Use std::thread::id in Fiber rather than a syscall

Summary: syscall() doesn't exist on Windows, but std::thread::id is standardized, and can do what we need it for, so use it instead.

Reviewed By: yfeldblum

Differential Revision: D3256064

fb-gh-sync-id: efddac82c117ccd8a53c8c93248529000b4326a5
fbshipit-source-id: efddac82c117ccd8a53c8c93248529000b4326a5

folly/experimental/fibers/Fiber.cpp

@@ -35,14 +35,8 @@ namespace fibers {
 namespace {
 static const uint64_t kMagic8Bytes = 0xfaceb00cfaceb00c;
 
-pid_t localThreadId() {
-  // __thread doesn't allow non-const initialization.
-  // OSX doesn't support thread_local.
-  static FOLLY_TLS pid_t threadId = 0;
-  if (UNLIKELY(threadId == 0)) {
-    threadId = syscall(FOLLY_SYS_gettid);
-  }
-  return threadId;
+std::thread::id localThreadId() {
+  return std::this_thread::get_id();
 }
 
 /* Size of the region from p + nBytes down to the last non-magic value */

folly/experimental/fibers/Fiber.h

@@ -184,7 +184,7 @@ class Fiber {
   folly::IntrusiveListHook listHook_; /**< list hook for different FiberManager
                                            queues */
   folly::IntrusiveListHook globalListHook_; /**< list hook for global list */
-  pid_t threadId_{0};
+  std::thread::id threadId_{};
 };
 }
 }