Add support for getting SO_COOKIE socket option on linux

Signed-off-by: Benjamin Leggett <benjamin.leggett@solo.io>

src/sys/unix.rs

@@ -2418,6 +2418,18 @@ impl crate::Socket {
         unsafe { setsockopt(self.as_raw(), libc::SOL_SOCKET, libc::SO_DETACH_FILTER, 0) }
     }
 
+    /// Gets the value for the `SO_COOKIE` option on this socket.
+    ///
+    /// The socket cookie is a unique, kernel-managed identifier tied to each socket.
+    /// Therefore, there is no corresponding `set` helper.
+    ///
+    /// For more information about this option, see [Linux patch](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5daab9db7b65df87da26fd8cfa695fb9546a1ddb)
+    #[cfg(all(feature = "all", any(target_os = "linux")))]
+    #[cfg_attr(docsrs, doc(cfg(all(feature = "all", any(target_os = "linux")))))]
+    pub fn cookie(&self) -> io::Result<u64> {
+        unsafe { getsockopt::<libc::c_ulonglong>(self.as_raw(), libc::SOL_SOCKET, libc::SO_COOKIE) }
+    }
+
     /// Get the value of the `IPV6_TCLASS` option for this socket.
     ///
     /// For more information about this option, see [`set_tclass_v6`].

tests/socket.rs

@@ -1528,3 +1528,21 @@ fn dccp() {
     let mut recv_buf = [0_u8; 64];
     assert!(accepted.read(&mut recv_buf).unwrap() == msg.len());
 }
+
+#[test]
+#[cfg(all(feature = "all", target_os = "linux"))]
+fn cookie() {
+    let socket = Socket::new(Domain::IPV4, Type::STREAM, None).unwrap();
+    let first_socket_cookie = socket.cookie();
+    match first_socket_cookie {
+        Ok(_) => {}
+        Err(err) => panic!("Could not get socket cookie, err: {err}"),
+    }
+
+    //Fetch cookie again and make sure it's the same value (ALWAYS should be, smoke test)
+    let second_socket_cookie = socket.cookie();
+    match second_socket_cookie {
+        Ok(cookie) => assert_eq!(cookie, first_socket_cookie.unwrap()),
+        Err(err) => panic!("Could not get socket cookie a second time, err: {err}"),
+    }
+}