Issue #2860788 by bkosborne: XML parser should clear XML error buffer before processing XML data

src/Plugin/migrate_plus/data_parser/SimpleXml.php

@@ -38,6 +38,12 @@ class SimpleXml extends DataParserPluginBase {
    * {@inheritdoc}
    */
   protected function openSourceUrl($url) {
+    // Clear XML error buffer. Other Drupal code that executed during the
+    // migration may have polluted the error buffer and could create false
+    // positives in our error check below. We are only concerned with errors
+    // that occur from attempting to load the XML string into an object here.
+    libxml_clear_errors();
+
     $xml_data = $this->getDataFetcherPlugin()->getResponseContent($url);
     $xml = simplexml_load_string($xml_data);
     $this->registerNamespaces($xml);

src/Plugin/migrate_plus/data_parser/Xml.php

@@ -174,6 +174,13 @@ class Xml extends DataParserPluginBase {
   protected function openSourceUrl($url) {
     // (Re)open the provided URL.
     $this->reader->close();
+
+    // Clear XML error buffer. Other Drupal code that executed during the
+    // migration may have polluted the error buffer and could create false
+    // positives in our error check below. We are only concerned with errors
+    // that occur from attempting to load the XML string into an object here.
+    libxml_clear_errors();
+
     return $this->reader->open($url, NULL, \LIBXML_NOWARNING);
   }