os::net: expanding TcpStreamExt for Linux with tcp_deferaccept.

allows for socket to process only when there is data to process,
the option sets a number of seconds until the data is ready.

library/std/src/os/net/linux_ext/tcp.rs

@@ -53,6 +53,51 @@ pub trait TcpStreamExt: Sealed {
     /// ```
     #[unstable(feature = "tcp_quickack", issue = "96256")]
     fn quickack(&self) -> io::Result<bool>;
+
+    /// A socket listener will be awakened solely when data arrives.
+    ///
+    /// The `accept` argument set the delay in seconds until the
+    /// data is available to read, reducing the number of short lived
+    /// connections without data to process.
+    /// Contrary to other platforms `SO_ACCEPTFILTER` feature equivalent, there is
+    /// no necessity to set it after the `listen` call.
+    ///
+    /// See [`man 7 tcp`](https://man7.org/linux/man-pages/man7/tcp.7.html)
+    ///
+    /// # Examples
+    ///
+    /// ```no run
+    /// #![feature(tcp_deferaccept)]
+    /// use std::net::TcpStream;
+    /// use std::os::linux::net::TcpStreamExt;
+    ///
+    /// let stream = TcpStream::connect("127.0.0.1:8080")
+    ///         .expect("Couldn't connect to the server...");
+    /// stream.set_deferaccept(1).expect("set_deferaccept call failed");
+    /// ```
+    #[unstable(feature = "tcp_deferaccept", issue = "119639")]
+    #[cfg(target_os = "linux")]
+    fn set_deferaccept(&self, accept: u32) -> io::Result<()>;
+
+    /// Gets the accept delay value (in seconds) of the `TCP_DEFER_ACCEPT` option.
+    ///
+    /// For more information about this option, see [`TcpStreamExt::set_deferaccept`].
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(tcp_deferaccept)]
+    /// use std::net::TcpStream;
+    /// use std::os::linux::net::TcpStreamExt;
+    ///
+    /// let stream = TcpStream::connect("127.0.0.1:8080")
+    ///         .expect("Couldn't connect to the server...");
+    /// stream.set_deferaccept(1).expect("set_deferaccept call failed");
+    /// assert_eq!(stream.deferaccept().unwrap_or(0), 1);
+    /// ```
+    #[unstable(feature = "tcp_deferaccept", issue = "119639")]
+    #[cfg(target_os = "linux")]
+    fn deferaccept(&self) -> io::Result<u32>;
 }
 
 #[unstable(feature = "tcp_quickack", issue = "96256")]
@@ -67,4 +112,14 @@ impl TcpStreamExt for net::TcpStream {
     fn quickack(&self) -> io::Result<bool> {
         self.as_inner().as_inner().quickack()
     }
+
+    #[cfg(target_os = "linux")]
+    fn set_deferaccept(&self, accept: u32) -> io::Result<()> {
+        self.as_inner().as_inner().set_deferaccept(accept)
+    }
+
+    #[cfg(target_os = "linux")]
+    fn deferaccept(&self) -> io::Result<u32> {
+        self.as_inner().as_inner().deferaccept()
+    }
 }

library/std/src/os/net/linux_ext/tests.rs

@@ -26,3 +26,29 @@ fn quickack() {
     t!(stream.set_quickack(false));
     assert_eq!(false, t!(stream.quickack()));
 }
+
+#[test]
+#[cfg(target_os = "linux")]
+fn deferaccept() {
+    use crate::{
+        net::{test::next_test_ip4, TcpListener, TcpStream},
+        os::net::linux_ext::tcp::TcpStreamExt,
+    };
+
+    macro_rules! t {
+        ($e:expr) => {
+            match $e {
+                Ok(t) => t,
+                Err(e) => panic!("received error for `{}`: {}", stringify!($e), e),
+            }
+        };
+    }
+
+    let addr = next_test_ip4();
+    let _listener = t!(TcpListener::bind(&addr));
+    let stream = t!(TcpStream::connect(&("localhost", addr.port())));
+    stream.set_deferaccept(1).expect("set_deferaccept failed");
+    assert_eq!(stream.deferaccept().unwrap(), 1);
+    stream.set_deferaccept(0).expect("set_deferaccept failed");
+    assert_eq!(stream.deferaccept().unwrap(), 0);
+}

library/std/src/sys/pal/unix/net.rs

@@ -441,6 +441,18 @@ impl Socket {
         Ok(raw != 0)
     }
 
+    // bionic libc makes no use of this flag
+    #[cfg(target_os = "linux")]
+    pub fn set_deferaccept(&self, accept: u32) -> io::Result<()> {
+        setsockopt(self, libc::IPPROTO_TCP, libc::TCP_DEFER_ACCEPT, accept as c_int)
+    }
+
+    #[cfg(target_os = "linux")]
+    pub fn deferaccept(&self) -> io::Result<u32> {
+        let raw: c_int = getsockopt(self, libc::IPPROTO_TCP, libc::TCP_DEFER_ACCEPT)?;
+        Ok(raw as u32)
+    }
+
     #[cfg(any(target_os = "android", target_os = "linux",))]
     pub fn set_passcred(&self, passcred: bool) -> io::Result<()> {
         setsockopt(self, libc::SOL_SOCKET, libc::SO_PASSCRED, passcred as libc::c_int)