What happens to abandoned coroutine in Lua

Question

I wonder what happens with the coroutine in this example:

function powersOfTwo(i)
    coroutine.yield(i)
    powersOfTwo(2*i)
end

function iterator()
    return coroutine.wrap(function () powersOfTwo(1) end)
end

for power in iterator() do
    print(power)
    if power > 1000 then break end
end

The coroutine does not finish its code. Is it garbage collected?

Answer 1

The coroutine does not finish its code. Is it garbage collected?

Yes, it is garbage-collected as there are no references to the coroutine left after the loop has finished execution. The fact that this involves a coroutine doesn't make a difference; if iterator() returned a closure, it would be garbage-collected all the same.

You can verify this by using collectgarbage:

-- Declare functions, which won't be garbage-collected as they're in the global table
function collectgarbage_full()
    for _ = 1, 10 do -- Run multiple cycles to ensure that all garbage is collected even if the generational GC of Lua 5.2 or later is used
        collectgarbage"collect"
    end
end
function powersOfTwo(i)
    coroutine.yield(i)
    powersOfTwo(2*i)
end
function iterator()
    return coroutine.wrap(function() powersOfTwo(1) end)
end
-- Count non-garbage used memory
collectgarbage_full()
local used = collectgarbage"count"
-- Now use the iterator, creating a garbage coroutine
for power in iterator() do
    print(power)
    if power > 1000 then break end
end
-- Collect garbage, compare used memory against used memory before the garbage coroutine
collectgarbage_full()
if used < collectgarbage"count" then
    error"Garbage coroutine was not collected!"
end
print"Coroutine was garbage-collected."

You should see the following output:

1
2
4
8
16
32
64
128
256
512
1024
Coroutine was garbage-collected.