Ensure empty error tables in scripts don't crash Valkey

Author: Fusl

src/lua/script_lua.c

@@ -1668,6 +1668,11 @@ void luaExtractErrorInformation(lua_State *lua, errorInfo *err_info) {
         err_info->ignore_err_stats_update = lua_toboolean(lua, -1);
     }
     lua_pop(lua, 1);
+
+    if (err_info->msg == NULL) {
+        // Ensure we never return a NULL msg.
+        err_info->msg = sdsnew("ERR unknown error");
+    }
 }
 
 /* This is the core of our Lua debugger, called each time Lua is about

tests/unit/scripting.tcl

@@ -2464,4 +2464,21 @@ start_server {tags {"scripting"}} {
         # Using a null byte never seemed to work with functions, so
         # we don't have a test for that case.
     }
+
+    test {EVAL - explicit error() call handling} {
+        # error("simple string error")
+        assert_error {ERR user_script:1: simple string error script: *} {
+            r eval "error('simple string error')" 0
+        }
+
+        # error({"err": "ERR table error"})
+        assert_error {ERR table error script: *} {
+            r eval "error({err='ERR table error'})" 0
+        }
+
+        # error({})
+        assert_error {ERR unknown error script: *} {
+            r eval "error({})" 0
+        }
+    }
 }