Prevent db_schema.xml declarations with comment="" from breaking zero downtime deployments

lib/internal/Magento/Framework/Setup/Declaration/Schema/Dto/ElementFactory.php

@@ -1,7 +1,7 @@
 <?php
 /**
- * Copyright © Magento, Inc. All rights reserved.
- * See COPYING.txt for license details.
+ * Copyright 2015 Adobe
+ * All Rights Reserved.
  */
 namespace Magento\Framework\Setup\Declaration\Schema\Dto;
 
@@ -22,6 +22,8 @@ class ElementFactory
      *
      * Where @key - is xsi:type of the object
      * Where @value - is instance class name
+     *
+     * @var array
      */
     private $typeFactories = [];
 
@@ -70,6 +72,25 @@ private function castGenericAttributes(array $elementStructuralData)
         return $elementStructuralData;
     }
 
+    /**
+     * Remove empty comments from the schema declaration
+     *
+     * Empty comments are never persisted in the database, they always end up being read back as null
+     *
+     * @see \Magento\Framework\Setup\Declaration\Schema\Db\MySQL\DbSchemaReader::readColumns
+     *
+     * @param array $elementStructuralData
+     * @return array
+     */
+    private function removeEmptyComments(array $elementStructuralData)
+    {
+        if (isset($elementStructuralData['comment']) && $elementStructuralData['comment'] === "") {
+            unset($elementStructuralData['comment']);
+        }
+
+        return $elementStructuralData;
+    }
+
     /**
      * Instantiate different types of elements, depends on their xsi:type.
      *
@@ -84,6 +105,7 @@ public function create($type, array $elementStructuralData)
         }
 
         $elementStructuralData = $this->castGenericAttributes($elementStructuralData);
+        $elementStructuralData = $this->removeEmptyComments($elementStructuralData);
         $elementStructuralData['type'] = $type;
         return $this->typeFactories[$type]->create($elementStructuralData);
     }