[processing] Fix random selection algorithm when filter is set on layer

(cherry-picked from 31330dd)

Author: nyalldawson

python/plugins/processing/algs/qgis/RandomSelection.py

@@ -93,11 +93,11 @@ def processAlgorithm(self, parameters, context, feedback):
         layer = self.parameterAsVectorLayer(parameters, self.INPUT, context)
         method = self.parameterAsEnum(parameters, self.METHOD, context)
 
-        featureCount = layer.featureCount()
+        ids = layer.allFeatureIds()
         value = self.parameterAsInt(parameters, self.NUMBER, context)
 
         if method == 0:
-            if value > featureCount:
+            if value > len(ids):
                 raise QgsProcessingException(
                     self.tr('Selected number is greater than feature count. '
                             'Choose a lower value and try again.'))
@@ -106,9 +106,9 @@ def processAlgorithm(self, parameters, context, feedback):
                 raise QgsProcessingException(
                     self.tr("Percentage can't be greater than 100. Set a "
                             "different value and try again."))
-            value = int(round(value / 100.0, 4) * featureCount)
+            value = int(round(value / 100.0, 4) * len(ids))
 
-        selran = random.sample(list(range(featureCount)), value)
+        selran = random.sample(ids, value)
 
         layer.selectByIds(selran)
         return {self.OUTPUT: parameters[self.INPUT]}