From 6769ba6a52ffb490f98c2c0c576191e33ce85efb Mon Sep 17 00:00:00 2001
From: Martin Frisk <martin.f@keska.io>
Date: Wed, 8 Apr 2026 12:29:16 +0200
Subject: [PATCH] Check and forward any trailers on stream end.

When errors occur on a stream it is sent as a trailer, but the current client
does not look for them. This makes an error sent from a server look like a
successfully completed stream.

This PR makes tonic-web-wasm-client behave the same as most grpc
implementations in that the errors in the trailer is forwarded.
---
 src/response_body.rs                  | 10 ++++++--
 test-suite/simple/client/tests/web.rs | 28 +++++++++++++++++++++
 test-suite/simple/proto/echo.proto    |  2 ++
 test-suite/simple/server/src/main.rs  | 36 +++++++++++++++++++++++++++
 4 files changed, 74 insertions(+), 2 deletions(-)

diff --git a/src/response_body.rs b/src/response_body.rs
index 86f7b45..1fbbd87 100644
--- a/src/response_body.rs
+++ b/src/response_body.rs
@@ -279,8 +279,11 @@ impl Body for ResponseBody {
             return Poll::Ready(Some(Ok(http_body::Frame::data(data.freeze()))));
         }
 
-        // If reading data is finished return `None`
+        // If reading data is finished, return trailers (if available) before ending
         if self.state.finished_data() {
+            if let Some(trailers) = self.trailer.take() {
+                return Poll::Ready(Some(Ok(http_body::Frame::trailers(trailers))));
+            }
             return Poll::Ready(None);
         }
 
@@ -300,7 +303,10 @@ impl Body for ResponseBody {
                 let data = self.data.take().unwrap();
                 return Poll::Ready(Some(Ok(http_body::Frame::data(data.freeze()))));
             } else if self.state.finished_data() {
-                // If we finished reading data continue return `None`
+                // If we finished reading data, return trailers before ending
+                if let Some(trailers) = self.trailer.take() {
+                    return Poll::Ready(Some(Ok(http_body::Frame::trailers(trailers))));
+                }
                 return Poll::Ready(None);
             } else if self.finished_stream {
                 // If stream is finished but data is not finished return error
diff --git a/test-suite/simple/client/tests/web.rs b/test-suite/simple/client/tests/web.rs
index 8fcaaa7..266441e 100644
--- a/test-suite/simple/client/tests/web.rs
+++ b/test-suite/simple/client/tests/web.rs
@@ -93,6 +93,34 @@ async fn test_infinite_echo_stream() {
     assert!(response.is_some());
 }
 
+#[wasm_bindgen_test]
+async fn test_echo_stream_error() {
+    let mut client = build_client();
+
+    let mut stream_response = client
+        .echo_stream_error(EchoRequest {
+            message: "John".to_string(),
+        })
+        .await
+        .expect("success stream response")
+        .into_inner();
+
+    // First two messages should succeed
+    for i in 0..2 {
+        let response = stream_response.message().await.expect("stream message");
+        assert!(response.is_some(), "message {} should be present", i);
+        assert_eq!(response.unwrap().message, "echo(John)");
+    }
+
+    // Third message should be an error from the trailer
+    let error = stream_response
+        .message()
+        .await
+        .expect_err("should receive error from trailer");
+    assert_eq!(error.code(), Code::Internal);
+    assert_eq!(error.message(), "stream error after 2 messages");
+}
+
 #[wasm_bindgen_test]
 async fn test_error_response() {
     let mut client = build_client();
diff --git a/test-suite/simple/proto/echo.proto b/test-suite/simple/proto/echo.proto
index aa0eaef..883b688 100644
--- a/test-suite/simple/proto/echo.proto
+++ b/test-suite/simple/proto/echo.proto
@@ -12,6 +12,8 @@ service Echo {
   rpc EchoErrorResponse (EchoRequest) returns (EchoResponse) {}
 
   rpc EchoTimeout (EchoRequest) returns (EchoResponse) {}
+
+  rpc EchoStreamError (EchoRequest) returns (stream EchoResponse) {}
 }
 
 message EchoRequest {
diff --git a/test-suite/simple/server/src/main.rs b/test-suite/simple/server/src/main.rs
index 4d875be..2fea4d2 100644
--- a/test-suite/simple/server/src/main.rs
+++ b/test-suite/simple/server/src/main.rs
@@ -61,6 +61,16 @@ impl Echo for EchoService {
         Ok(Response::new(InfiniteMessageStream::new(request.message)))
     }
 
+    type EchoStreamErrorStream = ErrorAfterMessagesStream;
+
+    async fn echo_stream_error(
+        &self,
+        request: Request<EchoRequest>,
+    ) -> Result<Response<Self::EchoStreamErrorStream>, Status> {
+        let request = request.into_inner();
+        Ok(Response::new(ErrorAfterMessagesStream::new(request.message)))
+    }
+
     async fn echo_error_response(
         &self,
         _: tonic::Request<EchoRequest>,
@@ -118,6 +128,32 @@ impl Stream for InfiniteMessageStream {
     }
 }
 
+pub struct ErrorAfterMessagesStream {
+    message: String,
+    count: u8,
+}
+
+impl ErrorAfterMessagesStream {
+    pub fn new(message: String) -> Self {
+        Self { message, count: 0 }
+    }
+}
+
+impl Stream for ErrorAfterMessagesStream {
+    type Item = Result<EchoResponse, Status>;
+
+    fn poll_next(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        if self.count < 2 {
+            self.count += 1;
+            Poll::Ready(Some(Ok(EchoResponse {
+                message: format!("echo({})", self.message),
+            })))
+        } else {
+            Poll::Ready(Some(Err(Status::internal("stream error after 2 messages"))))
+        }
+    }
+}
+
 const DEFAULT_MAX_AGE: Duration = Duration::from_secs(24 * 60 * 60);
 const DEFAULT_EXPOSED_HEADERS: [HeaderName; 3] = [
     HeaderName::from_static("grpc-status"),