IPC thread写法太晦涩

主要用到TLS，首次进入gHaveTLS为false，锁保护说明此函数很多其他函数在调用。
通过if (pthread_key_create(&gTLS, threadDestructor) != 0)，中threadDestructor(void *st)
调用IPCThreadState::IPCThreadState()创建IPCThreadState对象，并将对象的索引设置为gTLS。再次进入后通过gTLS获取到IPCThreadState对象。

 IPCThreadState* IPCThreadState::self()
 {
     if (gHaveTLS) {
 restart:
         const pthread_key_t k = gTLS;
         IPCThreadState* st = (IPCThreadState*)pthread_getspecific(k);
         if (st) return st;
         return new IPCThreadState;
     }

     if (gShutdown) return NULL;

     pthread_mutex_lock(&gTLSMutex);
     if (!gHaveTLS) {
         if (pthread_key_create(&gTLS, threadDestructor) != ) {
             pthread_mutex_unlock(&gTLSMutex);
             return NULL;
         }
         gHaveTLS = true;
     }
     pthread_mutex_unlock(&gTLSMutex);
     goto restart;
 }

通pthread_setspecific(gTLS, this);gTLS与IPCThreadState对象关联了。

 IPCThreadState::IPCThreadState()
     : mProcess(ProcessState::self()),
       mMyThreadId(androidGetTid()),
       mStrictModePolicy(),
       mLastTransactionBinderFlags()
 {
     pthread_setspecific(gTLS, this);
     clearCaller();
     mIn.setDataCapacity();
     mOut.setDataCapacity();
 }