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) != 0) {
            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(0),
      mLastTransactionBinderFlags(0)
{
    pthread_setspecific(gTLS, this);
    clearCaller();
    mIn.setDataCapacity(256);
    mOut.setDataCapacity(256);
}