database.php: add retry_on_deadlock utility function

inc/database.php

@@ -6,6 +6,9 @@
 
 defined('TINYBOARD') or exit;
 
+// https://dev.mysql.com/doc/mysql-errors/8.0/en/server-error-reference.html
+const MYSQL_ER_LOCK_DEADLOCK = 1213;
+
 class PreparedQueryDebug {
 	protected $query, $explain_query = false;
 
@@ -147,6 +150,32 @@ function query($query) {
 	return $pdo->query($query);
 }
 
+/**
+ * @param int $max_retries The maximum number of times the callable may be re-executed on deadlock detection.
+ * @param callable $query_func A callable that executes a query. Must throw a PDOException on deadlocks.
+ * @return mixed The value returned by $query_func.
+ * @throws PDOException Throws once the maximum number of retries have been attempted.
+ */
+function retry_on_deadlock(int $max_retries, callable $query_func) {
+	// Add the first original attempt to the max count.
+	$max_retries += 1;
+	$attempt = 0;
+
+	while (true) {
+		$attempt++;
+
+		try {
+			return $query_func();
+		} catch (\PDOException $e) {
+			if ($e->errorInfo === null || $e->errorInfo[1] != MYSQL_ER_LOCK_DEADLOCK || $attempt >= $max_retries) {
+				throw $e;
+			}
+			// Wait for 0.5s before retrying...
+			usleep(500000);
+		}
+	}
+}
+
 function db_error($PDOStatement = null) {
 	global $pdo, $db_error;