From 39a16eab7dc19f16bb81bc7a53b78b05a727b3ba Mon Sep 17 00:00:00 2001
From: Alan Somers <asomers@gmail.com>
Date: Fri, 15 Nov 2024 16:10:20 -0700
Subject: [PATCH] Add the TCP_FUNCTION_BLK socket option, on FreeBSD

---
 changelog/2539.added.md   |  1 +
 src/sys/socket/sockopt.rs | 11 +++++++++++
 test/sys/test_sockopt.rs  | 22 ++++++++++++++++++++++
 3 files changed, 34 insertions(+)
 create mode 100644 changelog/2539.added.md

diff --git a/changelog/2539.added.md b/changelog/2539.added.md
new file mode 100644
index 0000000000..c42b7dbd65
--- /dev/null
+++ b/changelog/2539.added.md
@@ -0,0 +1 @@
+Add the `TCP_FUNCTION_BLK` sockopt, on FreeBSD.
diff --git a/src/sys/socket/sockopt.rs b/src/sys/socket/sockopt.rs
index 408f8acb65..10704bbdcf 100644
--- a/src/sys/socket/sockopt.rs
+++ b/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.
diff --git a/test/sys/test_sockopt.rs b/test/sys/test_sockopt.rs
index 3cecbf7d7f..b6e78c1839 100644
--- a/test/sys/test_sockopt.rs
+++ b/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.to_bytes().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() {