Calling the callback when event registration fails

[Andrew Martin]

Digging through the different backends, it looks like only the kqueue
backend is even capable of returning False when modifyFd/modifyFdOnce is
called. This happens when kevent returns eINTR or eINVAL. Why do we call
the callback here instead of just throwing an error like we do in so many
other cases?

On Wed, Feb 6, 2019 at 5:44 PM Andrew Martin <andrew.thaddeus at gmail.com>
wrote:

> In the event manager's registerFd_, we find:
>
>     registerFd_ :: EventManager -> IOCallback -> Fd -> Event -> Lifetime
>                 -> IO (FdKey, Bool)
>     registerFd_ mgr@(EventManager{..}) cb !fd !evs lt = do
>       ... <- ...
>       (modify,ok) <- withMVar (callbackTableVar mgr fd) $ \tbl -> do
>         ... <- ...
>       -- this simulates behavior of old IO manager:
>       -- i.e. just call the callback if the registration fails.
>       when (not ok) (cb reg evs)
>       return (reg,modify)
>
> A comment and a question. Comment: registerFd_ is only ever called in
> contexts where exceptions are masked, so withMVar is doing some needless
> mask/restore. Question: why do we immidiately call the callback if event
> registration fails? This means that if event registration fails during
> something like `threadWaitRead`, the end result would be that
> `threadWaitRead` would just return immidiately. That doesn't seem good.
>
> --
> -Andrew Thaddeus Martin
>


-- 
-Andrew Thaddeus Martin
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