I ran into an issue with a migration with XML data parser plugin when using an XPath attribute in its item_selector like item_selector: /Items/Item[@type="foo"].
The exception is : "Migration failed with source plugin exception: Index is defined as a source ID but has no value".
Looking at the code in Xml::fetchNextRow(), when an element matching the selector is found, $target_element gets it as SimpleXml and then tests the Xpath predicate. If the predicate is not empty but does not match the element, variable $target_element should be unset, otherwise if no "better" element is found in the XML document, the one in $target_element would still be used.
This case may happen when looking for a /Items/Item[@type="foo"] when we only have <items><item type="bar"></item></items>.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I was hit by this as well, getting wrong total counts for some XML migrations whereby rows are filtered by a Xpath attribute. Patch is simple, straightforward and fixes the issue.
Since it's such a simple fix, setting this to RTBC.
Closing as duplicate of 🐛 Using the XML Data Parser with a Predicate pulls in the final xml item regardless of if the predicate matches Needs work