s2n-tls rust binding: expose selected application protocol (#4599)

Co-authored-by: Lindsay Stewart <slindsay@amazon.com>

bindings/rust/s2n-tls/src/connection.rs

@@ -947,6 +947,14 @@ impl Connection {
         })
     }
 
+    pub fn application_protocol(&self) -> Option<&[u8]> {
+        let protocol = unsafe { s2n_get_application_protocol(self.connection.as_ptr()) };
+        if protocol.is_null() {
+            return None;
+        }
+        Some(unsafe { CStr::from_ptr(protocol).to_bytes() })
+    }
+
     /// Provides access to the TLS-Exporter functionality.
     ///
     /// See https://datatracker.ietf.org/doc/html/rfc5705 and https://www.rfc-editor.org/rfc/rfc8446.

bindings/rust/s2n-tls/src/testing/s2n_tls.rs

@@ -998,4 +998,31 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn no_application_protocol() -> Result<(), Error> {
+        let config = config_builder(&security::DEFAULT)?.build()?;
+        let mut pair = tls_pair(config);
+        assert!(poll_tls_pair_result(&mut pair).is_ok());
+        assert!(pair.server.0.connection.application_protocol().is_none());
+        Ok(())
+    }
+
+    #[test]
+    fn application_protocol() -> Result<(), Error> {
+        let config = config_builder(&security::DEFAULT)?.build()?;
+        let mut pair = tls_pair(config);
+        pair.server
+            .0
+            .connection
+            .set_application_protocol_preference(["http/1.1", "h2"])?;
+        pair.client
+            .0
+            .connection
+            .append_application_protocol_preference(b"h2")?;
+        assert!(poll_tls_pair_result(&mut pair).is_ok());
+        let protocol = pair.server.0.connection.application_protocol().unwrap();
+        assert_eq!(protocol, b"h2");
+        Ok(())
+    }
 }