iterating tables using for loops wrapped in coroutines

Hi,

I'm trying to wrap my head around how to use coroutines properly.
A function in my program iterates over a directory and inserts all found files into a
table. Next, I need to do some stuff with this table, and iterate over it to insert the files into an SQL database.

--FUNCTIONAL NON-COROUTINE VERSION

local D,P=Database,ProgressBar
local Files=IterateDir() --returns the table

if D:transaction() then --prepare database for transaction
P:setMaximum(#Files) --set the progressbar maximum value to the amount of files in the table
for FileName,Extension in pairs(Files)do
D:exec('insert into Files("'..Filename..'","'..Extension..'")') --execute SQL query
P:setValue(ProgressBar:value()+1) --step the progressbar
end
if D:commit() then --commit the database
print'Operation finished, database updated succesfully'
P:reset() --reset the progressbar
end
end

Using some examples I've found on the internet, I'm trying to wrap the loop in a coroutine, however I'm not quite sure
how to implement coroutine.yield() in this case (hence Lua returns the error 'cannot resume dead coroutine')

--DYSFUNCTIONAL COROUTINE VERSION

local D,P=Database,ProgressBar
local Files=IterateDir() --returns the table

local Routine=coroutine.wrap(function(FileName,Extension)
D:exec('insert into Files("'..Filename..'","'..Extension..'")') --execute SQL query
P:setValue(ProgressBar:value()+1) --step the progressbar
coroutine.yield()
end)


if D:transaction() then --prepare database for transaction
P:setMaximum(#Files) --set the progressbar maximum value to the amount of files in the table

for FileName,Exension in pairs(Files)do Routine(File,Extension)end

if D:commit() then --commit the database
print'Operation finished, database updated succesfully'
P:reset() --reset the progressbar
end

end

Can somebody help me get the second version working?

Ultimately, I would like to use true OS threads but as I understand it I need to wait until LuaJIT FFI supports callbacks, or I need
to use an additional module (I would like to keep dependencies to a minimum)

Thanks in advance

Ralf