Memory leak in node

[sloosch]

Hi,

noticed that Aff-Bus is leaking slowly memory in a toy project i have. Here is a minimal working example which shows the leakage quite well (on my machine after 30sec 500MB and rising):

loopy :: BusRW Unit -> Aff _ Unit
loopy bus = do
  _ <- Bus.read bus
  _ <- forkAff $ loopy bus
  Bus.write unit bus
  
main :: forall e. Eff _ Unit
main = launchAff_ $ do
  bus <- Bus.make
  _ <- forkAff $ loopy bus
  Bus.write unit bus

I have only tested this with node (v8.11.1) not in a browser.

After playing around a bit, i noticed that the problem seems to be the traverse with Either here https://github.com/slamdata/purescript-aff-bus/blob/master/src/Control/Monad/Aff/Bus.purs#L64

Replacing the traverse with pattern-matching seems to fix the problem e.g.

attempt (takeVar cell) >>= case _ of
    Left _ -> pure unit
    Right res -> do ....

Can anyone confirm the leakage? Is this even a problem in Aff-Bus or is it Aff making problems?

[safareli]

@natefaubion might know if it's Aff related

[natefaubion]

Looping inside a traverse_ isn’t tail recursive (it’s equivalent to fix \loop -> loop *> pure unit) so it will continue to build continuations indefinitely. It’s unfortunately expected behavior.

[safareli]

so we should remove traverse_ and use case _ of or use forever which uses MonadRec:

launchAff_ $ forever $ attempt (takeVar cell) >>= traverse_ \res → do
  vars ← takeVar consumers
  putVar mempty consumers
  sequence_ (foldl (\xs a → putVar res a : xs) mempty vars)

update: actually forever is not fine as we want the loop to end if takeVar fails
update: if we remove attempt forever will be fine

[sloosch]

@safareli
Unrelated to the issue but maybe also consider replacing sequence_ (foldl (\xs a → putVar res a : xs) mempty vars) with traverse_ (putVar res) vars? Building an intermediate list of Affs seems unnecessary.

[natefaubion]

@sloosch It builds an intermediate list so that they are run in the order they were added. Reads are cons'ed. If we used traverse_ it would be LIFO instead FIFO.