[DATABASE][MariaDB] Fix index changes with foreign keys enabled

lib/database/mysqlschema.php

@@ -333,6 +333,21 @@ class MysqlSchema extends Schema
         $statements[] = "CREATE FULLTEXT INDEX $name ON $table " . $this->buildIndexList($def);
     }
 
+    /**
+     * Append an SQL statement with an index definition for an advisory
+     * index over one or more columns on a table.
+     *
+     * @param array $statements
+     * @param string $table
+     * @param string $name
+     * @param array $def
+     */
+    public function appendCreateIndex(array &$statements, $table, $name, array $def)
+    {
+        $statements[] = "ALTER TABLE {$this->quoteIdentifier($table)} "
+                      . "ADD INDEX {$name} {$this->buildIndexList($def)}";
+    }
+
     /**
      * Close out a 'create table' SQL statement.
      *
@@ -456,7 +471,8 @@ class MysqlSchema extends Schema
      */
     public function appendDropIndex(array &$statements, $table, $name)
     {
-        $statements[] = "DROP INDEX {$name} ON {$this->quoteIdentifier($table)}";
+        $statements[] = "ALTER TABLE {$this->quoteIdentifier($table)} "
+                      . "DROP INDEX {$name}";
     }
 
     private function isNumericType(array $cd): bool

lib/database/schema.php

@@ -660,6 +660,24 @@ class Schema
             $this->appendCreateFulltextIndex($statements, $tableName, $indexName, $colDef);
         }
 
+        /*
+         * Merges all consecutive ALTER TABLE's into one statement.
+         * This is necessary in MariaDB as foreign keys can disallow removal of
+         * an index if a replacement isn't provided instantly.
+         */
+        [$stmts_orig, $statements] = [$statements, []];
+        foreach ($stmts_orig as $stmt) {
+            $prev = array_slice($statements, -1)[0] ?? '';
+            $prefix = "ALTER TABLE {$this->quoteIdentifier($tableName)} ";
+            if (mb_substr($stmt, 0, mb_strlen($prefix)) === $prefix
+                && mb_substr($prev, 0, mb_strlen($prefix)) === $prefix) {
+                $statements[] = array_pop($statements) . ', '
+                              . mb_substr($stmt, mb_strlen($prefix));
+            } else {
+                $statements[] = $stmt;
+            }
+        }
+
         return $statements;
     }