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Long story short: calling provider's addFeatures
is implemented for some providers in a way that
will roll back all changes on errors, leaving
the backend storage unchanged.
Adding a QgsFeatureSink flag to control this
behavior allows certain providers to support
partial feature addition.
The issue comes from QgsVectorDataProvider::commitChanges
that is documented to leave the provider unchanged (roll
back) on any error, giving the client code the possibility
to fix errors (in the editing buffer) and re-commit.
Without a full rollback implementation in the memory
provider and after the type check introduction in this
PR we ended up with situations like this:
vl = ... an empty memory layer
self.assertTrue(vl.addFeatures([valid, invalid]))
self.assertFalse(vl.commitChanges())
self.assertEqual(vl.featureCount(), 1) <--- fails!
We actually had 3 features from vl.getFeatures():
[valid, invalid, valid] (the first from the provider
the second and third from the editing buffer).
On the other hand, QgsFeatureSink would probably assume
that addFeatures will allow partial additions.
BTW: This is for sure the longest commit message I've ever
written.