Drain result channel during server execution.

src/proto/server/testing.rs

@@ -296,9 +296,9 @@ impl Server {
           let (stream, peer_address) = result?;
           self.spawn_handler(stream, peer_address);
         },
-        // If shutdown_rx's sender is dropped and we receive an error, we take
-        // that as a signal to shut down immediately too.
         result = &mut self.shutdown_rx => {
+          // If shutdown_rx's sender is dropped and we receive an error, we take
+          // that as a signal to shut down immediately too.
           match result.unwrap_or(ShutdownType::Immediate) {
             ShutdownType::LameDuck => break,
             ShutdownType::Immediate => {
@@ -308,6 +308,14 @@ impl Server {
               break
             }
           }
+        },
+        optional_result = self.result_rx.recv() => {
+          // We can never exhaust the result channel because we hold onto a
+          // sender in `self.result_tx`.
+          let result = optional_result.unwrap();
+
+          // Return an error if a handler returns an error.
+          result?;
         }
       );
     }