Remove _DD_ROOT_CPP_SESSION_ID env var in favour of SHM carrier

SHM is now the sole propagation mechanism. The env var was only ever
written and read by C++ itself, so there was no cross-SDK interop value
in keeping it. Removes: the registry entry, environment::set(), the
write in Tracer constructor, the fallback read in finalize_config, and
the supported-configurations.json entry.

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

Author: khanayan123

include/datadog/environment.h

@@ -81,8 +81,7 @@ namespace environment {
   MACRO(DD_APM_TRACING_ENABLED, BOOLEAN, true)                                 \
   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, "")
+  MACRO(DD_EXTERNAL_ENV, STRING, "")
 
 #define ENV_DEFAULT_RESOLVED_IN_CODE(X) X
 #define WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) ARG,
@@ -111,9 +110,6 @@ StringView name(Variable variable);
 // `nullopt` if that variable is not set in the environment.
 Optional<StringView> lookup(Variable variable);
 
-// Set the specified environment `variable` to `value`. Does not overwrite if
-// already set (equivalent to setenv(..., 0) on POSIX).
-void set(Variable variable, StringView value);
 
 std::string to_json();
 

src/datadog/environment.cpp

@@ -19,14 +19,6 @@ Optional<StringView> lookup(Variable variable) {
   return StringView{value};
 }
 
-void set(Variable variable, StringView value) {
-  const char* name = variable_names[variable];
-#ifdef _WIN32
-  _putenv_s(name, value.data());
-#else
-  setenv(name, value.data(), /*overwrite=*/0);
-#endif
-}
 
 std::string to_json() {
   auto result = nlohmann::json::object({});

src/datadog/tracer.cpp

@@ -66,13 +66,10 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
       tracing_enabled_(config.tracing_enabled),
       resource_renaming_mode_(config.resource_renaming_mode) {
   // Create a shared memory carrier so exec'd children can inherit the root
-  // session ID without relying solely on environment variables.
+  // session ID.
   if (auto carrier = SessionCarrier::create(signature_.root_session_id)) {
     session_carrier_ = std::make_shared<SessionCarrier>(std::move(*carrier));
   }
-  // Also set the env var for interop with SDKs that still use it.
-  environment::set(environment::_DD_ROOT_CPP_SESSION_ID,
-                   signature_.root_session_id);
 
   telemetry::init(config.telemetry, signature_, logger_, config.http_client,
                   config.event_scheduler, config.agent_url);

src/datadog/tracer_config.cpp

@@ -407,12 +407,8 @@ Expected<FinalizedTracerConfig> finalize_config(const TracerConfig& user_config,
     final_config.runtime_id = user_config.runtime_id;
   }
 
-  // Prefer shared memory carrier (set by a C++ parent); fall back to the
-  // environment variable for interop with SDKs that still use env vars.
   if (auto shm = read_inherited_session_id()) {
     final_config.root_session_id = std::move(*shm);
-  } else if (auto env = lookup(environment::_DD_ROOT_CPP_SESSION_ID)) {
-    final_config.root_session_id = std::string{*env};
   }
 
   final_config.process_tags = user_config.process_tags;

supported-configurations.json

@@ -28,13 +28,6 @@
         "type": "STRING"
       }
     ],
-    "_DD_ROOT_CPP_SESSION_ID": [
-      {
-        "default": "",
-        "implementation": "A",
-        "type": "STRING"
-      }
-    ],
     "DD_INSTRUMENTATION_INSTALL_ID": [
       {
         "default": "",