Make http3 codec not encode empty trailers block

changelogs/current.yaml

@@ -107,7 +107,9 @@ bug_fixes:
 - area: http3
   change: |
     Fixed a bug where an empty trailers block could be sent. This would occur if a filter removed
-    the last trailer - a likely occurrence with the ``grpc_web_filter``.
+    the last trailer - a likely occurrence with the ``grpc_web_filter``. This change makes http3 codec
+    behave the same way http2 codec does, converting an empty trailers block to no trailers.
+    This behavior can be reverted by setting the runtime guard ``envoy.reloadable_features.http3_remove_empty_trailers`` to false.
 - area: http
   change: |
     Fixed a bug where an incomplete request (missing body or trailers) may be proxied to the upstream when the limit on

source/common/quic/envoy_quic_server_stream.cc

@@ -93,12 +93,14 @@ void EnvoyQuicServerStream::encodeHeaders(const Http::ResponseHeaderMap& headers
 }
 
 void EnvoyQuicServerStream::encodeTrailers(const Http::ResponseTrailerMap& trailers) {
-  if (trailers.empty()) {
-    ENVOY_STREAM_LOG(debug, "skipping submitting empty trailers", *this);
-    // Instead of submitting empty trailers, we send empty data with end_stream=true instead.
-    Buffer::OwnedImpl empty_buffer;
-    encodeData(empty_buffer, true);
-    return;
+  if (Runtime::runtimeFeatureEnabled("envoy.reloadable_features.http3_remove_empty_trailers")) {
+    if (trailers.empty()) {
+      ENVOY_STREAM_LOG(debug, "skipping submitting empty trailers", *this);
+      // Instead of submitting empty trailers, we send empty data with end_stream=true instead.
+      Buffer::OwnedImpl empty_buffer;
+      encodeData(empty_buffer, true);
+      return;
+    }
   }
   ENVOY_STREAM_LOG(debug, "encodeTrailers: {}.", *this, trailers);
   encodeTrailersImpl(envoyHeadersToHttp2HeaderBlock(trailers));

source/common/runtime/runtime_features.cc

@@ -59,6 +59,7 @@ RUNTIME_GUARD(envoy_reloadable_features_http2_use_oghttp2);
 RUNTIME_GUARD(envoy_reloadable_features_http2_use_visitor_for_data);
 RUNTIME_GUARD(envoy_reloadable_features_http2_validate_authority_with_quiche);
 RUNTIME_GUARD(envoy_reloadable_features_http3_happy_eyeballs);
+RUNTIME_GUARD(envoy_reloadable_features_http3_remove_empty_trailers);
 RUNTIME_GUARD(envoy_reloadable_features_http_filter_avoid_reentrant_local_reply);
 // Delay deprecation and decommission until UHV is enabled.
 RUNTIME_GUARD(envoy_reloadable_features_http_reject_path_with_fragment);