net: provide NamedPipe{Client, Server} types and builders (#3760)

This builds on https://github.com/tokio-rs/mio/pull/1351 and introduces the
tokio::net::windows::named_pipe module which provides low level types for
building and communicating asynchronously over windows named pipes.

Named pipes require the `net` feature flag to be enabled on Windows.

Co-authored-by: Alice Ryhl <[email protected]>

examples/named-pipe.rs

@@ -0,0 +1,60 @@
+use std::io;
+
+#[cfg(windows)]
+async fn windows_main() -> io::Result<()> {
+    use tokio::io::AsyncWriteExt;
+    use tokio::io::{AsyncBufReadExt, BufReader};
+    use tokio::net::windows::named_pipe::{ClientOptions, ServerOptions};
+
+    const PIPE_NAME: &str = r"\\.\pipe\named-pipe-single-client";
+
+    let server = ServerOptions::new().create(PIPE_NAME)?;
+
+    let server = tokio::spawn(async move {
+        // Note: we wait for a client to connect.
+        server.connect().await?;
+
+        let mut server = BufReader::new(server);
+
+        let mut buf = String::new();
+        server.read_line(&mut buf).await?;
+        server.write_all(b"pong\n").await?;
+        Ok::<_, io::Error>(buf)
+    });
+
+    let client = tokio::spawn(async move {
+        // There's no need to use a connect loop here, since we know that the
+        // server is already up - `open` was called before spawning any of the
+        // tasks.
+        let client = ClientOptions::new().open(PIPE_NAME)?;
+
+        let mut client = BufReader::new(client);
+
+        let mut buf = String::new();
+        client.write_all(b"ping\n").await?;
+        client.read_line(&mut buf).await?;
+        Ok::<_, io::Error>(buf)
+    });
+
+    let (server, client) = tokio::try_join!(server, client)?;
+
+    assert_eq!(server?, "ping\n");
+    assert_eq!(client?, "pong\n");
+
+    Ok(())
+}
+
+#[tokio::main]
+async fn main() -> io::Result<()> {
+    #[cfg(windows)]
+    {
+        windows_main().await?;
+    }
+
+    #[cfg(not(windows))]
+    {
+        println!("Named pipes are only supported on Windows!");
+    }
+
+    Ok(())
+}