Segfault when city removal causes boat displacement and unit removed with Lua during unit_moved event

[XHawk87]

Describe the bug
The server crashes if a city is disbanded, for example by migration, and the city contained a boat which was forced to move off of the city tile, and during the unit_moved Lua event the boat is destroyed.

To Reproduce
Steps to reproduce the behaviour:

1. Load the saved game
2. Start the game
3. See crash

Expected behaviour
No crash.

Screenshots

Platform and version (please complete the following information):

• OS: Linux
• Freeciv21 version: 3.1-rc.1
• Ruleset/Longturn game (if applicable): Chaos

Additional context
Discord discussion

[XHawk87]

I'm looking into this bug. I found that it's possible to use bounce_unit() for all but one feature of the custom bounce code, displaying a city tile link for the disbanded city.

freeciv21/server/citytools.cpp

Line 1688 in 74984fa

const char *ctl = city_tile_link(pcity);

freeciv21/server/citytools.cpp

Lines 1748 to 1753 in 74984fa

	notify_player(unit_owner(punit), tile1, E_UNIT_RELOCATED,
	ftc_server,
	_("Moved %s out of disbanded city %s "
	"since it cannot stay on %s."),
	unit_link(punit), ctl,
	terrain_name_translation(tile_terrain(pcenter)));

How would you go about solving this? Should I pass in some extra data with the bounce_reason somehow?

freeciv21/server/unittools.h

Lines 108 to 115 in 74984fa

	/// Why do we need to bounce a unit?
	enum class bounce_reason {
	generic, ///< We just need to do it
	terrain_change, ///< We need to do it because of changing terrain
	};
	void bounce_unit(struct unit *punit, bool verbose,
	bounce_reason reason = bounce_reason::generic,
	int max_distance = 2);

[XHawk87]

Actually, single-responsibility. I don't think bounce_unit() should also be trying to communicate, it should just be bouncing the unit and reporting success or failure. We should have something else that's solely responsible for communicating the outcome.

[XHawk87]

See related refactor #2478

[XHawk87]

See another related refactor #2481