Split test_unix_peercred into implicit and explicit versions. (#916)

The `test_unix_peercred` test tests passing credentials over a socket.
It uses `set_sockopt_passcred` to enable this, but when enabled,
credentials are received on all messages, not just those with explicit
credentials passed. So make two copies of this test, one where
credentials are passed explicitly, and one that relies on implicit
passing.

Author: sunfishcode

tests/net/unix.rs

@@ -582,12 +582,66 @@ fn test_unix_msg_with_scm_rights() {
 
 #[cfg(all(feature = "process", linux_kernel))]
 #[test]
-fn test_unix_peercred() {
+fn test_unix_peercred_explicit() {
     use rustix::io::{IoSlice, IoSliceMut};
     use rustix::net::{
         recvmsg, sendmsg, sockopt, RecvAncillaryBuffer, RecvAncillaryMessage, RecvFlags,
         SendAncillaryBuffer, SendAncillaryMessage, SendFlags, SocketFlags,
     };
+
+    let (send_sock, recv_sock) = rustix::net::socketpair(
+        AddressFamily::UNIX,
+        SocketType::STREAM,
+        SocketFlags::CLOEXEC,
+        None,
+    )
+    .unwrap();
+
+    sockopt::set_socket_passcred(&recv_sock, true).unwrap();
+
+    let ucred = sockopt::get_socket_peercred(&send_sock).unwrap();
+    let msg = SendAncillaryMessage::ScmCredentials(ucred);
+    let mut space = [0; rustix::cmsg_space!(ScmCredentials(1))];
+    let mut cmsg_buffer = SendAncillaryBuffer::new(&mut space);
+    assert!(cmsg_buffer.push(msg));
+
+    sendmsg(
+        &send_sock,
+        &[IoSlice::new(b"cred")],
+        &mut cmsg_buffer,
+        SendFlags::empty(),
+    )
+    .unwrap();
+
+    let mut cmsg_space = [0; rustix::cmsg_space!(ScmCredentials(1))];
+    let mut cmsg_buffer = RecvAncillaryBuffer::new(&mut cmsg_space);
+
+    let mut buffer = [0; BUFFER_SIZE];
+    recvmsg(
+        &recv_sock,
+        &mut [IoSliceMut::new(&mut buffer)],
+        &mut cmsg_buffer,
+        RecvFlags::empty(),
+    )
+    .unwrap();
+
+    match cmsg_buffer.drain().next().unwrap() {
+        RecvAncillaryMessage::ScmCredentials(ucred2) => assert_eq!(ucred2, ucred),
+        _ => panic!("Unexpected ancilliary message"),
+    };
+}
+
+/// Like `test_unix_peercred_explicit`, but relies on the fact that
+/// `set_socket_passcred` enables passing of the credentials implicitly
+/// instead of passing an explicit message to `sendmsg`.
+#[cfg(all(feature = "process", linux_kernel))]
+#[test]
+fn test_unix_peercred_implicit() {
+    use rustix::io::{IoSlice, IoSliceMut};
+    use rustix::net::{
+        recvmsg, sendmsg, sockopt, RecvAncillaryBuffer, RecvAncillaryMessage, RecvFlags,
+        SendAncillaryBuffer, SendFlags, SocketFlags,
+    };
     use rustix::process::{getgid, getpid, getuid};
 
     let (send_sock, recv_sock) = rustix::net::socketpair(
@@ -605,10 +659,7 @@ fn test_unix_peercred() {
     assert_eq!(ucred.uid, getuid());
     assert_eq!(ucred.gid, getgid());
 
-    let msg = SendAncillaryMessage::ScmCredentials(ucred);
-    let mut space = [0; rustix::cmsg_space!(ScmCredentials(1))];
-    let mut cmsg_buffer = SendAncillaryBuffer::new(&mut space);
-    assert!(cmsg_buffer.push(msg));
+    let mut cmsg_buffer = SendAncillaryBuffer::default();
 
     sendmsg(
         &send_sock,