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

Thu Feb 9 00:26:18 PST 2023

On Thu, 9 Feb 2023, 6:18 pm Richard Duivenvoorde via QGIS-Developer, <
qgis-developer at lists.osgeo.org> wrote:

> 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."
>

One thing you can do:

>From the thread which owns the object, when you've finished using that
object in the thread, call
object.moveToThread(QgsApplication.instance().thread())

You're allowed to push objects which belong to the current thread to
another thread, but never allowed to pull objects which belong to another
thread over to the current thread.

Nyall

> 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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