[QGIS-Developer] Cannot load WMS capabilities from WMS provider in QGIS3 from python

[David Marteau]

Here is a test script that illustrate the problem a bit further: if we run createProvider() twice, the first time 
the getCapbabilities will fail with a timeout. The second time, it will succeed. 

If you run this script, clear the Qgis network cache between two invocations. 

===================8<==============================
import os
import logging

from qgis.core import QgsApplication, QgsMessageLog
from qgis.core import QgsProject, QgsProviderRegistry

logger = logging.getLogger()
logger.setLevel(logging.DEBUG)

# No Display
os.environ['QT_QPA_PLATFORM'] = 'offscreen'

qgis_application = QgsApplication([], False )
qgis_application.setPrefixPath('/usr/local', True)
qgis_application.initQgis()

# Log any messages
def writelogmessage(message, tag, level):
    arg = '{}: {}'.format( tag, message )
    if level == QgsMessageLog.WARNING:
        logger.warning(arg)
    elif level == QgsMessageLog.CRITICAL:
        logger.error(arg)
    else:
        logger.info <http://logger.info/>(arg)

messageLog = QgsApplication.messageLog()
messageLog.messageReceived.connect( writelogmessage )

wmsuri = ("contextualWMSLegend=0&crs=EPSG:4326&dpiMode=7&featureCount=10&format=image/jpeg"
          "&layers=s2cloudless&styles&tileMatrixSet=s2cloudless-wmsc-14"
          "&url=http://tiles.maps.eox.at/wms? <http://tiles.maps.eox.at/wms?>" )

# GetCapabilities Fail with Request Timeout !!!
provider = QgsProviderRegistry.instance().createProvider( "wms", wmsuri )

# Run again => works !!!
provider = QgsProviderRegistry.instance().createProvider( "wms", wmsuri )

qgis_application.exitQgis()
del qgis_application
====================8<=============================

We have also tested simple QgsNetwork invocation from c++: tests shows that event loops have the running status, but the 
request is not processed at the  first invocation.

David


> Le 15 janv. 2018 à 15:05, David Marteau <dmarteau at 3liz.com> a écrit :
> 
>> 
>> 
>> 
>> Hi David, 
>> 
>> You  need an event loop to use a Q(qg)NetworkAccessManager, it's asynchronous.
>> 
> 
> Hi Alessandro,
> 
> Yes, we know that (see the second code in previous mail  ):  the  QgsNetworkAccessManager works perfeclty in python, this is not the point.
> 
> The point is: trying to instanciate  a  wmsprovider will issue a timeout when trying to request the capabilities: the code responsible for that creates its own QEventLoop and manage to do a blocking call (which is expected here). 
> Please have a look at https://github.com/qgis/QGIS/blob/master/src/providers/wms/qgswmscapabilities.cpp#L1964 <https://github.com/qgis/QGIS/blob/master/src/providers/wms/qgswmscapabilities.cpp#L1964>.  What we do not understand is why, despite the call of loop.exec() in the c++ code, the request is not processed (my example with the  Q(gs)etworkAccessManager
> shows that it should processed)
> 
> Actually you cannot instanciate properly a QgsProject with a WMS layer.
> 
> To convince yourself, try to instanciate a QgsProject in python (in a standalone python script) from a .qgs project having a single wms layer: it will fail to instanciate the layer.
> 
>> 
>> 
>> -- 
>> Alessandro Pasotti
>> w3:   www.itopen.it <http://www.itopen.it/>

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