Update of bug #21459 (project wesnoth):
Status: None => In Progress
Assigned to: None => involution
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Follow-up Comment #2:
I think that the source of the bug is the following (see
https://github.com/wesnoth/wesnoth-old/blob/master/src/playsingle_controller.cpp#L654
for reference)
Whenever a human side (controlled by local player, not a remote one) begins
its turn, the client will have 'current_team().is_human() = true' and will
execute the try/catch block there, including 'before_human_turn()' which
inserts the init_side command, and then play_turn which watches for and
executes actions.
When control of the current side is changed, we have to escape the play_turn
function and start over. So the function play_human_turn in
playmp_controller.cpp, which handles when the local human is moving, has to
throw an "end_turn_exception" when it detects that the turn needs to be
restarted.
https://github.com/wesnoth/wesnoth-old/blob/master/src/playmp_controller.cpp#L208
Since this is the mechanism to give control, it creates the possibility that a
side will be initialized twice and cause an error. There are various
mechanisms in the code right now to try to prevent that, but I believe that
the server errors you are seeing, even the ones ostensibly related to
whiteboard, are probably caused by sides being initialized twice, in an odd
edge case.
Most of the cases work out right now -- when a client drops before sending
their init signal, we are fine. When the host transfers control on their turn,
there are functions "maybe_do_init" that seem to try to check if they should
init again or not:
https://github.com/wesnoth/wesnoth-old/blob/master/src/playturn.cpp#L296
(But I'm not sure if that's the right place to do that, as the side will still
get reinitialized anyway if it goes to a human in playmp_controller.)
Right now I think that the bad scenario is probably that when the host leaves
and a new host is elected, that player immediately becomes the human
controller of the abandoned sides, and issues init sides commands. Then they
see the dialog to decide what to do with the sides, and if they give them to
another player then most likely that player will also initialize them.
I'm not sure if it will fix the bug, but I'm going to change the system so
that 1.) abandoned sides are idle rather than controlled by the host, 2.) idle
sides are not initialized on the start of their turn.
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