addressing comments

Signed-off-by: Yanjun Xiang <yanjunxiang@google.com>

source/extensions/filters/http/ext_proc/ext_proc.cc

@@ -118,18 +118,18 @@ void Filter::setEncoderFilterCallbacks(Http::StreamEncoderFilterCallbacks& callb
 }
 
 Filter::StreamOpenState Filter::openStream() {
-  // If processing already completes, no need to open a gRPC stream.
+  // External processing already completes. Return IgnoreError so the filter will return Continue.
   if (processing_complete_) {
-    return sent_immediate_response_ ? StreamOpenState::Error : StreamOpenState::IgnoreError;
+    return StreamOpenState::IgnoreError;
   }
   if (!stream_) {
     ENVOY_LOG(debug, "Opening gRPC stream to external processor");
     stream_ = client_->start(*this, grpc_service_, decoder_callbacks_->streamInfo());
-    if (stream_ == nullptr) {
-      return config_->failureModeAllow() ? StreamOpenState::IgnoreError : StreamOpenState::Error;
-    }
-
     stats_.streams_started_.inc();
+    if (processing_complete_) {
+      // Stream failed while starting and either onGrpcError or onGrpcClose was already called
+      return sent_immediate_response_ ? StreamOpenState::Error : StreamOpenState::IgnoreError;
+    }
     // For custom access logging purposes. Applicable only for Envoy gRPC as Google gRPC does not
     // have a proper implementation of streamInfo.
     if (grpc_service_.has_envoy_grpc()) {