SourcePluginBase has a protected property, called $mapRowAdded, which is never used.
Okay, well, that's not entirely true; it's used in exactly one place -- the very convoluted next() method:
if (!$this->mapRowAdded && ($id_map = $this->idMap->getRowBySource($this->currentSourceIds))) {
Um, yeah...$mapRowAdded's default value is FALSE, and nowhere else in core is that ever changed or touched in any way. I'm convinced that the property is vestigial and not actually doing anything. It's clutter.
Remove the property.
Patch, review, commit.
None.
None.
None.
Removed the $mapRowAdded property from \Drupal\migrate\Plugin\migrate\source\SourcePluginBase, as it was not being used for anything.
Closed: duplicate
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
#7 mentions this should be deprecated with tests so not ready for review.
There is a later issue that is removing this and other dead code in the file. I am closing this and moving credit.