Fix memory leak in handleToFd, fixes issue #4

[hasufell]

From comment #4 (comment)

I'm not really sure this is the correct thing to do.

@rwbarton @hs-viktor

[Bodigrim]

You probably want to replace Safe with Trustworthy, otherwise downstream consumers will complain.
(I personally have zero sympathy to Safe Haskell, but such changes do break other packages in a very frustrating way)

[hs-viktor]

Doesn't this change lose any unwritten data in the write buffer? It used to be flushed, but now is just dropped?

With hClose this happens before the call to hClose_handle_ handle_ (after which the above code is modelled):

hClose_help :: Handle__ -> IO (Handle__, Maybe SomeException)
hClose_help handle_ =
  case haType handle_ of
      ClosedHandle -> return (handle_,Nothing)
      _ -> do mb_exc1 <- trymaybe $ flushWriteBuffer handle_ -- interruptible
                    -- it is important that hClose doesn't fail and
                    -- leave the Handle open (#3128), so we catch
                    -- exceptions when flushing the buffer.
              (h_, mb_exc2) <- hClose_handle_ handle_
              return (h_, if isJust mb_exc1 then mb_exc1 else mb_exc2)

[hasufell]

I'm really not too sure about this change. I think it's not necessary for 2.8.0.0, since people have lived with it for over a decade.

[hs-viktor]

If the memory leak can be safely avoided, I'm inclined to fix anyway, but replacing flush with just dropping the buffer is not correct. Otherwise, I don't see any issues.

[hasufell]

@hs-viktor I'm not too familiar with this part of the code. Feel free to adjust this PR.

[hasufell]

Btw, I tried to reproduce the original bug and the only GHC I could reproduce it with is 8.0.2 (I didn't try older ones).

But with 8.0.2 is builds up pretty fast.

[hasufell]

Since current unix cannot be built with 8.0.2 I suggest to just close this as obsolete.

[hasufell]

Or we try to fix it for 2.7.2.3, but it won't be shipped with any affected GHC.

[Bodigrim]

I don't see any point in unix-2.7.2.3, let's just close the issue as non-reproducible.