Implement stable session identifier headers for telemetry

Adds DD-Session-ID and DD-Root-Session-ID HTTP headers to all telemetry
requests per the Stable Service Instance Identifier RFC.

- DD-Session-ID is always set to the tracer's runtime_id
- DD-Root-Session-ID is only set when it differs from the session ID
  (i.e. when running as a child process)
- Root session ID is propagated to exec'd children via
  _DD_ROOT_CPP_SESSION_ID env var, read in finalize_config()
- _DD_ROOT_CPP_SESSION_ID registered in the environment variable
  registry and supported-configurations.json

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

Author: khanayan123

include/datadog/environment.h

@@ -81,7 +81,8 @@ 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_EXTERNAL_ENV, STRING, "")                                           \
+  MACRO(_DD_ROOT_CPP_SESSION_ID, STRING, "")
 
 #define ENV_DEFAULT_RESOLVED_IN_CODE(X) X
 #define WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) ARG,
@@ -95,7 +96,7 @@ enum Variable { DD_LIST_ENVIRONMENT_VARIABLES(WITH_COMMA) };
 #define QUOTED_WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) \
   WITH_COMMA(QUOTED(ARG), TYPE, DEFAULT_VALUE)
 
-inline const char *const variable_names[] = {
+inline const char* const variable_names[] = {
     DD_LIST_ENVIRONMENT_VARIABLES(QUOTED_WITH_COMMA)};
 
 #undef QUOTED
@@ -110,6 +111,10 @@ 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();
 
 }  // namespace environment

include/datadog/tracer_config.h

@@ -225,6 +225,7 @@ class FinalizedTracerConfig final {
   bool log_on_startup;
   bool generate_128bit_trace_ids;
   Optional<RuntimeID> runtime_id;
+  Optional<std::string> root_session_id;
   Clock clock;
   std::string integration_name;
   std::string integration_version;

include/datadog/tracer_signature.h

@@ -33,15 +33,18 @@ namespace tracing {
 
 struct TracerSignature {
   RuntimeID runtime_id;
+  std::string root_session_id;
   std::string default_service;
   std::string default_environment;
   std::string library_version;
   StringView library_language;
   StringView library_language_version;
 
   TracerSignature() = delete;
-  TracerSignature(RuntimeID id, std::string service, std::string environment)
+  TracerSignature(RuntimeID id, std::string root_session, std::string service,
+                  std::string environment)
       : runtime_id(id),
+        root_session_id(std::move(root_session)),
         default_service(std::move(service)),
         default_environment(std::move(environment)),
         library_version(tracer_version),

src/datadog/environment.cpp

@@ -11,19 +11,28 @@ namespace environment {
 StringView name(Variable variable) { return variable_names[variable]; }
 
 Optional<StringView> lookup(Variable variable) {
-  const char *name = variable_names[variable];
-  const char *value = std::getenv(name);
+  const char* name = variable_names[variable];
+  const char* value = std::getenv(name);
   if (!value) {
     return nullopt;
   }
   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({});
 
-  for (const char *name : variable_names) {
-    if (const char *value = std::getenv(name)) {
+  for (const char* name : variable_names) {
+    if (const char* value = std::getenv(name)) {
       result[name] = value;
     }
   }

src/datadog/telemetry/telemetry.cpp

@@ -53,10 +53,13 @@ void init(FinalizedConfiguration configuration,
           std::shared_ptr<tracing::HTTPClient> client,
           std::shared_ptr<tracing::EventScheduler> event_scheduler,
           tracing::HTTPClient::URL agent_url, tracing::Clock clock) {
-  instance(Ctor_param{configuration,
-                      tracing::TracerSignature(tracing::RuntimeID::generate(),
-                                               tracing::get_process_name(), ""),
-                      logger, client, event_scheduler, agent_url, clock});
+  auto runtime_id = tracing::RuntimeID::generate();
+  auto root_session_id = runtime_id.string();
+  instance(Ctor_param{
+      configuration,
+      tracing::TracerSignature(runtime_id, std::move(root_session_id),
+                               tracing::get_process_name(), ""),
+      logger, client, event_scheduler, agent_url, clock});
 }
 
 void init(FinalizedConfiguration configuration,

src/datadog/telemetry/telemetry_impl.cpp

@@ -302,13 +302,18 @@ void Telemetry::app_started() {
   auto payload = app_started_payload();
 
   auto on_headers = [payload_size = payload.size(),
-                     debug_enabled = config_.debug](DictWriter& headers) {
+                     debug_enabled = config_.debug,
+                     &sig = tracer_signature_](DictWriter& headers) {
     headers.set("Content-Type", "application/json");
     headers.set("Content-Length", std::to_string(payload_size));
     headers.set("DD-Telemetry-API-Version", "v2");
     headers.set("DD-Client-Library-Language", "cpp");
     headers.set("DD-Client-Library-Version", tracer_version);
     headers.set("DD-Telemetry-Request-Type", "app-started");
+    headers.set("DD-Session-ID", sig.runtime_id.string());
+    if (sig.root_session_id != sig.runtime_id.string()) {
+      headers.set("DD-Root-Session-ID", sig.root_session_id);
+    }
     if (debug_enabled) {
       headers.set("DD-Telemetry-Debug-Enabled", "true");
     }
@@ -356,14 +361,18 @@ void Telemetry::app_closing() {
 
 void Telemetry::send_payload(StringView request_type, std::string payload) {
   auto set_telemetry_headers = [request_type, payload_size = payload.size(),
-                                debug_enabled =
-                                    config_.debug](DictWriter& headers) {
+                                debug_enabled = config_.debug,
+                                &sig = tracer_signature_](DictWriter& headers) {
     headers.set("Content-Type", "application/json");
     headers.set("Content-Length", std::to_string(payload_size));
     headers.set("DD-Telemetry-API-Version", "v2");
     headers.set("DD-Client-Library-Language", "cpp");
     headers.set("DD-Client-Library-Version", tracer_version);
     headers.set("DD-Telemetry-Request-Type", request_type);
+    headers.set("DD-Session-ID", sig.runtime_id.string());
+    if (sig.root_session_id != sig.runtime_id.string()) {
+      headers.set("DD-Root-Session-ID", sig.root_session_id);
+    }
     if (debug_enabled) {
       headers.set("DD-Telemetry-Debug-Enabled", "true");
     }

src/datadog/tracer.cpp

@@ -48,8 +48,9 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
     : logger_(config.logger),
       runtime_id_(config.runtime_id ? *config.runtime_id
                                     : RuntimeID::generate()),
-      signature_{runtime_id_, config.defaults.service,
-                 config.defaults.environment},
+      signature_{runtime_id_,
+                 config.root_session_id.value_or(runtime_id_.string()),
+                 config.defaults.service, config.defaults.environment},
       config_manager_(std::make_shared<ConfigManager>(config)),
       collector_(/* see constructor body */),
       span_sampler_(
@@ -64,6 +65,9 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
       baggage_extraction_enabled_(false),
       tracing_enabled_(config.tracing_enabled),
       resource_renaming_mode_(config.resource_renaming_mode) {
+  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);
   if (config.report_hostname) {

src/datadog/tracer_config.cpp

@@ -407,6 +407,10 @@ Expected<FinalizedTracerConfig> finalize_config(const TracerConfig &user_config,
     final_config.runtime_id = user_config.runtime_id;
   }
 
+  if (auto val = lookup(environment::_DD_ROOT_CPP_SESSION_ID)) {
+    final_config.root_session_id = std::string{*val};
+  }
+
   final_config.process_tags = user_config.process_tags;
 
   auto agent_finalized =

supported-configurations.json

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

test/remote_config/test_remote_config.cpp

@@ -47,8 +47,10 @@ auto logger = std::make_shared<NullLogger>();
 
 REMOTE_CONFIG_TEST("initial state payload") {
   // Verify the initial payload structure for a remote configuration instance.
+  auto runtime_id = RuntimeID::generate();
   const TracerSignature tracer_signature{
-      /* runtime_id = */ RuntimeID::generate(),
+      /* runtime_id = */ runtime_id,
+      /* root_session_id = */ runtime_id.string(),
       /* service = */ "testsvc",
       /* environment = */ "test"};
 
@@ -92,8 +94,10 @@ REMOTE_CONFIG_TEST("initial state payload") {
 // TODO: test all combination of product and capabilities generation
 
 REMOTE_CONFIG_TEST("response processing") {
+  auto runtime_id = RuntimeID::generate();
   const TracerSignature tracer_signature{
-      /* runtime_id = */ RuntimeID::generate(),
+      /* runtime_id = */ runtime_id,
+      /* root_session_id = */ runtime_id.string(),
       /* service = */ "testsvc",
       /* environment = */ "test"};