[QGIS-Developer] Load a QgsMeshLayer within a QgsTask... different Threads?

[Richard Duivenvoorde]

Ok after reading https://docs.qgis.org/3.28/en/docs/pyqgis_developer_cookbook/tasks.html again, reading the warning:

"Any background task (regardless of how it is created) must NEVER use any QObject that lives on the main thread, such as accessing QgsVectorLayer, QgsProject or perform any GUI based operations like creating new widgets or interacting with existing widgets. Qt widgets must only be accessed or modified from the main thread. Data that is used in a task must be copied before the task is started. Attempting to use them from background threads will result in crashes."

Seems to make it pretty clear :-(

Before rereading that, I even tried to pass a QgsMeshLayer-'reference' to the QgsTask, but that did not work either.

I think I must conclude that I just have to wait for the load... :-)

Regards,

Richard Duivenvoorde



On 2/8/23 21:40, Richard Duivenvoorde via QGIS-Developer wrote:
> Hi,
> 
> Having to juggle with rather large/long loading netcdf files, I thought to off load the loading to a QgsTask, so while the netcdf was loaded user could do other things...
> 
> But I get "...is run from a different thread than the object  lives in ..." warnings.
> 
> In short:
> 
>      def run(self) -> bool:
>          log.info(f'LOADING... {self.netcdf_file}')
>          try:
>              self.layer = QgsMeshLayer(self.netcdf_file, 'PythonLoadedMeshLayer',  'mdal')
>      ....
> 
> BUT: while it looks like all is fine, I get a Qt warning in my messages, and on my terminal I get:
> 
> [INFO] (Dummy-1   ) LOADING... /tmp/kees/aoi/aoi1/areaOfInterest1.nc
> Warning: fileName (/home/richard/git/qgis/src/core/project/qgsproject.cpp:812) is run from a different thread than the object  lives in [0x5593004f1aa0 vs 0x5592ff7e6440]
> Stacktrace (piped through c++filt):
> /home/richard/bin/qgis_/master/debug/bin/qgis(+0xe26a)[0x5592fe9ba26a]
> /home/richard/bin/qgis_/master/debug/bin/qgis(+0xea29)[0x5592fe9baa29]
> /lib/x86_64-linux-gnu/libQt5Core.so.5(+0xc3b50)[0x7fc594ec3b50]
> /lib/x86_64-linux-gnu/libQt5Core.so.5(qt_message_output(QtMsgType, QMessageLogContext const&, QString const&)+0xd)[0x7fc594ec50fd]
> /lib/x86_64-linux-gnu/libQt5Core.so.5(QDebug::~QDebug()+0x68)[0x7fc594fc6fa8]
> /home/richard/bin/qgis_/master/debug/lib/libqgis_core.so.3.29.0(QgsProject::fileName() const+0x267)[0x7fc592f1af2f]
> /home/richard/bin/qgis_/master/debug/lib/libqgis_core.so.3.29.0(QgsMapLayer::loadNamedProperty(QString const&, QgsMapLayer::PropertyType, bool&, QFlags<QgsMapLayer::StyleCategory>)+0x752)[0x7fc592a215c0]
> /home/richard/bin/qgis_/master/debug/lib/libqgis_core.so.3.29.0(QgsMapLayer::loadNamedStyle(QString const&, bool&, QFlags<QgsMapLayer::StyleCategory>)+0x2b1)[0x7fc592a20b8f]
> /home/richard/bin/qgis_/master/debug/lib/libqgis_core.so.3.29.0(QgsMapLayer::loadDefaultStyle(bool&)+0x2dc)[0x7fc592a1f6ec]
> /home/richard/bin/qgis_/master/debug/lib/libqgis_core.so.3.29.0(QgsMeshLayer::loadDefaultStyle(bool&)+0x685)[0x7fc5930a84bb]
> /home/richard/bin/qgis_/master/debug/lib/libqgis_core.so.3.29.0(QgsMeshLayer::QgsMeshLayer(QString const&, QString const&, QString const&, QgsMeshLayer::LayerOptions const&)+0x2b9)[0x7fc5930a5ab1]
> /home/richard/bin/qgis_/master/debug/share/qgis/python/qgis/_core.so(sipQgsMeshLayer::sipQgsMeshLayer(QString const&, QString const&, QString const&, QgsMeshLayer::LayerOptions const&)+0x3b)[0x7fc43040b9c7]
> /home/richard/bin/qgis_/master/debug/share/qgis/python/qgis/_core.so(+0x121a0bc)[0x7fc43041a0bc]
> /usr/lib/python3/dist-packages/PyQt5/sip.cpython-311-x86_64-linux-gnu.so(+0x19f60)[0x7fc4dc271f60]
> /lib/x86_64-linux-gnu/libpython3.11.so.1.0(+0x1e02fe)[0x7fc47a5e02fe]
> /lib/x86_64-linux-gnu/libpython3.11.so.1.0(_PyObject_MakeTpCall+0x7d)[0x7fc47a57baed]
> /lib/x86_64-linux-gnu/libpython3.11.so.1.0(_PyEval_EvalFrameDefault+0x4a63)[0x7fc47a509c63]
> /lib/x86_64-linux-gnu/libpython3.11.so.1.0(+0x26a6da)[0x7fc47a66a6da]
> /lib/x86_64-linux-gnu/libpython3.11.so.1.0(+0x17f30c)[0x7fc47a57f30c]
> /usr/lib/python3/dist-packages/PyQt5/sip.cpython-311-x86_64-linux-gnu.so(+0x1196c)[0x7fc4dc26996c]
> [INFO] (MainThread) finished OK
> 
> My Questions:
> - should I NOT do this?
> - or is the loading/construction of a QgsMeshLayer having pointers to places it should not?
> 
> (mmm loading a QgsVectorLayer (shape via ogr) also results in about the same warnings...
> so next question: is there another way to load these large netcdf's without blocking QGIS ?)
> 
> Thanks for any info.
> 
> Regards,
> 
> Richard Duivenvoorde
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