[Qgis-developer] Connection reusing in postgres provider

[Marco Hugentobler]

Hi Jürgen,

Sorry to spam you with another mail today.

There is a problem with the reusage of connections in postgres provider (via 
static member variable QgsPostgresProvider::connections). Two postgres layers 
that share the same connection might influence each others binary cursors. As 
a binary cursors is valid inside a transaction, calling "end work" on a 
connection will also influence the cursor of the second layer.

Independence of the binary cursors is particularly important in geoprocessing, 
where situations like in the following Pseudocode happen very often:

QgsVectorDataProvider* pa;
QgsVectorDataProvider*pb;

pa->select
pb->select

while(pb->getNextFeature)
{
	QgsRect boundingBox = feature.boundingBox;
	pa->select(boundingBox)
	while(pa->getNextFeature)
	{

	}
}

For a test, I changed the static connections variable to a nonstatic one (so 
disabled the connection sharing) and the problems in the geoprocessing plugin 
went away.

Do you think connection sharing in postgres provider should be disabled?
Or is there a possibility to prevent the binary cursor problem with keeping 
connection sharing?

Regards,
Marco

-- 
Dr. Marco Hugentobler
Institute of Cartography
ETH Zurich
Technical Advisor QGIS Project Steering Committee