router: add two header formatter operators (#11242)

This commit updates the router's header formatter to support
the RESPONSE_FLAGS and RESPONSE_CODE_DETAILS operators.

Signed-off-by: Spencer Lewis <slewis@squareup.com>

docs/root/configuration/http/http_conn_man/headers.rst

@@ -644,3 +644,11 @@ Supported variable names are:
             key: "x-request-start"
             value: "%START_TIME(%s.%3f)%"
           append: true
+
+%RESPONSE_FLAGS%
+    Additional details about the response or connection, if any. Possible values and their meanings
+    are listed in the access log formatter :ref:`documentation<config_access_log_format_response_flags>`.
+
+%RESPONSE_CODE_DETAILS%
+    Response code details provides additional information about the HTTP response code, such as
+    who set it (the upstream or envoy) and why.

docs/root/version_history/current.rst

@@ -73,6 +73,8 @@ New Features
 * request_id: added to :ref:`always_set_request_id_in_response setting <envoy_v3_api_field_extensions.filters.network.http_connection_manager.v3.HttpConnectionManager.always_set_request_id_in_response>`
   to set :ref:`x-request-id <config_http_conn_man_headers_x-request-id>` header in response even if
   tracing is not forced.
+* router: add support for RESPONSE_FLAGS and RESPONSE_CODE_DETAILS :ref:`header formatters
+  <config_http_conn_man_headers_custom_request_headers>`.
 * router: more fine grained internal redirect configs are added to the :ref`internal_redirect_policy
   <envoy_api_field_router.RouterAction.internal_redirect_policy>` field.
 * runtime: add new gauge :ref:`deprecated_feature_seen_since_process_start <runtime_stats>` that gets reset across hot restarts.

source/common/router/header_formatter.cc

@@ -344,6 +344,17 @@ StreamInfoHeaderFormatter::StreamInfoHeaderFormatter(absl::string_view field_nam
   } else if (field_name == "HOSTNAME") {
     std::string hostname = Envoy::AccessLog::AccessLogFormatUtils::getHostname();
     field_extractor_ = [hostname](const StreamInfo::StreamInfo&) { return hostname; };
+  } else if (field_name == "RESPONSE_FLAGS") {
+    field_extractor_ = [](const StreamInfo::StreamInfo& stream_info) {
+      return StreamInfo::ResponseFlagUtils::toShortString(stream_info);
+    };
+  } else if (field_name == "RESPONSE_CODE_DETAILS") {
+    field_extractor_ = [](const StreamInfo::StreamInfo& stream_info) -> std::string {
+      if (stream_info.responseCodeDetails().has_value()) {
+        return stream_info.responseCodeDetails().value();
+      }
+      return "";
+    };
   } else {
     throw EnvoyException(fmt::format("field '{}' not supported as custom header", field_name));
   }

test/common/router/header_formatter_test.cc

@@ -750,6 +750,8 @@ TEST(HeaderParserTest, TestParseInternal) {
       {"%PER_REQUEST_STATE(testing)%", {"test_value"}, {}},
       {"%REQ(x-request-id)%", {"123"}, {}},
       {"%START_TIME%", {"2018-04-03T23:06:09.123Z"}, {}},
+      {"%RESPONSE_FLAGS%", {"LR"}, {}},
+      {"%RESPONSE_CODE_DETAILS%", {"via_upstream"}, {}},
 
       // Unescaped %
       {"%", {}, {"Invalid header configuration. Un-escaped % at position 0"}},
@@ -875,6 +877,12 @@ TEST(HeaderParserTest, TestParseInternal) {
   ON_CALL(stream_info, filterState()).WillByDefault(ReturnRef(filter_state));
   ON_CALL(Const(stream_info), filterState()).WillByDefault(ReturnRef(*filter_state));
 
+  ON_CALL(stream_info, hasResponseFlag(StreamInfo::ResponseFlag::LocalReset))
+      .WillByDefault(Return(true));
+
+  absl::optional<std::string> rc_details{"via_upstream"};
+  ON_CALL(stream_info, responseCodeDetails()).WillByDefault(ReturnRef(rc_details));
+
   for (const auto& test_case : test_cases) {
     Protobuf::RepeatedPtrField<envoy::config::core::v3::HeaderValueOption> to_add;
     envoy::config::core::v3::HeaderValueOption* header = to_add.Add();