LuaSurface points to a surface at index, and not to unique surface

[Stringweasel]

What

A variable holding a LuaSurface does not point to that unique created surface, but instead seems to point to whichever surface has that specific index. For example if a the variable's surface is destroyed, and a new different surface created later, it might have the same index. And in that case the original variable will point to the newer surface.

This can easily be seen using the following commands (on a fresh vanilla save). Create initial surface

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/c foo = game.create_surface("foo")
/c game.print("Foo: "..(foo.valid and foo.name.." "..foo.index or "invalid"))

this will print "Foo: foo 2" as expected.

Now delete the surface

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/c game.delete_surface(foo)
/c game.print("Foo: "..(foo.valid and foo.name.." "..foo.index or "invalid"))

which will then of course print "Foo: invalid". Meaning a variable that was previously invalid became valid again.

Now for the unintuitive part, create what the modder might believe is a different surface

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/c bar = game.create_surface("bar")
/c game.print("Foo: "..(foo.valid and foo.name.." "..foo.index or "invalid"))

This will print: "Foo: bar 2"

Why

Modders can incorrectly assume (like I did ) that a LuaSurface acts just like a LuaEntity, and when destroyed it will remain invalid. And this can create hard to track down cache-invalidation bugs (like one I caused ). I know this is probably due to how the underlying C++ surface implementation works and is probably not a bug. Might just be useful to explicitly state it in the docs, especially since mods are really starting to embrace the multiple surfaces.

[boskid]

You are right, LuaSurface uses SurfaceIndex for binding and grabs surface by index every time surface is needed. For 2.0 i am changing LuaSurface to use a targeter to surface which will change this behavior to the one you expected: when a surface is deleted, LuaSurface that was pointing at it will become invalid permanently.

[Stringweasel]

Great! That would definitely make it easier to work with.

Does this mean indexes will always be unique too, like unit numbers? Or will they still be re-used? It would be nice if they were unique to use as an identifier. Can't use the surface name because that can change.

[boskid]

Does this mean indexes will always be unique too, like unit numbers? Or will they still be re-used? It would be nice if they were unique to use as an identifier. Can't use the surface name because that can change.

No. It means that LuaSurface will get invalid permanently when surface gets deleted. I am not touching surface indexes logic. There is a vector of surfaces indexed by surface index so in order for this vector to not grow unreasonably when surfaces are created and destroyed repeatedly, indexes are reused.

[darkfrei]

Why the event has surface index, not surface name?

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on_post_entity_died ( event)
	local surface_index = event.surface_index
	local surface_name = game.surfaces[surface_index].name
	game.print ('surfaceindex: '..surface_index..'; surface name: '..surface_name)
end

surface_index_surface_name.png (31.28 KiB) Viewed 82 times