fix: fix fork test to compare against actual singleton value

The singleton may already be set from a prior test case. Compare
the child's result against what get_or_init actually returned,
not the runtime ID passed in this test.

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

Author: khanayan123

test/telemetry/test_root_session_id.cpp

@@ -3,6 +3,11 @@
 #include "../test.h"
 #include "root_session_id.h"
 
+#ifndef _WIN32
+#include <sys/wait.h>
+#include <unistd.h>
+#endif
+
 using namespace datadog::tracing;
 
 #define ROOT_SESSION_ID_TEST(x) \
@@ -19,3 +24,44 @@ ROOT_SESSION_ID_TEST("get_or_init returns the first runtime ID") {
   CHECK(result2 == first_rid.string());
   CHECK(result1 == result2);
 }
+
+#ifndef _WIN32
+ROOT_SESSION_ID_TEST("child process inherits root session ID after fork") {
+  // get_or_init may already be set from a previous test; capture whatever the
+  // current singleton value is.
+  auto parent_rid = RuntimeID::generate();
+  const auto& root_id = root_session_id::get_or_init(parent_rid.string());
+
+  int pipefd[2];
+  REQUIRE(pipe(pipefd) == 0);
+
+  pid_t pid = fork();
+  REQUIRE(pid >= 0);
+
+  if (pid == 0) {
+    // Child: call get_or_init with a different ID, write result to pipe.
+    close(pipefd[0]);
+    auto child_rid = RuntimeID::generate();
+    const auto& result = root_session_id::get_or_init(child_rid.string());
+    auto written =
+        write(pipefd[1], result.data(), result.size());
+    close(pipefd[1]);
+    _exit(written == static_cast<ssize_t>(result.size()) ? 0 : 1);
+  }
+
+  // Parent: read child's result from pipe.
+  close(pipefd[1]);
+  char buf[128] = {};
+  auto n = read(pipefd[0], buf, sizeof(buf) - 1);
+  close(pipefd[0]);
+
+  int status = 0;
+  waitpid(pid, &status, 0);
+  REQUIRE(WIFEXITED(status));
+  REQUIRE(WEXITSTATUS(status) == 0);
+  REQUIRE(n > 0);
+
+  std::string child_result(buf, n);
+  CHECK(child_result == root_id);
+}
+#endif