os/unix/stream: stop into_raw_fd from closing the fd

`UnixStream::into_raw_fd` calls `as_raw_fd`, which doesn't take the
ownership of the file descriptor, so the file descriptor is closed when
`self` is dropped upon returning from the function.

Because `UnixStream` uses a `Arc` to support Clone, there could be an
arbitrary number of instances around. We cannot take ownership of the
descriptor from all of the instances. Therefore we have no choice but to
duplicate the file descriptor and return that.

Fixes #855

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://book.async.rs/overview
 
 ## [Unreleased]
 
+## Fixed
+
+- Ensure `UnixStream::into_raw_fd` doesn't close the file descriptor ([#855](https://github.com/async-rs/async-std/issues/855))
+
 # [1.6.3] - 2020-07-31
 
 ## Added

src/os/unix/net/stream.rs

@@ -252,6 +252,6 @@ impl FromRawFd for UnixStream {
 
 impl IntoRawFd for UnixStream {
     fn into_raw_fd(self) -> RawFd {
-        self.as_raw_fd()
+        (*self.watcher).get_ref().try_clone().unwrap().into_raw_fd()
     }
 }