docs: improve some comments and remove infallible condition from metr…

…ics (#2773)

rs/p2p/quic_transport/src/metrics.rs

@@ -17,7 +17,6 @@ pub(crate) const CONNECTION_RESULT_SUCCESS_LABEL: &str = "success";
 pub(crate) const CONNECTION_RESULT_FAILED_LABEL: &str = "failed";
 pub(crate) const ERROR_TYPE_APP: &str = "app";
 pub(crate) const INFALIBBLE: &str = "infallible";
-const ERROR_CLOSED_STREAM: &str = "closed_stream";
 const ERROR_RESET_STREAM: &str = "reset_stream";
 const ERROR_STOPPED_STREAM: &str = "stopped_stream";
 const ERROR_APP_CLOSED_CONN: &str = "app_closed_conn";
@@ -205,15 +204,17 @@ impl QuicTransportMetrics {
 
 pub fn observe_conn_error(err: &ConnectionError, op: &str, counter: &IntCounterVec) {
     match err {
+        // This can occur during a topology change or when the connection manager attempts to replace an old, broken connection with a new one.
         ConnectionError::LocallyClosed => counter
             .with_label_values(&[op, ERROR_LOCALLY_CLOSED_CONN])
             .inc(),
+        // This can occur during a topology change or when the connection manager attempts to replace an old, broken connection with a new one.
         ConnectionError::ApplicationClosed(_) => counter
             .with_label_values(&[op, ERROR_APP_CLOSED_CONN])
             .inc(),
-        // Can happen if peer crashes or there are connectivity problems.
+        // This can occur if the peer crashes or experiences connectivity issues.
         ConnectionError::TimedOut => counter.with_label_values(&[op, ERROR_TIMED_OUT_CONN]).inc(),
-        // A connection was closed by the QUIC protocol.
+        // A connection was closed by the QUIC protocol. Overall should be infallible.
         _ => counter
             .with_label_values(&[op, ERROR_QUIC_CLOSED_CONN])
             .inc(),
@@ -225,9 +226,11 @@ pub fn observe_write_error(err: &WriteError, op: &str, counter: &IntCounterVec)
         // This should be infallible. The peer will never stop a stream, it can only reset it.
         WriteError::Stopped(_) => counter.with_label_values(&[op, ERROR_STOPPED_STREAM]).inc(),
         WriteError::ConnectionLost(conn_err) => observe_conn_error(conn_err, op, counter),
-        // This should be infallible
-        WriteError::ClosedStream => counter.with_label_values(&[op, ERROR_CLOSED_STREAM]).inc(),
-        _ => counter.with_label_values(&[op, INFALIBBLE]).inc(),
+        // If any of the following errors occur it means that we have a bug in the protocol implementation or
+        // there is malicious peer on the other side.
+        WriteError::ClosedStream | WriteError::ZeroRttRejected => {
+            counter.with_label_values(&[op, INFALIBBLE]).inc()
+        }
     }
 }