QGISserver qgis/QGIS#42063 issue

1st QGIS server

First server (http://localhost:8080/ows/) serves the qgis_projects_1/test.gqs project, which contains one single vector layer named vector_bat.

Here is a simple GetMap query.

http://localhost:8080/ows/?service=WMS&map=/io/data/test.qgs&request=GetMap&version=1.3.0&width=512&height=512&crs=EPSG:2056&BBOX=2539106,1184111,2539880,1184814&LAYERS=vector_bat

The LegendGraphic is fine.

http://localhost:8080/ows/?service=WMS&map=/io/data/test.qgs&request=GetLegendGraphic&version=1.3.0&LAYERS=vector_bat

2nd QGIS server

The second QGIS server publishes the qgis_projects_2/test_2.gqs project. It contains one WMS layer, which is the one provided by the first QGIS server.

The GetMap is working as expected.

http://localhost:8081/ows/?service=WMS&map=/io/data/test_2.qgs&request=GetMap&version=1.3.0&width=512&height=512&crs=EPSG:2056&BBOX=2539106,1184111,2539880,1184814&LAYERS=vector_bat_as_wms

The cascaded WMS query is clearly visible in the logs:

2nd server receives the request:

qgis-issue-42063-qgisserver2-1  | 10:39:01 INFO Server[25]: ******************** New request ***************
qgis-issue-42063-qgisserver2-1  | 10:39:01 INFO Server[25]: Request URL: http://localhost:8081/ows/?service=WMS&map=/io/data/test_2.qgs&request=GetMap&version=1.3.0&width=512&height=512&crs=EPSG:2056&BBOX=2539106,1184111,2539880,1184814&LAYERS=vector_bat_as_wms
...

2nd server cascades request to the first server:

qgis-issue-42063-qgisserver1-1  | 10:39:01 INFO Server[25]: ******************** New request ***************
qgis-issue-42063-qgisserver1-1  | 10:39:01 INFO Server[25]: Request URL: http://172.18.0.1:8080/ows/?MAP=/io/data/test.qgs&SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&BBOX=6.640780999999999601%2C46.80535700000000077%2C6.651016000000000261%2C46.81175499999999801&CRS=CRS%3A84&WIDTH=702&HEIGHT=439&LAYERS=vector_bat&STYLES=&FORMAT=image%2Fpng&DPI=90&MAP_RESOLUTION=90&FORMAT_OPTIONS=dpi%3A90&TRANSPARENT=TRUE
...

first server response:

qgis-issue-42063-qgisserver1-1  | 10:39:01 INFO Server[25]: Request finished in 34 ms

second server response:

qgis-issue-42063-qgisserver2-1  | 10:39:01 INFO Server[25]: Request finished in 391 ms

GetLegengGraphic query does not work: generated images only contains the layer title.

http://localhost:8081/ows/?service=WMS&map=/io/data/test_2.qgs&request=GetLegendGraphic&version=1.3.0&LAYERS=vector_bat_as_wms

It's seems the GetLegengGraphic query is cascaded as expected, but the server the cascaded request seems asynchronous. Here are the relevant logs:

initial query received by QGIS#2:

qgis-issue-42063-qgisserver2-1  | 10:47:36 INFO Server[25]: ******************** New request ***************
qgis-issue-42063-qgisserver2-1  | 10:47:36 INFO Server[25]: Request URL: http://localhost:8081/ows/?service=WMS&map=/io/data/test_2.qgs&request=GetLegendGraphic&version=1.3.0&LAYERS=vector_bat_as_wms

Cascaded query received by QGIS#1:

qgis-issue-42063-qgisserver1-1  | 10:47:36 INFO Server[25]: ******************** New request ***************
qgis-issue-42063-qgisserver1-1  | 10:47:36 INFO Server[25]: Request URL: http://172.18.0.1:8080/ows/?MAP=/io/data/test.qgs&SERVICE=WMS&VERSION=1.3.0&REQUEST=GetLegendGraphic&LAYER=vector_bat&FORMAT=image/png&STYLE=default&SLD_VERSION=1.1.0&TRANSPARENT=true

Initial query response:

qgis-issue-42063-qgisserver2-1  | 10:47:36 INFO Server[25]: Request finished in 3 ms

Cascaded query response:

qgis-issue-42063-qgisserver1-1  | 10:47:36 INFO Server[25]: Request finished in 17 ms

Sniffing the traffic with wireshark confirmed this asynchronous behaviour.

Code analysis

Looking in the code, I guess QgsWmsLegendNode::getLegendGraphic is called asynchronously.

https://github.com/qgis/QGIS/blob/5b2c0e4fa658eb92dafc605230b60320ec369b18/src/core/layertree/qgslayertreemodellegendnode.cpp#L1272

The sync/async behaviour is driven by QgsLegendSettings::mSynchronousLegendRequests flag.

It seems the only time this setting is true is when printing layouts: https://github.com/qgis/QGIS/blob/5b2c0e4fa658eb92dafc605230b60320ec369b18/src/core/layout/qgslayoutitemlegend.cpp#L133.

In my opinion, this setting should also be set to true somewhere in src/server/: https://github.com/qgis/QGIS/blob/master/src/server/services/wms/qgswmsgetlegendgraphics.cpp https://github.com/qgis/QGIS/blob/master/src/server/services/wms/qgswmsrenderer.cpp