Attempts to add test coverage.

Signed-off-by: Biren Roy <birenroy@google.com>

test/common/http/http2/codec_impl_test.cc

@@ -597,6 +597,51 @@ TEST_P(Http2CodecImplTest, SimpleRequestResponse) {
   }
 }
 
+TEST_P(Http2CodecImplTest, SimpleRequestResponseOldApi) {
+  scoped_runtime_.mergeValues({{"envoy.reloadable_features.http2_use_visitor_for_data", "false"}});
+  initialize();
+
+  InSequence s;
+  TestRequestHeaderMapImpl request_headers;
+  HttpTestUtility::addDefaultHeaders(request_headers);
+  request_headers.setMethod("POST");
+
+  // Encode request headers.
+  EXPECT_CALL(request_decoder_, decodeHeaders_(_, false));
+  EXPECT_TRUE(request_encoder_->encodeHeaders(request_headers, false).ok());
+
+  // Queue request body.
+  Buffer::OwnedImpl request_body(std::string(1024, 'a'));
+  request_encoder_->encodeData(request_body, true);
+
+  // Flush request body.
+  EXPECT_CALL(request_decoder_, decodeData(_, true)).Times(AtLeast(1));
+  driveToCompletion();
+
+  TestResponseHeaderMapImpl response_headers{{":status", "200"}};
+
+  // Encode response headers.
+  EXPECT_CALL(response_decoder_, decodeHeaders_(_, false));
+  response_encoder_->encodeHeaders(response_headers, false);
+
+  // Queue response body.
+  Buffer::OwnedImpl response_body(std::string(1024, 'b'));
+  response_encoder_->encodeData(response_body, true);
+
+  // Flush response body.
+  EXPECT_CALL(response_decoder_, decodeData(_, true)).Times(AtLeast(1));
+  driveToCompletion();
+
+  EXPECT_TRUE(client_wrapper_->status_.ok());
+  EXPECT_TRUE(server_wrapper_->status_.ok());
+
+  if (http2_implementation_ == Http2Impl::Nghttp2) {
+    // Regression test for issue #19761.
+    EXPECT_EQ(0, getClientDataSourcesSize());
+    EXPECT_EQ(0, getServerDataSourcesSize());
+  }
+}
+
 TEST_P(Http2CodecImplTest, ShutdownNotice) {
   initialize();
   EXPECT_EQ(absl::nullopt, request_encoder_->http1StreamEncoderOptions());