Clear any messages from RecvAncillaryBuffer at start of recv

This should prevent leaks of file descriptors if the `RecvAncillaryBuffer`
is re-used for multiple reads.

src/backend/libc/net/msghdr.rs

@@ -21,6 +21,8 @@ pub(crate) fn with_recv_msghdr<R>(
     control: &mut RecvAncillaryBuffer<'_>,
     f: impl FnOnce(&mut c::msghdr) -> io::Result<R>,
 ) -> io::Result<R> {
+    control.clear();
+
     let namelen = size_of::<c::sockaddr_storage>() as c::socklen_t;
     let mut msghdr = {
         let mut h: c::msghdr = unsafe { zeroed() };

src/backend/linux_raw/net/msghdr.rs

@@ -33,6 +33,8 @@ pub(crate) fn with_recv_msghdr<R>(
     control: &mut RecvAncillaryBuffer<'_>,
     f: impl FnOnce(&mut c::msghdr) -> io::Result<R>,
 ) -> io::Result<R> {
+    control.clear();
+
     let namelen = size_of::<c::sockaddr_storage>() as c::c_int;
     let mut msghdr = c::msghdr {
         msg_name: name.as_mut_ptr().cast(),

src/net/send_recv/msg.rs

@@ -248,6 +248,11 @@ impl<'buf> RecvAncillaryBuffer<'buf> {
         self.read = 0;
     }
 
+    /// Delete all messages from the buffer.
+    pub(crate) fn clear(&mut self) {
+        self.drain().for_each(drop);
+    }
+
     /// Drain all messages from the buffer.
     pub fn drain(&mut self) -> AncillaryDrain<'_> {
         AncillaryDrain {
@@ -260,7 +265,7 @@ impl<'buf> RecvAncillaryBuffer<'buf> {
 
 impl Drop for RecvAncillaryBuffer<'_> {
     fn drop(&mut self) {
-        self.drain().for_each(drop);
+        self.clear();
     }
 }