Add the TCP_FUNCTION_BLK socket option, on FreeBSD

Cargo.toml

@@ -107,3 +107,6 @@ path = "test/test_clearenv.rs"
 [[test]]
 name = "test-prctl"
 path = "test/sys/test_prctl.rs"
+
+[patch.crates-io]
+libc = { git = "https://github.com/asomers/libc.git", rev = "5344e1a71517b265e7c2fe14ada8cb6c56e152e0" }

changelog/2539.added.md

@@ -0,0 +1 @@
+Add the `TCP_FUNCTION_BLK` sockopt, on FreeBSD.

src/sys/socket/sockopt.rs

@@ -300,7 +300,18 @@ sockopt_impl!(
     libc::SO_REUSEPORT_LB,
     bool
 );
+#[cfg(target_os = "freebsd")]
 #[cfg(feature = "net")]
+sockopt_impl!(
+    #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
+    /// Select or query the set of functions that TCP will use for this connection.  This allows a
+    /// user to select an alternate TCP stack.
+    TcpFunctionBlk,
+    Both,
+    libc::IPPROTO_TCP,
+    libc::TCP_FUNCTION_BLK,
+    libc::tcp_function_set
+);
 sockopt_impl!(
     #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
     /// Used to disable Nagle's algorithm.

test/sys/test_sockopt.rs

@@ -281,6 +281,28 @@ fn test_tcp_congestion() {
     assert_eq!(getsockopt(&fd, sockopt::TcpCongestion).unwrap(), val);
 }
 
+#[test]
+#[cfg(target_os = "freebsd")]
+fn test_tcp_function_blk() {
+    use std::ffi::CStr;
+
+    let fd = socket(
+        AddressFamily::Inet,
+        SockType::Stream,
+        SockFlag::empty(),
+        None,
+    )
+    .unwrap();
+
+    let tfs = getsockopt(&fd, sockopt::TcpFunctionBlk).unwrap();
+    let name = unsafe{ CStr::from_ptr(tfs.function_set_name.as_ptr()) };
+    assert!(!name.is_empty());
+
+    // We can't know at compile time what options are available.  So just test the setter by a
+    // no-op set.
+    setsockopt(&fd, sockopt::TcpFunctionBlk, &tfs).unwrap();
+}
+
 #[test]
 #[cfg(linux_android)]
 fn test_bindtodevice() {