udp: add support for ECN on Windows

quinn-proto/src/connection/mod.rs

@@ -1209,6 +1209,7 @@ impl Connection {
     /// Retrieving the local IP address is currently supported on the following
     /// platforms:
     /// - Linux
+    /// - Windows
     ///
     /// On all non-supported platforms the local IP address will not be available,
     /// and the method will return `None`.

quinn-udp/Cargo.toml

@@ -28,4 +28,5 @@ socket2 = "0.5"
 tracing = "0.1.10"
 
 [target.'cfg(windows)'.dependencies]
-windows-sys = { version = "0.52.0", features = ["Win32_Networking_WinSock"] }
+windows-sys = { version = "0.52.0", features = ["Win32_Foundation", "Win32_System_IO", "Win32_Networking_WinSock"] }
+once_cell = "1.19.0"

quinn-udp/src/cmsg/mod.rs

@@ -7,6 +7,10 @@ use std::{
 #[path = "unix.rs"]
 mod imp;
 
+#[cfg(windows)]
+#[path = "windows.rs"]
+mod imp;
+
 pub(crate) use imp::Aligned;
 
 /// Helper to encode a series of control messages (native "cmsgs") to a buffer for use in `sendmsg`

quinn-udp/src/cmsg/windows.rs

@@ -0,0 +1,83 @@
+use std::{
+    ffi::{c_int, c_uchar},
+    mem, ptr,
+};
+
+use windows_sys::Win32::Networking::WinSock;
+
+use super::{CMsgHdr, MsgHdr};
+
+#[derive(Copy, Clone)]
+#[repr(align(8))] // Conservative bound for align_of<WinSock::CMSGHDR>
+pub(crate) struct Aligned<T>(pub(crate) T);
+
+/// Helpers for [`WinSock::WSAMSG`]
+// https://learn.microsoft.com/en-us/windows/win32/api/ws2def/ns-ws2def-wsamsg
+// https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/Networking/WinSock/struct.WSAMSG.html
+impl MsgHdr for WinSock::WSAMSG {
+    type ControlMessage = WinSock::CMSGHDR;
+
+    fn cmsg_first_hdr(&self) -> *mut Self::ControlMessage {
+        if self.Control.len as usize >= mem::size_of::<WinSock::CMSGHDR>() {
+            self.Control.buf as *mut WinSock::CMSGHDR
+        } else {
+            ptr::null_mut::<WinSock::CMSGHDR>()
+        }
+    }
+
+    fn cmsg_nxt_hdr(&self, cmsg: &Self::ControlMessage) -> *mut Self::ControlMessage {
+        let next =
+            (cmsg as *const _ as usize + cmsghdr_align(cmsg.cmsg_len)) as *mut WinSock::CMSGHDR;
+        let max = self.Control.buf as usize + self.Control.len as usize;
+        if unsafe { next.offset(1) } as usize > max {
+            ptr::null_mut()
+        } else {
+            next
+        }
+    }
+
+    fn set_control_len(&mut self, len: usize) {
+        self.Control.len = len as _;
+    }
+
+    fn control_len(&self) -> usize {
+        self.Control.len as _
+    }
+}
+
+/// Helpers for [`WinSock::CMSGHDR`]
+// https://learn.microsoft.com/en-us/windows/win32/api/ws2def/ns-ws2def-wsacmsghdr
+// https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/Networking/WinSock/struct.CMSGHDR.html
+impl CMsgHdr for WinSock::CMSGHDR {
+    fn cmsg_len(length: usize) -> usize {
+        cmsgdata_align(mem::size_of::<Self>()) + length
+    }
+
+    fn cmsg_space(length: usize) -> usize {
+        cmsgdata_align(mem::size_of::<Self>() + cmsghdr_align(length))
+    }
+
+    fn cmsg_data(&self) -> *mut c_uchar {
+        (self as *const _ as usize + cmsgdata_align(mem::size_of::<Self>())) as *mut c_uchar
+    }
+
+    fn set(&mut self, level: c_int, ty: c_int, len: usize) {
+        self.cmsg_level = level as _;
+        self.cmsg_type = ty as _;
+        self.cmsg_len = len as _;
+    }
+
+    fn len(&self) -> usize {
+        self.cmsg_len as _
+    }
+}
+
+// Helpers functions for `WinSock::WSAMSG` and `WinSock::CMSGHDR` are based on C macros from
+// https://github.com/microsoft/win32metadata/blob/main/generation/WinSDK/RecompiledIdlHeaders/shared/ws2def.h#L741
+fn cmsghdr_align(length: usize) -> usize {
+    (length + mem::align_of::<WinSock::CMSGHDR>() - 1) & !(mem::align_of::<WinSock::CMSGHDR>() - 1)
+}
+
+fn cmsgdata_align(length: usize) -> usize {
+    (length + mem::align_of::<usize>() - 1) & !(mem::align_of::<usize>() - 1)
+}

quinn-udp/src/lib.rs

@@ -15,8 +15,9 @@ use std::{
 use bytes::Bytes;
 use tracing::warn;
 
-#[cfg(unix)]
+#[cfg(any(unix, windows))]
 mod cmsg;
+
 #[cfg(unix)]
 #[path = "unix.rs"]
 mod imp;