MDL-51245 mssql: Diagnose missing READ_COMMITTED_SNAPSHOT mode

Without it, transactions are executed in standard READ_COMMITED
mode, without snapshoting/row versioning, leading to update conflicts
under high concurrency.

lang/en/error.php

@@ -388,6 +388,7 @@
 $string['movecategorynotpossible'] = 'You cannot move category \'{$a}\' into the selected category.';
 $string['movecategoryownparent'] = 'You cannot make category \'{$a}\' a parent of itself.';
 $string['movecategoryparentconflict'] = 'You cannot make category \'{$a}\' a subcategory of one of its own subcategories.';
+$string['mssqlrcsmodemissing'] = 'The database is not using the expected READ_COMMITTED_SNAPSHOT mode which can lead to wrong results, especially under high concurrency scenarios. Please enable it for correct behaviour. You can find more information in the <a href="https://docs.moodle.org/en/Installing_MSSQL_for_PHP#Configuration">Moodle Docs</a>.';
 $string['multiplerecordsfound'] = 'Multiple records found, only one record expected.';
 $string['multiplerestorenotallow'] = 'Multiple restore execution not allowed!';
 $string['mustbeloggedin'] = 'You must be logged in to do this';

lib/dml/mssql_native_moodle_database.php

@@ -98,6 +98,37 @@ public function get_configuration_help() {
         return get_string('nativemssqlhelp', 'install');
     }
 
+    /**
+     * Diagnose database and tables, this function is used
+     * to verify database and driver settings, db engine types, etc.
+     *
+     * @return string null means everything ok, string means problem found.
+     */
+    public function diagnose() {
+        // Verify the database is running with READ_COMMITTED_SNAPSHOT enabled.
+        // (that's required to get snapshots/row versioning on READ_COMMITED mode).
+        $correctrcsmode = false;
+        $sql = "SELECT is_read_committed_snapshot_on
+                  FROM sys.databases
+                 WHERE name = '{$this->dbname}'";
+        $this->query_start($sql, null, SQL_QUERY_AUX);
+        $result = mssql_query($sql, $this->mssql);
+        $this->query_end($result);
+        if ($result) {
+            if ($row = mssql_fetch_assoc($result)) {
+                $correctrcsmode = (bool)reset($row);
+            }
+        }
+        $this->free_result($result);
+
+        if (!$correctrcsmode) {
+            return get_string('mssqlrcsmodemissing', 'error');
+        }
+
+        // Arrived here, all right.
+        return null;
+    }
+
     /**
      * Connect to db
      * Must be called before other methods.