Add accept_from returning remote address

Author: gnaaman-dn

changelog/2609.added.md

@@ -0,0 +1 @@
+Add `accept_from` function returning the remote-address.

src/sys/socket/mod.rs

@@ -2364,6 +2364,41 @@ pub fn accept4(sockfd: RawFd, flags: SockFlag) -> Result<RawFd> {
     Errno::result(res)
 }
 
+/// Accept a connection on a socket, returning the remote address.
+///
+/// [Further reading](https://man7.org/linux/man-pages/man2/accept.2.html)
+#[cfg(any(
+    all(
+        target_os = "android",
+        any(
+            target_arch = "aarch64",
+            target_arch = "x86",
+            target_arch = "x86_64"
+        )
+    ),
+    freebsdlike,
+    netbsdlike,
+    target_os = "emscripten",
+    target_os = "fuchsia",
+    solarish,
+    target_os = "linux",
+))]
+pub fn accept_from<S: SockaddrLike>(sockfd: RawFd, flags: SockFlag) -> Result<(RawFd, Option<S>)> {
+    let mut storage = std::mem::MaybeUninit::<libc::sockaddr_storage>::uninit();
+    let mut socklen = std::mem::size_of::<libc::sockaddr_storage>() as libc::socklen_t;
+    let res = unsafe {
+        libc::accept4(sockfd, storage.as_mut_ptr().cast(), &mut socklen as *mut _, flags.bits())
+    };
+
+    let sock = Errno::result(res)?;
+    let addr = unsafe {
+        let storage = storage.assume_init();
+        S::from_raw((&storage as *const libc::sockaddr_storage).cast(), Some(socklen))
+    };
+
+    Ok((sock, addr))
+}
+
 /// Initiate a connection on a socket
 ///
 /// [Further reading](https://pubs.opengroup.org/onlinepubs/9699919799/functions/connect.html)

test/sys/test_socket.rs

@@ -3161,3 +3161,42 @@ fn can_open_routing_socket() {
         socket(AddressFamily::Route, SockType::Raw, SockFlag::empty(), None)
             .expect("Failed to open routing socket");
 }
+
+// cfg needed for capability check.
+#[cfg(linux_android)]
+#[test]
+fn test_accept_from() {
+    use nix::sys::socket::{accept_from, bind, connect, listen, socket};
+    use nix::sys::socket::{Backlog, SockFlag, SockType, SockaddrIn};
+    use std::net::Ipv4Addr;
+
+    let sock_addr = SockaddrIn::from_str("127.0.0.1:6780").unwrap();
+    let listener = socket(
+        AddressFamily::Inet,
+        SockType::Stream,
+        SockFlag::empty(),
+        None,
+    )
+    .expect("listener socket failed");
+    bind(listener.as_raw_fd(), &sock_addr).expect("bind failed");
+    listen(&listener, Backlog::MAXCONN).expect("listen failed");
+
+    let connector = socket(
+        AddressFamily::Inet,
+        SockType::Stream,
+        SockFlag::empty(),
+        None,
+    )
+    .expect("connector socket failed");
+
+    let send_thread =
+        std::thread::spawn(move || connect(connector.as_raw_fd(), &sock_addr));
+    let (_peer, address) =
+        accept_from::<SockaddrIn>(listener.as_raw_fd(), SockFlag::empty())
+            .unwrap();
+    let address = address.expect("no address");
+
+    assert_eq!(address.ip(), Ipv4Addr::LOCALHOST);
+
+    send_thread.join().unwrap().unwrap();
+}