From 029e8080bd9fe5f254f7783881f619c0cb95ed4c Mon Sep 17 00:00:00 2001
From: Xuyang Tao <taoxuy@google.com>
Date: Wed, 25 Sep 2024 21:46:56 +0000
Subject: [PATCH] cel: add response.backend_latency in CEL attributes (#36292)

We will depend on CEL to get those information inside HTTP wasm
- unit tests added
- doc added
Signed-off-by: Xuyang Tao <taoxuy@google.com>
---
 docs/root/intro/arch_overview/advanced/attributes.rst | 1 +
 source/extensions/filters/common/expr/context.cc      | 9 +++++++++
 source/extensions/filters/common/expr/context.h       | 2 ++
 test/extensions/filters/common/expr/context_test.cc   | 8 ++++++++
 4 files changed, 20 insertions(+)

diff --git a/docs/root/intro/arch_overview/advanced/attributes.rst b/docs/root/intro/arch_overview/advanced/attributes.rst
index eae665abf3b5..f3f8e6fb84c0 100644
--- a/docs/root/intro/arch_overview/advanced/attributes.rst
+++ b/docs/root/intro/arch_overview/advanced/attributes.rst
@@ -93,6 +93,7 @@ Response attributes are only available after the request completes.
    response.trailers, "map<string, string>", All response trailers indexed by the lower-cased trailer name
    response.size, int, Size of the response body
    response.total_size, int, Total size of the response including the approximate uncompressed size of the headers and the trailers
+   response.backend_latency, duration, Duration between the first byte sent to and the last byte received from the upstream backend
 
 Connection attributes
 ---------------------
diff --git a/source/extensions/filters/common/expr/context.cc b/source/extensions/filters/common/expr/context.cc
index cc30fb794a95..10438dff4df2 100644
--- a/source/extensions/filters/common/expr/context.cc
+++ b/source/extensions/filters/common/expr/context.cc
@@ -184,6 +184,15 @@ absl::optional<CelValue> ResponseWrapper::operator[](CelValue key) const {
       return CelValue::CreateString(&details.value());
     }
     return {};
+  } else if (value == BackendLatency) {
+    Envoy::StreamInfo::TimingUtility timing(info_);
+    const auto last_upstream_rx_byte_received = timing.lastUpstreamRxByteReceived();
+    const auto first_upstream_tx_byte_sent = timing.firstUpstreamTxByteSent();
+    if (last_upstream_rx_byte_received.has_value() && first_upstream_tx_byte_sent.has_value()) {
+      return CelValue::CreateDuration(absl::FromChrono(last_upstream_rx_byte_received.value() -
+                                                       first_upstream_tx_byte_sent.value()));
+    }
+    return {};
   }
   return {};
 }
diff --git a/source/extensions/filters/common/expr/context.h b/source/extensions/filters/common/expr/context.h
index 7e5f5d1defec..867776641ee2 100644
--- a/source/extensions/filters/common/expr/context.h
+++ b/source/extensions/filters/common/expr/context.h
@@ -8,6 +8,7 @@
 #include "source/common/http/headers.h"
 #include "source/common/runtime/runtime_features.h"
 #include "source/common/singleton/const_singleton.h"
+#include "source/common/stream_info/utility.h"
 
 #include "eval/public/cel_value.h"
 #include "eval/public/containers/container_backed_list_impl.h"
@@ -47,6 +48,7 @@ constexpr absl::string_view CodeDetails = "code_details";
 constexpr absl::string_view Trailers = "trailers";
 constexpr absl::string_view Flags = "flags";
 constexpr absl::string_view GrpcStatus = "grpc_status";
+constexpr absl::string_view BackendLatency = "backend_latency";
 
 // Per-request or per-connection metadata
 constexpr absl::string_view Metadata = "metadata";
diff --git a/test/extensions/filters/common/expr/context_test.cc b/test/extensions/filters/common/expr/context_test.cc
index 733c2413ce2c..c766d47e258d 100644
--- a/test/extensions/filters/common/expr/context_test.cc
+++ b/test/extensions/filters/common/expr/context_test.cc
@@ -411,6 +411,14 @@ TEST(Context, ResponseAttributes) {
     EXPECT_FALSE(value.has_value());
   }
 
+  {
+    info.setUpstreamInfo(std::make_shared<StreamInfo::UpstreamInfoImpl>());
+    StreamInfo::UpstreamTiming& upstream_timing = info.upstreamInfo()->upstreamTiming();
+    upstream_timing.onFirstUpstreamTxByteSent(info.timeSource());
+    upstream_timing.onLastUpstreamRxByteReceived(info.timeSource());
+    EXPECT_TRUE(response[CelValue::CreateStringView(BackendLatency)].has_value());
+  }
+
   {
     Http::TestResponseHeaderMapImpl header_map{{header_name, "a"}, {grpc_status, "7"}};
     Http::TestResponseTrailerMapImpl trailer_map{{trailer_name, "b"}};