Is it possible to deadlock memory in Lua like this?

Question

Both of these tables have references to each other, but neither are referenced anywhere else outside of the function. Will these tables remain in memory after a call to f() or will they be garbage collected?

function f()
     local t1 = {}
     local t2 = {}
     t1[1] = t2
     t2[1] = t1
end

f()

Answer 1

Lua's garbage collection checks for reachability rather than just using reference counting, so circular references will not prevent collection. Here's a modified version of your program to demonstrate this:

function f()
     local t1 = setmetatable({}, {__gc = function() print "Collected t1" end})
     local t2 = setmetatable({}, {__gc = function() print "Collected t2" end})
     t1[1] = t2
     t2[1] = t1
end

f()
print "Before collectgarbage()"
collectgarbage()
print "After collectgarbage()"

The result I get:

Before collectgarbage()
Collected t2
Collected t1
After collectgarbage()

Note that the __gc metamethod on tables is new to Lua 5.2. If you want to try this demo on older versions of Lua, you'll need to make use of newproxy or otherwise acquire a userdata to use instead.