MDL-60915 core_dml: fix miscellaneous incorrect recordset usage

The new recordset support for Postgres requires transactions and
will cause errors if recordsets are not closed correctly. This
commit fixes problems that were identified during unit tests, and
via some basic code analysis, across all core code. Most of these
are incorrect usage of recordset (forgetting to close them).

lib/tablelib.php

@@ -1499,9 +1499,9 @@ class table_sql extends flexible_table {
      * method or if other_cols returns NULL then put the data straight into the
      * table.
      *
-     * @return void
+     * After calling this function, don't forget to call close_recordset.
      */
-    function build_table() {
+    public function build_table() {
 
         if ($this->rawdata instanceof \Traversable && !$this->rawdata->valid()) {
             return;
@@ -1515,10 +1515,16 @@ class table_sql extends flexible_table {
             $this->add_data_keyed($formattedrow,
                 $this->get_row_class($row));
         }
+    }
 
-        if ($this->rawdata instanceof \core\dml\recordset_walk ||
-                $this->rawdata instanceof moodle_recordset) {
+    /**
+     * Closes recordset (for use after building the table).
+     */
+    public function close_recordset() {
+        if ($this->rawdata && ($this->rawdata instanceof \core\dml\recordset_walk ||
+                $this->rawdata instanceof moodle_recordset)) {
             $this->rawdata->close();
+            $this->rawdata = null;
         }
     }
 
@@ -1629,6 +1635,7 @@ class table_sql extends flexible_table {
         $this->setup();
         $this->query_db($pagesize, $useinitialsbar);
         $this->build_table();
+        $this->close_recordset();
         $this->finish_output();
     }
 }