Move autowrap from ThreadManager constructor to Thread::Current.

This is in preparation for a WrapCurrentThread method,
or AutoThread constructor, taking a socket
server as argument, to eliminate the need for the
MessageQueue::set_socketserver method and its lock.

No intended change in behaviour; the ThreadManager constructor
records the calling thread id, and autowrap is still restricted to
that thread. Behavior when NO_MAIN_THREAD_WRAPPING is defined is
also unchanged.

Also makes the ThreadManager a singleton, with private constructor
and destructor. And marks its destructor as RTC_NOTREACHED, since
by the documentation for RTC_DEFINE_STATIC_LOCAL, the intention is to
leak the object and never destruct it.

BUG=webrtc:7501

Review-Url: https://codereview.webrtc.org/2833993003
Cr-Commit-Position: refs/heads/master@{#17879}

webrtc/base/thread.cc

@@ -31,23 +31,32 @@ ThreadManager* ThreadManager::Instance() {
   return &thread_manager;
 }
 
+ThreadManager::~ThreadManager() {
+  // By above RTC_DEFINE_STATIC_LOCAL.
+  RTC_NOTREACHED() << "ThreadManager should never be destructed.";
+}
+
 // static
 Thread* Thread::Current() {
-  return ThreadManager::Instance()->CurrentThread();
+  ThreadManager* manager = ThreadManager::Instance();
+  Thread* thread = manager->CurrentThread();
+
+#ifndef NO_MAIN_THREAD_WRAPPING
+  // Only autowrap the thread which instantiated the ThreadManager.
+  if (!thread && manager->IsMainThread()) {
+    thread = new Thread();
+    thread->WrapCurrentWithThreadManager(manager, true);
+  }
+#endif
+
+  return thread;
 }
 
 #if defined(WEBRTC_POSIX)
 #if !defined(WEBRTC_MAC)
 ThreadManager::ThreadManager() {
+  main_thread_ref_ = CurrentThreadRef();
   pthread_key_create(&key_, nullptr);
-#ifndef NO_MAIN_THREAD_WRAPPING
-  WrapCurrentThread();
-#endif
-}
-
-ThreadManager::~ThreadManager() {
-  UnwrapCurrentThread();
-  pthread_key_delete(key_);
 }
 #endif
 
@@ -62,15 +71,8 @@ void ThreadManager::SetCurrentThread(Thread *thread) {
 
 #if defined(WEBRTC_WIN)
 ThreadManager::ThreadManager() {
+  main_thread_ref_ = CurrentThreadRef();
   key_ = TlsAlloc();
-#ifndef NO_MAIN_THREAD_WRAPPING
-  WrapCurrentThread();
-#endif
-}
-
-ThreadManager::~ThreadManager() {
-  UnwrapCurrentThread();
-  TlsFree(key_);
 }
 
 Thread *ThreadManager::CurrentThread() {
@@ -99,6 +101,10 @@ void ThreadManager::UnwrapCurrentThread() {
   }
 }
 
+bool ThreadManager::IsMainThread() {
+  return IsThreadRefEqual(CurrentThreadRef(), main_thread_ref_);
+}
+
 Thread::ScopedDisallowBlockingCalls::ScopedDisallowBlockingCalls()
   : thread_(Thread::Current()),
     previous_state_(thread_->SetAllowBlockingCalls(false)) {
@@ -142,6 +148,10 @@ Thread::~Thread() {
   DoDestroy();
 }
 
+bool Thread::IsCurrent() const {
+  return ThreadManager::Instance()->CurrentThread() == this;
+}
+
 std::unique_ptr<Thread> Thread::CreateWithSocketServer() {
   return std::unique_ptr<Thread>(new Thread(SocketServer::CreateDefault()));
 }

webrtc/base/thread.h

@@ -23,6 +23,7 @@
 #include "webrtc/base/constructormagic.h"
 #include "webrtc/base/event.h"
 #include "webrtc/base/messagequeue.h"
+#include "webrtc/base/platform_thread_types.h"
 
 #if defined(WEBRTC_WIN)
 #include "webrtc/base/win32.h"
@@ -36,9 +37,7 @@ class ThreadManager {
  public:
   static const int kForever = -1;
 
-  ThreadManager();
-  ~ThreadManager();
-
+  // Singleton, constructor and destructor are private.
   static ThreadManager* Instance();
 
   Thread* CurrentThread();
@@ -60,7 +59,12 @@ class ThreadManager {
   Thread *WrapCurrentThread();
   void UnwrapCurrentThread();
 
+  bool IsMainThread();
+
  private:
+  ThreadManager();
+  ~ThreadManager();
+
 #if defined(WEBRTC_POSIX)
   pthread_key_t key_;
 #endif
@@ -69,6 +73,9 @@ class ThreadManager {
   DWORD key_;
 #endif
 
+  // The thread to potentially autowrap.
+  PlatformThreadRef main_thread_ref_;
+
   RTC_DISALLOW_COPY_AND_ASSIGN(ThreadManager);
 };
 
@@ -123,9 +130,7 @@ class LOCKABLE Thread : public MessageQueue {
     const bool previous_state_;
   };
 
-  bool IsCurrent() const {
-    return Current() == this;
-  }
+  bool IsCurrent() const;
 
   // Sleeps the calling thread for the specified number of milliseconds, during
   // which time no processing is performed. Returns false if sleeping was

webrtc/base/thread_darwin.mm

@@ -39,22 +39,13 @@ void InitCocoaMultiThreading() {
 namespace rtc {
 
 ThreadManager::ThreadManager() {
+  main_thread_ref_ = CurrentThreadRef();
   pthread_key_create(&key_, nullptr);
-#ifndef NO_MAIN_THREAD_WRAPPING
-  WrapCurrentThread();
-#endif
   // This is necessary to alert the cocoa runtime of the fact that
   // we are running in a multithreaded environment.
   InitCocoaMultiThreading();
 }
 
-ThreadManager::~ThreadManager() {
-  @autoreleasepool {
-    UnwrapCurrentThread();
-    pthread_key_delete(key_);
-  }
-}
-
 // static
 void* Thread::PreRun(void* pv) {
   ThreadInit* init = static_cast<ThreadInit*>(pv);