Implement getField CEL function

buf/validate/internal/BUILD.bazel

@@ -139,6 +139,7 @@ cc_library(
         "@com_google_absl//absl/status",
         "@com_google_cel_cpp//eval/public:cel_function_adapter",
         "@com_google_cel_cpp//eval/public:cel_function_registry",
+        "@com_google_cel_cpp//eval/public/containers:field_access",
         "@com_google_cel_cpp//eval/public/containers:container_backed_map_impl",
         "@com_googlesource_code_re2//:re2",
     ],

buf/validate/internal/extra_func.cc

@@ -26,6 +26,7 @@
 #include "eval/public/cel_function_adapter.h"
 #include "eval/public/cel_value.h"
 #include "eval/public/containers/container_backed_map_impl.h"
+#include "eval/public/containers/field_access.h"
 #include "google/protobuf/arena.h"
 #include "re2/re2.h"
 
@@ -52,6 +53,32 @@ bool isPathValid(const std::string_view path) {
 
 namespace cel = google::api::expr::runtime;
 
+cel::CelValue getField(
+    google::protobuf::Arena* arena, cel::CelValue msgval, cel::CelValue nameval) {
+  if (!msgval.IsMessage()) {
+    auto* error = google::protobuf::Arena::Create<cel::CelError>(
+        arena, absl::StatusCode::kInvalidArgument, "expected a message value for first argument");
+    return cel::CelValue::CreateError(error);
+  }
+  if (!nameval.IsString()) {
+    auto* error = google::protobuf::Arena::Create<cel::CelError>(
+        arena, absl::StatusCode::kInvalidArgument, "expected a string value for second argument");
+    return cel::CelValue::CreateError(error);
+  }
+  const auto* message = msgval.MessageOrDie();
+  auto name = nameval.StringOrDie();
+  const auto* field = message->GetDescriptor()->FindFieldByName(name.value());
+  if (field == nullptr) {
+    auto* error = google::protobuf::Arena::Create<cel::CelError>(
+        arena, absl::StatusCode::kInvalidArgument, "no such field");
+    return cel::CelValue::CreateError(error);
+  }
+  if (cel::CelValue result; cel::CreateValueFromSingleField(message, field, arena, &result).ok()) {
+    return result;
+  }
+  return cel::CelValue::CreateNull();
+}
+
 cel::CelValue isNan(google::protobuf::Arena* arena, cel::CelValue rhs) {
   if (!rhs.IsDouble()) {
     auto* error = google::protobuf::Arena::Create<cel::CelError>(
@@ -352,6 +379,12 @@ absl::Status RegisterExtraFuncs(
   if (!status.ok()) {
     return status;
   }
+  auto getFieldStatus =
+      cel::FunctionAdapter<cel::CelValue, cel::CelValue, cel::CelValue>::CreateAndRegister(
+          "getField", false, &getField, &registry);
+  if (!getFieldStatus.ok()) {
+    return getFieldStatus;
+  }
   auto isNanStatus = cel::FunctionAdapter<cel::CelValue, cel::CelValue>::CreateAndRegister(
       "isNan", true, &isNan, &registry);
   if (!isNanStatus.ok()) {