✨ zb: Add Socket::auth_mechanism

The socket impl will now inform us which authentication mechanism should
be used with it. The implementation can choose this based on the socket
type and the target platform.

This will allow us to switch to a single mechanism for each connection
in a following commit.

zbus/src/connection/socket/channel.rs

@@ -2,7 +2,7 @@ use std::io;
 
 use async_broadcast::{broadcast, Receiver, Sender};
 
-use crate::{fdo::ConnectionCredentials, Message};
+use crate::{fdo::ConnectionCredentials, AuthMechanism, Message};
 
 /// An in-process channel-based socket.
 ///
@@ -72,6 +72,10 @@ impl super::ReadHalf for Reader {
     async fn peer_credentials(&mut self) -> io::Result<ConnectionCredentials> {
         self_credentials().await
     }
+
+    fn auth_mechanism(&self) -> AuthMechanism {
+        AuthMechanism::Anonymous
+    }
 }
 
 /// The writer half of a [`Channel`].

zbus/src/connection/socket/mod.rs

@@ -23,7 +23,7 @@ use crate::{
         header::{MAX_MESSAGE_SIZE, MIN_MESSAGE_SIZE},
         PrimaryHeader,
     },
-    padding_for_8_bytes, Message,
+    padding_for_8_bytes, AuthMechanism, Message,
 };
 #[cfg(unix)]
 use std::os::fd::{AsFd, BorrowedFd, OwnedFd};
@@ -237,6 +237,13 @@ pub trait ReadHalf: std::fmt::Debug + Send + Sync + 'static {
     async fn peer_credentials(&mut self) -> io::Result<ConnectionCredentials> {
         Ok(ConnectionCredentials::default())
     }
+
+    /// The authentication mechanism to use for this socket on the target OS.
+    ///
+    /// Default is `AuthMechanism::External`.
+    fn auth_mechanism(&self) -> AuthMechanism {
+        AuthMechanism::External
+    }
 }
 
 /// The write half of a socket.
@@ -354,6 +361,10 @@ impl ReadHalf for Box<dyn ReadHalf> {
     async fn peer_credentials(&mut self) -> io::Result<ConnectionCredentials> {
         (**self).peer_credentials().await
     }
+
+    fn auth_mechanism(&self) -> AuthMechanism {
+        (**self).auth_mechanism()
+    }
 }
 
 #[async_trait::async_trait]

zbus/src/connection/socket/tcp.rs

@@ -48,6 +48,11 @@ impl ReadHalf for Arc<Async<TcpStream>> {
         )
         .await
     }
+
+    #[cfg(not(windows))]
+    fn auth_mechanism(&self) -> crate::AuthMechanism {
+        crate::AuthMechanism::Anonymous
+    }
 }
 
 #[cfg(not(feature = "tokio"))]
@@ -120,6 +125,11 @@ impl ReadHalf for tokio::net::tcp::OwnedReadHalf {
         )
         .await
     }
+
+    #[cfg(not(windows))]
+    fn auth_mechanism(&self) -> crate::AuthMechanism {
+        crate::AuthMechanism::Anonymous
+    }
 }
 
 #[cfg(feature = "tokio")]

zbus/src/connection/socket/vsock.rs

@@ -26,6 +26,10 @@ impl super::ReadHalf for std::sync::Arc<async_io::Async<vsock::VsockStream>> {
             }
         }
     }
+
+    fn auth_mechanism(&self) -> crate::AuthMechanism {
+        crate::AuthMechanism::Anonymous
+    }
 }
 
 #[cfg(all(feature = "vsock", not(feature = "tokio")))]
@@ -86,6 +90,10 @@ impl super::ReadHalf for tokio_vsock::ReadHalf {
             ret
         })
     }
+
+    fn auth_mechanism(&self) -> crate::AuthMechanism {
+        crate::AuthMechanism::Anonymous
+    }
 }
 
 #[cfg(feature = "tokio-vsock")]