From 007635360a3b35ac1cf211488a82f7743b89a8f5 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 15 Apr 2026 15:56:59 +0300
Subject: [PATCH] db.mysql: fix malformed error messages from real_query
 failing to encode to json (fixes #19842)

---
 vlib/db/mysql/utils.c.v    | 12 ++++++++++--
 vlib/db/mysql/utils_test.v | 20 ++++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)
 create mode 100644 vlib/db/mysql/utils_test.v

diff --git a/vlib/db/mysql/utils.c.v b/vlib/db/mysql/utils.c.v
index ea48daa3b..ca0ef215e 100644
--- a/vlib/db/mysql/utils.c.v
+++ b/vlib/db/mysql/utils.c.v
@@ -2,7 +2,7 @@ module mysql
 
 // get_error_msg returns error message from MySQL instance.
 fn get_error_msg(conn &C.MYSQL) string {
-	return unsafe { C.mysql_error(conn).vstring() }
+	return clone_mysql_cstring(C.mysql_error(conn))
 }
 
 // get_errno returns error number from MySQL instance.
@@ -12,7 +12,7 @@ fn get_errno(conn &C.MYSQL) int {
 
 // get_stmt_error_msg returns error message from a MySQL statement instance.
 fn get_stmt_error_msg(stmt &C.MYSQL_STMT) string {
-	return unsafe { cstring_to_vstring(&char(C.mysql_stmt_error(stmt))) }
+	return clone_mysql_cstring(&u8(C.mysql_stmt_error(stmt)))
 }
 
 // get_stmt_errno returns error number from a MySQL statement instance.
@@ -20,6 +20,14 @@ fn get_stmt_errno(stmt &C.MYSQL_STMT) int {
 	return C.mysql_stmt_errno(stmt)
 }
 
+@[inline]
+fn clone_mysql_cstring(ptr &u8) string {
+	if isnil(ptr) {
+		return ''
+	}
+	return unsafe { tos_clone(ptr) }
+}
+
 // resolve_nil_str returns an empty string if passed value is a nil pointer.
 fn resolve_nil_str(ptr &u8) string {
 	if isnil(ptr) {
diff --git a/vlib/db/mysql/utils_test.v b/vlib/db/mysql/utils_test.v
new file mode 100644
index 000000000..0760ff2be
--- /dev/null
+++ b/vlib/db/mysql/utils_test.v
@@ -0,0 +1,20 @@
+module mysql
+
+import json
+
+struct MysqlErrorPayload {
+	data string
+}
+
+fn test_clone_mysql_cstring_keeps_json_encodable_error_text() {
+	mut backing := "Incorrect string value: 'undefined' for function uuid_to_bin; code: 1411".bytes()
+	backing << u8(0)
+	msg := clone_mysql_cstring(backing.data)
+
+	backing[0] = 0xff
+
+	assert msg == "Incorrect string value: 'undefined' for function uuid_to_bin; code: 1411"
+	assert json.encode(MysqlErrorPayload{
+		data: msg
+	}) == '{"data":"Incorrect string value: \'undefined\' for function uuid_to_bin; code: 1411"}'
+}
-- 
2.39.5