fix: revert unrelated formatting, deduplicate session headers, fix Windows setenv

- Revert whitespace-only changes (const char * style) in environment.cpp
  and environment.h to keep the diff focused on the feature.
- Extract set_session_headers() helper in telemetry_impl.cpp to avoid
  duplicating session header logic in app_started() and send_payload().
- Fix Windows _putenv_s path to check getenv() first, matching the
  POSIX setenv(..., 0) "don't overwrite" semantics.

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

Author: khanayan123

include/datadog/environment.h

@@ -96,7 +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

src/datadog/environment.cpp

@@ -11,18 +11,20 @@ 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];
+  const char *name = variable_names[variable];
 #ifdef _WIN32
-  _putenv_s(name, value.data());
+  if (!std::getenv(name)) {
+    _putenv_s(name, value.data());
+  }
 #else
   setenv(name, value.data(), /*overwrite=*/0);
 #endif
@@ -31,8 +33,8 @@ void set(Variable variable, StringView value) {
 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_impl.cpp

@@ -60,6 +60,14 @@ HTTPClient::URL make_telemetry_endpoint(HTTPClient::URL url) {
   return url;
 }
 
+void set_session_headers(DictWriter& headers,
+                         const tracing::TracerSignature& sig) {
+  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);
+  }
+}
+
 void cancel_tasks(std::vector<tracing::EventScheduler::Cancel>& tasks) {
   for (auto& cancel_task : tasks) {
     cancel_task();
@@ -310,10 +318,7 @@ void Telemetry::app_started() {
     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);
-    }
+    set_session_headers(headers, sig);
     if (debug_enabled) {
       headers.set("DD-Telemetry-Debug-Enabled", "true");
     }
@@ -369,10 +374,7 @@ void Telemetry::send_payload(StringView request_type, std::string payload) {
     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);
-    }
+    set_session_headers(headers, sig);
     if (debug_enabled) {
       headers.set("DD-Telemetry-Debug-Enabled", "true");
     }