vmm_vhost: mark Endpoint as !Sync

It isn't safe to send or recv messages concurrently. The byte stream
could get mixed up.

Change-Id: I27039d3f44d5d8ec609ce7974c6f2f3fcbd9768c
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/5068397
Commit-Queue: Frederick Mayle <fmayle@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>

third_party/vmm_vhost/src/connection.rs

@@ -88,18 +88,26 @@ fn advance_slices_mut(bufs: &mut &mut [&mut [u8]], mut count: usize) {
 ///
 /// Builds on top of `PlatformEndpoint`, which provides methods for sending and receiving raw bytes
 /// and file descriptors (a thin cross-platform abstraction for unix domain sockets).
-pub struct Endpoint<R: Req>(pub(crate) PlatformEndpoint<R>);
+pub struct Endpoint<R: Req>(
+    pub(crate) PlatformEndpoint<R>,
+    // Mark `Endpoint` as `!Sync` because message sends and recvs cannot safely be done
+    // concurrently.
+    std::marker::PhantomData<std::cell::Cell<()>>,
+);
 
 impl<R: Req> From<SystemStream> for Endpoint<R> {
     fn from(sock: SystemStream) -> Self {
-        Self(PlatformEndpoint::from(sock))
+        Self(PlatformEndpoint::from(sock), std::marker::PhantomData)
     }
 }
 
 impl<R: Req> Endpoint<R> {
     /// Create a new stream by connecting to server at `path`.
     pub fn connect<P: AsRef<Path>>(path: P) -> Result<Self> {
-        PlatformEndpoint::connect(path).map(Self)
+        Ok(Self(
+            PlatformEndpoint::connect(path)?,
+            std::marker::PhantomData,
+        ))
     }
 
     /// Constructs the slave request endpoint for self.