Add tcp_listenfd_server example with target_os = "wasi" support

Use the `LISTEN_FDS` mechanism to use pre-opened sockets.

Especially for `wasm32-wasi` there is no other way to get access to sockets,
than to use pre-opened sockets.

Because `wasm32-wasi` does not yet return `TcpListener::local_addr()`, an
unspecified IP address and port will be returned and displayed.

```
$ cargo +nightly build --target wasm32-wasi --release --example tcp_listenfd_server --features="os-poll net"
   Compiling cfg-if v1.0.0
   Compiling wasi v0.10.2+wasi-snapshot-preview1
   Compiling log v0.4.14
   Compiling libc v0.2.112
   Compiling ppv-lite86 v0.2.15
   Compiling wasi v0.11.0+wasi-snapshot-preview1
   Compiling getrandom v0.2.3
   Compiling rand_core v0.6.3
   Compiling env_logger v0.8.4
   Compiling rand_chacha v0.3.1
   Compiling mio v0.8.0 (/home/harald/git/mio)
   Compiling rand v0.8.4
    Finished release [optimized] target(s) in 2.92s

$ wasmtime run --tcplisten 127.0.0.1:9000 --env 'LISTEN_FDS=1' target/wasm32-wasi/release/examples/tcp_listenfd_server.wasm
Using preopened socket FD 3
You can connect to the server using `nc`:
 $ nc <IP> <PORT>
You'll see our welcome message and anything you type will be printed here.
```

Signed-off-by: Harald Hoyer <harald@profian.com>

Cargo.toml

@@ -80,6 +80,10 @@ features = ["os-poll", "os-ext", "net"]
 name = "tcp_server"
 required-features = ["os-poll", "net"]
 
+[[example]]
+name = "tcp_listenfd_server"
+required-features = ["os-poll", "net"]
+
 [[example]]
 name = "udp_server"
 required-features = ["os-poll", "net"]

examples/tcp_listenfd_server.rs

@@ -0,0 +1,210 @@
+// You can run this example from the root of the mio repo:
+// cargo run --example tcp_listenfd_server --features="os-poll net"
+// or with wasi:
+// cargo +nightly build --target wasm32-wasi  --example tcp_listenfd_server --features="os-poll net"
+// wasmtime run --tcplisten 127.0.0.1:9000 --env 'LISTEN_FDS=1' target/wasm32-wasi/debug/examples/tcp_server.wasm
+
+use mio::event::Event;
+use mio::net::{TcpListener, TcpStream};
+use mio::{Events, Interest, Poll, Registry, Token};
+use std::collections::HashMap;
+use std::io::{self, Read, Write};
+use std::str::from_utf8;
+
+// Setup some tokens to allow us to identify which event is for which socket.
+const SERVER: Token = Token(0);
+
+// Some data we'll send over the connection.
+const DATA: &[u8] = b"Hello world!\n";
+
+#[cfg(not(windows))]
+fn get_first_listen_fd_listener() -> Option<std::net::TcpListener> {
+    #[cfg(unix)]
+    use std::os::unix::io::FromRawFd;
+    #[cfg(target_os = "wasi")]
+    use std::os::wasi::io::FromRawFd;
+
+    let stdlistener = unsafe { std::net::TcpListener::from_raw_fd(3) };
+    stdlistener.set_nonblocking(true).unwrap();
+    Some(stdlistener)
+}
+
+#[cfg(windows)]
+fn get_first_listen_fd_listener() -> Option<std::net::TcpListener> {
+    // Windows does not support `LISTEN_FDS`
+    None
+}
+
+fn main() -> io::Result<()> {
+    env_logger::init();
+
+    std::env::var("LISTEN_FDS").expect("LISTEN_FDS environment variable unset");
+
+    // Create a poll instance.
+    let mut poll = Poll::new()?;
+    // Create storage for events.
+    let mut events = Events::with_capacity(128);
+
+    // Setup the TCP server socket.
+    let mut server = {
+        let stdlistener = get_first_listen_fd_listener().unwrap();
+        stdlistener.set_nonblocking(true)?;
+        println!("Using preopened socket FD 3");
+        println!("You can connect to the server using `nc`:");
+        match stdlistener.local_addr() {
+            Ok(a) => println!(" $ nc {} {}", a.ip(), a.port()),
+            Err(_) => println!(" $ nc <IP> <PORT>"),
+        }
+        println!("You'll see our welcome message and anything you type will be printed here.");
+        TcpListener::from_std(stdlistener)
+    };
+
+    // Register the server with poll we can receive events for it.
+    poll.registry()
+        .register(&mut server, SERVER, Interest::READABLE)?;
+
+    // Map of `Token` -> `TcpStream`.
+    let mut connections = HashMap::new();
+    // Unique token for each incoming connection.
+    let mut unique_token = Token(SERVER.0 + 1);
+
+    loop {
+        poll.poll(&mut events, None)?;
+
+        for event in events.iter() {
+            match event.token() {
+                SERVER => loop {
+                    // Received an event for the TCP server socket, which
+                    // indicates we can accept an connection.
+                    let (mut connection, address) = match server.accept() {
+                        Ok((connection, address)) => (connection, address),
+                        Err(e) if e.kind() == io::ErrorKind::WouldBlock => {
+                            // If we get a `WouldBlock` error we know our
+                            // listener has no more incoming connections queued,
+                            // so we can return to polling and wait for some
+                            // more.
+                            break;
+                        }
+                        Err(e) => {
+                            // If it was any other kind of error, something went
+                            // wrong and we terminate with an error.
+                            return Err(e);
+                        }
+                    };
+
+                    println!("Accepted connection from: {}", address);
+
+                    let token = next(&mut unique_token);
+                    poll.registry()
+                        .register(&mut connection, token, Interest::WRITABLE)?;
+
+                    connections.insert(token, connection);
+                },
+                token => {
+                    // Maybe received an event for a TCP connection.
+                    let done = if let Some(connection) = connections.get_mut(&token) {
+                        handle_connection_event(poll.registry(), connection, event)?
+                    } else {
+                        // Sporadic events happen, we can safely ignore them.
+                        false
+                    };
+                    if done {
+                        if let Some(mut connection) = connections.remove(&token) {
+                            poll.registry().deregister(&mut connection)?;
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
+fn next(current: &mut Token) -> Token {
+    let next = current.0;
+    current.0 += 1;
+    Token(next)
+}
+
+/// Returns `true` if the connection is done.
+fn handle_connection_event(
+    registry: &Registry,
+    connection: &mut TcpStream,
+    event: &Event,
+) -> io::Result<bool> {
+    if event.is_writable() {
+        // We can (maybe) write to the connection.
+        match connection.write(DATA) {
+            // We want to write the entire `DATA` buffer in a single go. If we
+            // write less we'll return a short write error (same as
+            // `io::Write::write_all` does).
+            Ok(n) if n < DATA.len() => return Err(io::ErrorKind::WriteZero.into()),
+            Ok(_) => {
+                // After we've written something we'll reregister the connection
+                // to only respond to readable events.
+                registry.reregister(connection, event.token(), Interest::READABLE)?
+            }
+            // Would block "errors" are the OS's way of saying that the
+            // connection is not actually ready to perform this I/O operation.
+            Err(ref err) if would_block(err) => {}
+            // Got interrupted (how rude!), we'll try again.
+            Err(ref err) if interrupted(err) => {
+                return handle_connection_event(registry, connection, event)
+            }
+            // Other errors we'll consider fatal.
+            Err(err) => return Err(err),
+        }
+    }
+
+    if event.is_readable() {
+        let mut connection_closed = false;
+        let mut received_data = vec![0; 4096];
+        let mut bytes_read = 0;
+        // We can (maybe) read from the connection.
+        loop {
+            match connection.read(&mut received_data[bytes_read..]) {
+                Ok(0) => {
+                    // Reading 0 bytes means the other side has closed the
+                    // connection or is done writing, then so are we.
+                    connection_closed = true;
+                    break;
+                }
+                Ok(n) => {
+                    bytes_read += n;
+                    if bytes_read == received_data.len() {
+                        received_data.resize(received_data.len() + 1024, 0);
+                    }
+                }
+                // Would block "errors" are the OS's way of saying that the
+                // connection is not actually ready to perform this I/O operation.
+                Err(ref err) if would_block(err) => break,
+                Err(ref err) if interrupted(err) => continue,
+                // Other errors we'll consider fatal.
+                Err(err) => return Err(err),
+            }
+        }
+
+        if bytes_read != 0 {
+            let received_data = &received_data[..bytes_read];
+            if let Ok(str_buf) = from_utf8(received_data) {
+                println!("Received data: {}", str_buf.trim_end());
+            } else {
+                println!("Received (none UTF-8) data: {:?}", received_data);
+            }
+        }
+
+        if connection_closed {
+            println!("Connection closed");
+            return Ok(true);
+        }
+    }
+
+    Ok(false)
+}
+
+fn would_block(err: &io::Error) -> bool {
+    err.kind() == io::ErrorKind::WouldBlock
+}
+
+fn interrupted(err: &io::Error) -> bool {
+    err.kind() == io::ErrorKind::Interrupted
+}