Re: Coroutine bug?

[Lorenzo Donati <lorenzodonatibz@...>]

On 20/06/2011 6.08, Ian Godin wrote:
> I am trying to use threads/coroutines, but it looks like local
> variables are getting overwritten... it's behaving as if a single
> local variable exists, when I expected each coroutine to have it's own
> local variables.  The original was much more complicate (using the Lua
> C API, etc...), but I've managed to simplify it to a Lua-only script
> that reproduces the problem.  It's obviously just a toy example, but
> it replicates my problem.

[snip]

You forgot a "local" in front of function "check". This way "check" is a 
global function and you are redefining it every time you create a coroutine.

Remember that:

function check()
...
end

is just syntactic sugar for:

check = function()
...
end

and, without that "local" declaration, the variable "check" is global. 
Every time the statement "function check()" is executed, you assign a 
new closure (with obj as upvalue) to global var "check".

Therefore you overwrite "check" when the second coroutine is first 
started (second "resume"), so the "obj" upvalue for the first coroutine 
is lost together with the first closure assigned to "check".

When you resume the second coroutine for the second time (third call to 
resume), its execution resumes where it yielded (in the while loop), but 
then you call check (global), which retains the closure created _in the 
second coroutine_ (with ITS obj upvalue, i.e. that created in the second 
call to resume), so "check" prints out the data for the "obj" argument 
of the second coroutine.



-- Lorenzo