add getOSThreadID()

Summary:
Add a getOSThreadID() call.  This is similar to getCurrentThreadID(), but
returns the OS-level thread ID.

Reviewed By: yfeldblum

Differential Revision: D5173627

fbshipit-source-id: 0e8695ecfc1e382d12526c1b7d1717be114c9f4a

folly/ThreadId.h

@@ -19,10 +19,25 @@
 #include <cstdint>
 
 #include <folly/portability/PThread.h>
+#include <folly/portability/SysSyscall.h>
 #include <folly/portability/Windows.h>
 
 namespace folly {
 
+/**
+ * Get a process-specific identifier for the current thread.
+ *
+ * The return value will uniquely identify the thread within the current
+ * process.
+ *
+ * Note that the return value does not necessarily correspond to an operating
+ * system thread ID.  The return value is also only unique within the current
+ * process: getCurrentThreadID() may return the same value for two concurrently
+ * running threads in separate processes.
+ *
+ * The thread ID may be reused once the thread it corresponds to has been
+ * joined.
+ */
 inline uint64_t getCurrentThreadID() {
 #if __APPLE__
   return uint64_t(pthread_mach_thread_np(pthread_self()));
@@ -32,4 +47,44 @@ inline uint64_t getCurrentThreadID() {
   return uint64_t(pthread_self());
 #endif
 }
+
+/**
+ * Get the operating-system level thread ID for the current thread.
+ *
+ * The returned value will uniquely identify this thread on the system.
+ *
+ * This makes it more suitable for logging or displaying in user interfaces
+ * than the result of getCurrentThreadID().
+ *
+ * There are some potential caveats about this API, however:
+ *
+ * - In theory there is no guarantee that application threads map one-to-one to
+ *   kernel threads.  An application threading implementation could potentially
+ *   share one OS thread across multiple application threads, and/or it could
+ *   potentially move application threads between different OS threads over
+ *   time.  However, in practice all of the platforms we currently support have
+ *   a one-to-one mapping between userspace threads and operating system
+ *   threads.
+ *
+ * - This API may also be slightly slower than getCurrentThreadID() on some
+ *   platforms.  This API may require a system call, where getCurrentThreadID()
+ *   may only need to read thread-local memory.
+ *
+ * On Linux the returned value is a pid_t, and can be used in contexts
+ * requiring a thread pid_t.
+ *
+ * The thread ID may be reused once the thread it corresponds to has been
+ * joined.
+ */
+inline uint64_t getOSThreadID() {
+#if __APPLE__
+  uint64_t tid;
+  pthread_threadid_np(nullptr, &tid);
+  return tid;
+#elif _WIN32
+  return uint64_t(GetCurrentThreadId());
+#else
+  return uint64_t(syscall(FOLLY_SYS_gettid));
+#endif
+}
 }