Issue #3045211 by nlisgo, yogeshmpawar, Nebel54, heddn, quietone: Prevent link field migration from creating invalid link attributes

core/modules/link/src/Plugin/migrate/process/FieldLink.php

@@ -119,7 +119,9 @@ class FieldLink extends ProcessPluginBase {
       $attributes = unserialize($attributes);
     }
 
-    if (!$attributes) {
+    // In rare cases Drupal 6/7 link attributes are triple serialized. To avoid
+    // further problems with them we set them to an empty array in this case.
+    if (!is_array($attributes)) {
       $attributes = [];
     }
 

core/modules/link/tests/src/Unit/Plugin/migrate/process/FieldLinkTest.php

@@ -89,4 +89,27 @@ class FieldLinkTest extends UnitTestCase {
     ];
   }
 
+  /**
+   * Test the attributes that are deeply serialized are discarded.
+   */
+  public function testCanonicalizeUriSerialized() {
+    $link_plugin = new FieldLink([], '', [], $this->getMock(MigrationInterface::class));
+    $migrate_executable = $this->getMock(MigrateExecutableInterface::class);
+    $row = new Row();
+
+    $transformed = $link_plugin->transform([
+      'url' => '',
+      'title' => '',
+      'attributes' => serialize(serialize(['not too deep'])),
+    ], $migrate_executable, $row, NULL);
+    $this->assertEquals(['not too deep'], $transformed['options']['attributes']);
+
+    $transformed = $link_plugin->transform([
+      'url' => '',
+      'title' => '',
+      'attributes' => serialize(serialize(serialize(['too deep']))),
+    ], $migrate_executable, $row, NULL);
+    $this->assertEmpty($transformed['options']['attributes']);
+  }
+
 }