This section of the official lua modding documentation does not recommend keeping pointers to object refs (I understand a player to be an example of one - perhaps I'm wrong). This is to avoid the objectref going invalid.

Looking over this code, pretty much every single reference to a "player" is not the player's name - but an objectref. This is only a problem where the objectref (and not the username) is stored in the vehicle (for the driver and all passengers), and the potentially completely useless lib_mount.passengers value (which is even more scary - you're indexing with a playerref to get the same playerref, and the same playerref might not even == itself all the time)

This is brought up as an issue instead of a PR because fixing this will may break backwards compatibility.

See below. There's a way, at cost of performance, to not break compatibility.

Hello, im not coder, idk how to fix this, and if it can be fixed..

But i found bug, when player is in the car/boat from this mod and simple leave game.

Server will crash with this error:
2021-01-30 18:39:58: ACTION[Server]: milan right-clicks object 120: LuaEntitySAO "vehicle_mash:rowboat" at (-3124,2,267)
2021-01-30 18:40:02: ACTION[Server]: milan leaves game. List of players:
2021-01-30 18:40:03: ACTION[Main]: Server: Shutting down
2021-01-30 18:40:29: ACTION[Main]: [Tubelib] Store data...
2021-01-30 18:40:29: ACTION[Main]: [Tubelib] Data stored. t=1312us
2021-01-30 18:40:31: ERROR[Main]: ServerError: AsyncErr: ServerThread::run Lua: Runtime error from mod 'vehicle_mash' in callback luaentity_Step(): /home/minetest/.minetest/mods/lib_mount/init.lua:261: attempt t
o perform arithmetic on a nil value
2021-01-30 18:40:31: ERROR[Main]: stack traceback:
2021-01-30 18:40:31: ERROR[Main]: /home/minetest/.minetest/mods/lib_mount/init.lua:261: in function 'drive'
2021-01-30 18:40:31: ERROR[Main]: /home/minetest/.minetest/mods/vehicle_mash/framework.lua:143: in function </home/minetest/.minetest/mods/vehicle_mash/framework.lua:142>

2021-11-14 17:40:43: WARNING[Server]: Deprecated call to get_look_yaw, use get_look_horizontal instead (at ...gger\minetest-5.4.1-win64\bin..\mods\lib_mount\init.lua:274)
This error occurred a few thousand times in a few seconds. easy to fix.

change line 274 to:
entity.object:set_yaw(entity.driver:get_look_horizontal() - rot_steer)