Simplify thread-local static initialization

Summary:
The call stack used to look like this during static initialization:

  #0  0x00000000008032d1 in rocksdb::ThreadLocalPtr::StaticMeta::StaticMeta() (this=0x7ffff683b300) at util/thread_local.cc:172
  #1  0x00000000008030a7 in rocksdb::ThreadLocalPtr::Instance() () at util/thread_local.cc:135
  #2  0x000000000080310f in rocksdb::ThreadLocalPtr::StaticMeta::Mutex() () at util/thread_local.cc:141
  #3  0x0000000000803103 in rocksdb::ThreadLocalPtr::StaticMeta::InitSingletons() () at util/thread_local.cc:139
  #4  0x000000000080305d in rocksdb::ThreadLocalPtr::InitSingletons() () at util/thread_local.cc:106

It involves outer/inner classes and the call stacks goes
outer->inner->outer->inner, which is too difficult to understand. We can avoid
a level of back-and-forth by skipping StaticMeta::InitSingletons(), which
doesn't initialize anything beyond what ThreadLocalPtr::Instance() already
initializes.

Now the call stack looks like this during static initialization:

  #0  0x00000000008032c5 in rocksdb::ThreadLocalPtr::StaticMeta::StaticMeta() (this=0x7ffff683b300) at util/thread_local.cc:170
  #1  0x00000000008030a7 in rocksdb::ThreadLocalPtr::Instance() () at util/thread_local.cc:135
  #2  0x000000000080305d in rocksdb::ThreadLocalPtr::InitSingletons() () at util/thread_local.cc:106

Test Plan:
unit tests

verify StaticMeta::mutex_ is still initialized in DefaultEnv() (StaticMeta::mutex_ is the only variable intended to be initialized via StaticMeta::InitSingletons() which I removed)

  #0  0x00000000005cee17 in rocksdb::port::Mutex::Mutex(bool) (this=0x7ffff69500b0, adaptive=false) at port/port_posix.cc:52
  #1  0x0000000000769cf8 in rocksdb::ThreadLocalPtr::StaticMeta::StaticMeta() (this=0x7ffff6950000) at util/thread_local.cc:168
  #2  0x0000000000769a53 in rocksdb::ThreadLocalPtr::Instance() () at util/thread_local.cc:133
  #3  0x0000000000769a09 in rocksdb::ThreadLocalPtr::InitSingletons() () at util/thread_local.cc:105
  #4  0x0000000000647d98 in rocksdb::Env::Default() () at util/env_posix.cc:845

Reviewers: lightmark, yhchiang, sdong

Reviewed By: sdong

Subscribers: arahut, IslamAbdelRahman, yiwu, andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D60813

util/thread_local.cc

@@ -102,9 +102,7 @@ PIMAGE_TLS_CALLBACK p_thread_callback_on_exit = wintlscleanup::WinOnThreadExit;
 
 #endif  // OS_WIN
 
-void ThreadLocalPtr::InitSingletons() {
-  ThreadLocalPtr::StaticMeta::InitSingletons();
-}
+void ThreadLocalPtr::InitSingletons() { ThreadLocalPtr::Instance(); }
 
 ThreadLocalPtr::StaticMeta* ThreadLocalPtr::Instance() {
   // Here we prefer function static variable instead of global
@@ -136,8 +134,6 @@ ThreadLocalPtr::StaticMeta* ThreadLocalPtr::Instance() {
   return inst;
 }
 
-void ThreadLocalPtr::StaticMeta::InitSingletons() { Mutex(); }
-
 port::Mutex* ThreadLocalPtr::StaticMeta::Mutex() { return &Instance()->mutex_; }
 
 void ThreadLocalPtr::StaticMeta::OnThreadExit(void* ptr) {

util/thread_local.h

@@ -133,15 +133,6 @@ class ThreadLocalPtr {
     // Register the UnrefHandler for id
     void SetHandler(uint32_t id, UnrefHandler handler);
 
-    // Initialize all the singletons associated with StaticMeta.
-    //
-    // If this function is not called, then the singletons will be
-    // automatically initialized when they are used.
-    //
-    // Calling this function twice or after the singletons have been
-    // initialized will be no-op.
-    static void InitSingletons();
-
     // protect inst, next_instance_id_, free_instance_ids_, head_,
     // ThreadData.entries
     //