Moritz Kiefer authored Dec 12, 2019 and Marge Bot committed Dec 19, 2019

In our codebase we have some code along the lines of

```
newStdout <- hDuplicate stdout
stderr `hDuplicateTo` stdout
```

to avoid stray `putStrLn`s from corrupting a protocol (LSP) that is
run over stdout.

On CI we have seen a bunch of issues where `dup2` returned `EBUSY` so
this fails with `ResourceExhausted` in Haskell.

I’ve spent some time looking at the docs for `dup2` and the code in
`base` and afaict the following race condition is being triggered
here:

1. The user calls `hDuplicateTo stderr stdout`.
2. `hDuplicateTo` calls `hClose_help stdout_`, this closes the file
handle for stdout.
3. The file handle for stdout is now free, so another thread
allocating a file might get stdout.
4. If `dup2` is called while `stdout` (now pointing to something
else) is half-open, it returns EBUSY.

I think there might actually be an even worse case where `dup2` is run
after FD 1 is fully open again. In that case, you will end up not just
redirecting the original stdout to stderr but also the whatever
resulted in that file handle being allocated.

As far as I can tell, `dup2` takes care of closing the file handle
itself so there is no reason to do this in `hDuplicateTo`. So this PR
replaces the call to `hClose_help` by the only part of `hClose_help`
that we actually care about, namely, `flushWriteBuffer`.

I tested this on our codebase fairly extensively and haven’t been able
to reproduce the issue with this patch.