epoll_wait SIGSTOP/SIGCONT - attempt to call nil value

[houseofkodai]

when SIGSTOP CTRL+Z is given to a running program and SIGCONT (bg/fg) is given - epoll_wait goes for a toss - groks with "attempt to call a nil value"

replicable by running the following code - hitting CTRL+Z and then bg/fg from shell

local S = require 'syscall'

local ep = S.epoll_create()
ep:epoll_ctl('add', 0, 'in') --stdin
local epevents = S.t.epoll_events(2)
local epwait = function(timeout) return ep:epoll_wait(epevents, timeout) end 
while true do
  for i,evnt in epwait() do
    if evnt.IN then io.read() end
  end
end

[justincormack]

The issue is the slightly non intuitive iterator interface of epoll_wait (and some other similar functions). Maybe I should change this, but I am not sure how best to. The following works:

local S = require 'syscall'

local ep = S.epoll_create()
ep:epoll_ctl('add', 0, 'in') --stdin
local epevents = S.t.epoll_events(2)
local epwait = function(timeout) return ep:epoll_wait(epevents, timeout) end 
while true do
  local f, a, r = epwait()
  if f then
    for i,evnt in f, a, r do
      if evnt.IN then io.read() end
    end
  else print(a) end
end

The issue is if epoll_wait gets an error it returns nil, error (like all other syscalls do), but this is not a valid iterator (which must start with a function, hence the calling nil). You could wrap it to return an iterator that just terminates if you dont really care about errors, like this:

local S = require 'syscall'

local ep = S.epoll_create()
ep:epoll_ctl('add', 0, 'in') --stdin
local epevents = S.t.epoll_events(2)
local epwait = function(timeout)
  local function nilf() return nil end
  local f, a, r = ep:epoll_wait(epevents, timeout)
  if f then return f, a, r else return nilf end
end 
while true do
  for i,evnt in epwait() do
    if evnt.IN then io.read() end
  end
end

[houseofkodai]

Thanks for the prompt response - this works.

Yes the iterator logic is a bit confusing; but the lack of sample code made discovering its' proper usage more difficult.

May I suggest that this logic be used in the sample/testing code (epoll.lua) - which is what I referred to first, and I'm guessing that others would too. of course, the next thing I did was search the issues and posted this, since I did not find something similar.

• ka