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

[Mateusz Loskot]

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   // Problem #1: Overview's connection no longer valid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.

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