Ensure consistent label positioning by sorting LabelProviders

[PatrikSylve]

Problem

I have a QGIS project, containing vector data with rule base labeling that i publish in WMS in QGIS Server. When requesting a map, the labels will render on different positions each request. So the output image will vary even though the input is remains the same.

Example, two identical GetMap requests generating different output (notice the labels for example Oslo & Göteborg):

Solution

The issue stemmed from features being processed in seemingly random orders during each run of Pal::extractProblem. This was due to Pal::mLayers storing LabelProviders in an unordered_map.

To resolve this, I:

• Change the type from unordered_map to map
• Added a custom comparator to order QgsAbstractLabelProvider by providerId, or layerId as fallback.

Related issue

This potentially resolves #28174

Note

The issue is still present when having a vector tile web service as data source. In that case one soulution was to sort the layer->mFeatureParts before iterating the features in Pal::extractProblem.

    std::sort( layer->mFeatureParts.begin(), layer->mFeatureParts.end(), []( const std::unique_ptr<FeaturePart> &a, const std::unique_ptr<FeaturePart> &b ) {
      return a->featureId() < b->featureId();
    } );

But I suspect the feature order is influenced by the order the tiles are received from the vector tile service, so this may require additional handling specific to that use case.

[github-actions]

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[nyalldawson]

@PatrikSylve GREAT fix -- this will make a lot of people happy!

Are you confident fixing the test failures here?

[PatrikSylve]

Thanks, glad to hear!

I am not familiar with the test suite, but I will look into it and give it a try

[PatrikSylve]

@nyalldawson

Got the tests up and it would fail occasionally due to the layerId being generated randomly. Assigned a custom ID for the layers in the test and it seems to succeed consistently.

[nyalldawson]

@PatrikSylve I have an alternative idea -- see #60369. Instead of storing the layers in a map in the first place, we can just use a vector. This ensures that the layers will ALWAYS be processed in their actual layer order from the project, so things are no longer dependent on the layer ID (which, as you've noted, is itself non-deterministic).

I'm keen to hear your thoughts!

[PatrikSylve]

@nyalldawson

I continued on your work in #60369 and attempted to also use vector instead of map for QgsRuleBasedLabelProvider::mSubProviders. This also ensured that the subProviders were handled in consistent order when having rule base labeling.

What do you think?

[nyalldawson]

can you move this to private? it shouldn't be exposed to the python bindings