refactor: simplify root session ID to single get_or_init() singleton

Replace separate set()/get() with a single get_or_init(runtime_id) that
initializes on first call and returns the stored value on subsequent
calls. Remove root_session_id from TracerConfig/FinalizedTracerConfig
— the singleton is now the sole source of truth, read directly in
Tracer construction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: khanayan123

include/datadog/tracer_config.h

@@ -189,10 +189,6 @@ struct TracerConfig {
   // This option is ignored if `resource_renaming_enabled` is not `true`.
   Optional<bool> resource_renaming_always_simplified_endpoint;
 
-  // Populated internally from the root_session_id singleton. Tracks the
-  // runtime ID of the first C++ process in a fork/spawn chain.
-  Optional<std::string> root_session_id;
-
   /// A mapping of process-specific tags used to uniquely identify processes.
   ///
   /// The `process_tags` map allows associating arbitrary string-based keys and
@@ -229,7 +225,6 @@ 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;

src/datadog/root_session_id.cpp

@@ -20,15 +20,14 @@ std::atomic<bool>& initialized() {
 
 }  // namespace
 
-void set(const std::string& id) {
+const std::string& get_or_init(const std::string& runtime_id) {
   bool expected = false;
   if (initialized().compare_exchange_strong(expected, true)) {
-    instance() = id;
+    instance() = runtime_id;
   }
+  return instance();
 }
 
-const std::string& get() { return instance(); }
-
 }  // namespace root_session_id
 }  // namespace tracing
 }  // namespace datadog

src/datadog/root_session_id.h

@@ -6,11 +6,10 @@ namespace datadog {
 namespace tracing {
 namespace root_session_id {
 
-// Thread-safe singleton for the root session ID. The first call to `set`
-// captures the value; subsequent calls are no-ops. Immutable after init,
-// so fork-safe.
-void set(const std::string& id);
-const std::string& get();
+// Meyer's singleton for the root session ID. The first call initializes the
+// value; subsequent calls return the same value regardless of the argument.
+// Thread-safe via atomic CAS. Immutable after init, so fork-safe.
+const std::string& get_or_init(const std::string& runtime_id);
 
 }  // namespace root_session_id
 }  // namespace tracing

src/datadog/tracer.cpp

@@ -50,7 +50,7 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
       runtime_id_(config.runtime_id ? *config.runtime_id
                                     : RuntimeID::generate()),
       signature_{runtime_id_,
-                 config.root_session_id.value_or(runtime_id_.string()),
+                 root_session_id::get_or_init(runtime_id_.string()),
                  config.defaults.service, config.defaults.environment},
       config_manager_(std::make_shared<ConfigManager>(config)),
       collector_(/* see constructor body */),
@@ -66,8 +66,6 @@ Tracer::Tracer(const FinalizedTracerConfig& config,
       baggage_extraction_enabled_(false),
       tracing_enabled_(config.tracing_enabled),
       resource_renaming_mode_(config.resource_renaming_mode) {
-  root_session_id::set(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

@@ -14,7 +14,6 @@
 #include "null_logger.h"
 #include "parse_util.h"
 #include "platform_util.h"
-#include "root_session_id.h"
 #include "string_util.h"
 #include "threaded_event_scheduler.h"
 
@@ -262,11 +261,6 @@ Expected<TracerConfig> load_tracer_env_config(Logger &logger) {
     return std::move(error);
   }
 
-  const auto &root_sid = root_session_id::get();
-  if (!root_sid.empty()) {
-    env_cfg.root_session_id = root_sid;
-  }
-
   return env_cfg;
 }