[postgis-devel] [gdal-dev] Re: [SoC] GDAL WKTRaster weekly report #9

[Jorge Arévalo]

2009/8/2 Mateusz Loskot <mateusz at loskot.net>:
> Jorge Arévalo wrote:
>> Hello,
>>
>> 2009/8/2 Mateusz Loskot <mateusz at loskot.net>:
>>> Jorge Arévalo wrote:
>>>> When I create an overview (a children dataset), its properties are
>>>> the same properties of the general dataset (his parent), apart from
>>>> pixel size, table name and column_name, of course. The problem
>>>> appears when sharing the PGconn object.
>>>>
>>>> My code looks like this:
>>>>
>>>> poDS->overview[i]->hPGconn = poDS->hPGconn
>>>>
>>>> If each dataset's destructor (from the general one and from the
>>>> overviews) calls PQfinish over the same object, I get a segmentation
>>>> fault after the first time, of course. So, I have 2 options: - Detect
>>>> when a connection object has been freed, and perform only one call.
>>>> How should I do this?
>>> If I understand it correctly, the problem can be summarised in some
>>> kind of pseudo-code as follows:
>>>
>>> struct Overview
>>> {
>>>   PGconn* c;
>>>   Overview(PGconn* c) : c(c) {}
>>>   ~Overview() { PQfinish(c); }
>>>
>>> };
>>>
>>> struct Dataset
>>> {
>>>   PGconn* c;
>>>   Dataset(char const* connstr) { /*connect to pgsql*/ }
>>>   ~Dataset() {  PQfinish(c) ; c = 0; }
>>>   Overview get_overview() { return Overview(c); }
>>> };
>>>
>>>
>>> {
>>>   Overview ov;
>>>   {
>>>      Dataset d(...); // connect
>>>      ov = d.get_overview(); // side-effect: share connection
>>>   }  // Dataset dctor calls PQfinish
>>>
>>>   // Problem #1: Overview's connection no longer valid
>>> } // Problem #2: Overview dctor tries to close and fails
>>>
>>> Note: Overview here may be a subdataset, no difference between the two
>>> for the problem analysis.
>>
>> Yes, the problem looks like that.
>>
>>> Assuming this example reproduces the problem correctly, then
>>> there is no way to detect in Overview's dctor that
>>> connection is still valid.
>>
>> Ok, but maybe the Even's idea of a boolean var set to TRUE in dataset
>> and to FALSE in overviews could be a good way to simulate this
>> situation.
>
> Sure, it is but as long as you're confident that you
> can determine who's the base - master dataset/overview/subdataset.
>
> Clearly I lack of details and I've not read your code.
> Thus, I may have been able to see the issue only partially,
> so can't see & suggest best option.

Anyway, things are more clear now, many thanks. Your analysis is valid
for my situation.

Best regards,
Jorge

>
> Best regards,
> --
> Mateusz Loskot, http://mateusz.loskot.net
> Charter Member of OSGeo, http://osgeo.org
>