Match registry: _DD_ROOT_CPP_SESSION_ID implementation B, default ""

The central registry has this config as implementation B with an empty
string default. Update environment.h to use "" and teach config-inversion
to emit "B" for this var via an override map.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: khanayan123

include/datadog/environment.h

@@ -82,7 +82,7 @@ namespace environment {
   MACRO(DD_TRACE_RESOURCE_RENAMING_ENABLED, BOOLEAN, false)                    \
   MACRO(DD_TRACE_RESOURCE_RENAMING_ALWAYS_SIMPLIFIED_ENDPOINT, BOOLEAN, false) \
   MACRO(DD_EXTERNAL_ENV, STRING, "")                                           \
-  MACRO(_DD_ROOT_CPP_SESSION_ID, STRING, nullptr)
+  MACRO(_DD_ROOT_CPP_SESSION_ID, STRING, "")
 
 #define ENV_DEFAULT_RESOLVED_IN_CODE(X) X
 #define WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) ARG,

supported-configurations.json

@@ -282,8 +282,8 @@
     ],
     "_DD_ROOT_CPP_SESSION_ID": [
       {
-        "default": null,
-        "implementation": "A",
+        "default": "",
+        "implementation": "B",
         "type": "STRING"
       }
     ]

tools/config-inversion/main.cpp

@@ -4,23 +4,24 @@
 #include <iostream>
 #include <nlohmann/json.hpp>
 #include <sstream>
+#include <unordered_map>
 
 #include "datadog/environment.h"
 
 namespace fs = std::filesystem;
 namespace env = datadog::tracing::environment;
 
 template <typename T>
-nlohmann::json to_json_default(const T& value) {
+std::string to_string_any(const T& value) {
   std::ostringstream oss;
   oss << value;
   return oss.str();
 }
 
-template <>
-nlohmann::json to_json_default(std::nullptr_t const&) {
-  return nullptr;
-}
+// Variables whose registry implementation letter differs from the default "A".
+static const std::unordered_map<std::string, const char*> implementation_overrides = {
+    {"_DD_ROOT_CPP_SESSION_ID", "B"},
+};
 
 nlohmann::json build_configuration() {
   nlohmann::json j;
@@ -33,13 +34,14 @@ nlohmann::json build_configuration() {
 
 #define ENV_DEFAULT_RESOLVED_IN_CODE(X) ""
 
-#define X(NAME, TYPE, DEFAULT_VALUE)                                       \
-  do {                                                                     \
-    auto obj = nlohmann::json::object();                                   \
-    obj["default"] = to_json_default(DEFAULT_VALUE);                         \
-    obj["implementation"] = "A";                                           \
-    obj["type"] = QUOTED(TYPE);                                            \
-    supported_configurations[QUOTED(NAME)] = nlohmann::json::array({obj}); \
+#define X(NAME, TYPE, DEFAULT_VALUE)                                           \
+  do {                                                                         \
+    auto obj = nlohmann::json::object();                                       \
+    obj["default"] = to_string_any(DEFAULT_VALUE);                             \
+    auto it = implementation_overrides.find(QUOTED(NAME));                     \
+    obj["implementation"] = (it != implementation_overrides.end()) ? it->second : "A"; \
+    obj["type"] = QUOTED(TYPE);                                                \
+    supported_configurations[QUOTED(NAME)] = nlohmann::json::array({obj});     \
   } while (0);
 
   DD_LIST_ENVIRONMENT_VARIABLES(X)