base/src: Add Unix StreamChannel Message test

Prior to this CL, the StreamChannel in base/src/sys/unix had no tests
for FramingMode::Message. This CL adds a test for that framing mode.

Bug: b:231641496
Change-Id: I75e13709d558258eca838c3190aaf0237e39513b
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3793688
Reviewed-by: Noah Gold <nkgold@google.com>
Commit-Queue: Clarissa Garvey <clarissagarvey@chromium.org>
Tested-by: Clarissa Garvey <clarissagarvey@chromium.org>

base/src/sys/unix/stream_channel.rs

@@ -181,7 +181,7 @@ mod test {
     }
 
     #[test]
-    fn test_non_blocking_pair() {
+    fn test_non_blocking_pair_byte() {
         let (mut sender, mut receiver) =
             StreamChannel::pair(BlockingMode::Nonblocking, FramingMode::Byte).unwrap();
 
@@ -220,6 +220,43 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_non_blocking_pair_message() {
+        let (mut sender, mut receiver) =
+            StreamChannel::pair(BlockingMode::Nonblocking, FramingMode::Message).unwrap();
+
+        sender.write_all(&[75, 77, 54, 82, 76, 65]).unwrap();
+
+        // Wait for the data to arrive.
+        let event_ctx: EventContext<Token> =
+            EventContext::build_with(&[(receiver.get_read_notifier(), Token::ReceivedData)])
+                .unwrap();
+        let events = event_ctx.wait().unwrap();
+        let tokens: Vec<Token> = events
+            .iter()
+            .filter(|e| e.is_readable)
+            .map(|e| e.token)
+            .collect();
+        assert_eq!(tokens, vec! {Token::ReceivedData});
+
+        // Unlike Byte format, Message mode panics if the buffer is smaller than the packet size;
+        // make the buffer the right size.
+        let mut recv_buffer: [u8; 6] = [0; 6];
+
+        let size = receiver.read(&mut recv_buffer).unwrap();
+        assert_eq!(size, 6);
+        assert_eq!(recv_buffer, [75, 77, 54, 82, 76, 65]);
+
+        // Now that we've polled for & received all data, polling again should show no events.
+        assert_eq!(
+            event_ctx
+                .wait_timeout(std::time::Duration::new(0, 0))
+                .unwrap()
+                .len(),
+            0
+        );
+    }
+
     #[test]
     fn test_non_blocking_pair_error_no_data() {
         let (mut sender, mut receiver) =