Capture errors from connecting to broker

src/connection.rs

@@ -470,6 +470,7 @@ where
     let mut backoff = Backoff::new(backoff_config);
     backoff
         .retry_with_backoff("broker_connect", || async {
+            let mut errors = Vec::new();
             for broker in &brokers {
                 let conn = broker
                     .connect(
@@ -485,16 +486,24 @@ where
                     Ok(transport) => transport,
                     Err(e) => {
                         warn!(%e, "Failed to connect to broker");
+                        errors.push(e);
                         continue;
                     }
                 };
 
                 return ControlFlow::Break(connection);
             }
 
-            let err = Box::<dyn std::error::Error + Send + Sync>::from(
-                "Failed to connect to any broker, backing off".to_string(),
-            );
+            // errors should always contain at least 1 element here.
+            let errors_string = errors
+                .into_iter()
+                .map(|e| e.to_string())
+                .collect::<Vec<String>>()
+                .join(", ");
+
+            let err = Box::<dyn std::error::Error + Send + Sync>::from(format!(
+                "Failed to connect to any broker, backing off. Errors: {errors_string}"
+            ));
             let err: Arc<dyn std::error::Error + Send + Sync> = err.into();
             ControlFlow::Continue(ErrorOrThrottle::Error(err))
         })

tests/client.rs

@@ -721,7 +721,12 @@ async fn test_client_backoff_terminates() {
 
     match client_builder.build().await {
         Err(rskafka::client::error::Error::Connection(e)) => {
-            assert_eq!(e.to_string(), "all retries failed: Retry exceeded deadline");
+            // Error can be slightly different depending on the exact underlying error.
+            assert!(e.to_string().starts_with(concat!(
+                "all retries failed: Retry exceeded deadline. ",
+                "Source: Failed to connect to any broker, backing off. ",
+                "Errors: error connecting to broker \"localhost:9000\""
+            )));
         }
         _ => {
             unreachable!();