postProcessLayer() of QgsProcessingLayerPostProcessorInterface producing inconsistent results

[ar-siddiqui]

What is the bug or the crash?

In a processing script, the recommended practice to rename the layer and apply style is through postProcessLayer() of QgsProcessingLayerPostProcessorInterface.

I want to apply styles to all layers in my processing script and rename them. I am using the above-recommended method to achieve this.

I have written a script that works most of the time but not all the time. I ran the script multiple times to ensure that nothing in the code was changing yet the QGIS applied style sometimes and sometimes not.

Here is my function that applies styles, full code in the attachment below.

def run_alg_styler(display_name, layer_color1, layer_color2):
    """Create a New Post Processor class and returns it"""
    # Just simply creating a new instance of the class was not working
    # for details see https://gis.stackexchange.com/questions/423650/qgsprocessinglayerpostprocessorinterface-only-processing-the-last-layer
    class LayerPostProcessor(QgsProcessingLayerPostProcessorInterface):
        instance = None
        name = display_name
        color1 = layer_color1
        color2 = layer_color2

        def postProcessLayer(self, layer, context, feedback):
            feedback.pushInfo(f'Inside postProcessLayer: {self.name}')
            if layer.isValid():
                feedback.pushInfo(f'Layer valid: {self.name}')
                layer.setName(self.name)

                prov = layer.dataProvider()
                stats = prov.bandStatistics(
                    1, QgsRasterBandStats.All, layer.extent(), 0
                )
                min = stats.minimumValue
                max = stats.maximumValue
                renderer = QgsSingleBandPseudoColorRenderer(
                    layer.dataProvider(), band=1
                )
                color_ramp = QgsGradientColorRamp(
                    QColor(*self.color1), QColor(*self.color2)
                )
                renderer.setClassificationMin(min)
                renderer.setClassificationMax(max)
                renderer.createShader(color_ramp)
                layer.setRenderer(renderer)
            else:
                feedback.pushInfo(f'Layer not valid: {self.name}')

        # Hack to work around sip bug!
        @staticmethod
        def create() -> "LayerPostProcessor":
            LayerPostProcessor.instance = LayerPostProcessor()
            return LayerPostProcessor.instance

    return LayerPostProcessor.create()

Please see this demo:

2022-04-04.18-03-42_Trim.online-video-cutter.com.1.mp4

Steps to reproduce the issue

1. Open the project in the attached zip
2. Run the script in the attached zip
3. Run multiple times again until you see styles are not applied

style_issue.zip

Versions

3.22.4

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

https://gis.stackexchange.com/questions/423650/qgsprocessinglayerpostprocessorinterface-only-processing-the-last-layer

[alexbruy]

Duplicates #47533.