Setup tests for libtrusty-rs

Test logic is based on the tests in the original tipc-test C binary,
but adapted to use the Rust unit test infrastructure to make running
tests easier.

Test: Ran the tests
Bug: 226659377
Change-Id: I998013b2f8b304299acb09d58beb49330747802a

trusty/libtrusty-rs/Android.bp

@@ -26,3 +26,12 @@ rust_library {
         "libnix",
     ],
 }
+
+rust_test {
+    name: "libtrusty-rs-tests",
+    crate_name: "trusty_test",
+    srcs: ["tests/test.rs"],
+    rustlibs: [
+        "libtrusty-rs",
+    ]
+}

trusty/libtrusty-rs/src/lib.rs

@@ -128,15 +128,15 @@ impl TipcChannel {
     /// Receives a message from the connected service.
     ///
     /// Returns the number of bytes in the received message, or any error that
-    /// occurred when reading the message. A return value of 0 indicates that
-    /// there were no incoming messages to receive.
+    /// occurred when reading the message. Blocks until there is a message to
+    /// receive if none is already ready to read.
     ///
     /// # Errors
     ///
-    /// Returns an error with native error code 90 (`EMSGSIZE`) if `buf` isn't
-    /// large enough to contain the incoming message. Use
-    /// [`raw_os_error`][std::io::Error::raw_os_error] to check the error code
-    /// to determine if you need to increase the size of `buf`.
+    /// Returns an error with native error code 90 (`EMSGSIZE`) if `buf` isn't large
+    /// enough to contain the incoming message. Use
+    /// [`raw_os_error`][std::io::Error::raw_os_error] to check the error code to
+    /// determine if you need to increase the size of `buf`.
     pub fn recv(&mut self, buf: &mut [u8]) -> io::Result<usize> {
         self.0.read(buf)
     }

trusty/libtrusty-rs/tests/test.rs

@@ -0,0 +1,26 @@
+use trusty::{TipcChannel, DEFAULT_DEVICE};
+
+const ECHO_NAME: &str = "com.android.ipc-unittest.srv.echo";
+
+#[test]
+fn echo() {
+    let mut connection = TipcChannel::connect(DEFAULT_DEVICE, ECHO_NAME)
+        .expect("Failed to connect to Trusty service");
+
+    // Send a message to the echo TA.
+    let send_buf = [7u8; 32];
+    connection.send(send_buf.as_slice()).unwrap();
+
+    // Receive the response message from the TA.
+    let mut recv_buf = [0u8; 32];
+    let read_len = connection.recv(&mut recv_buf).expect("Failed to read from connection");
+
+    assert_eq!(
+        send_buf.len(),
+        read_len,
+        "Received data was wrong size (expected {} bytes, received {})",
+        send_buf.len(),
+        read_len,
+    );
+    assert_eq!(send_buf, recv_buf, "Received data does not match sent data");
+}