QgsVectorLayer 's addFeatures function crashes the app

[qgib]

Author Name: Mehmet Selim BILGIN (@MSBilgin)
Original Redmine Issue: 20694
Affected QGIS version: 3.4.1
Redmine category:python_bindings_/_sipify

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I generated Spatialite point layer that has 1M features by using random point plugin. In python I tried to copy all features to memory layer. Here is the sample code:

@sourceFeatures = self.sourceLayer.getSelectedFeatures()
self.targetLayer.startEditing()
self.targetLayer.addFeatures(sourceFeatures)
@---crashed---

But when i try with 10.000 features this time it successfully adds features. Interestingly, I tried to add 1M features with while loop, this time it succeed. And note that adding operation runs on another QThread.

[nyalldawson]

Likely a result of editing a layer from a background thread. This isn't thread safe and is always going to cause random crashes like this.