深入了解mediaserver-2

 

4.2 BnServiceManager<servicemanager进程>

      上面说了，defaultServiceManager返回的是一个BpServiceManager，通过它可以把命令请求发送到binder设备，而且handle的值为0。那么，系统的另外一端肯定有个接收命令的，那又是谁呢？

      很可惜啊，BnServiceManager不存在，但确实有一个程序完成了BnServiceManager的工作，那就是/system/bin/servicemanager进程.虽然service_manager没有从BnServiceManager中派生，但是它肯定完成了BnServiceManager的功能。

//service_manager.c
int main(int argc, char **argv)
{
    struct binder_state *bs;
    void *svcmgr = BINDER_SERVICE_MANAGER;

    bs = binder_open(128*1024);

    if (binder_become_context_manager(bs)) {
        LOGE("cannot become context manager (%s)\n", strerror(errno));
        return -1;
    }

    svcmgr_handle = svcmgr;
    binder_loop(bs, svcmgr_handler);
    return 0;
}

void binder_loop(struct binder_state *bs, binder_handler func)
{
    int res;
    struct binder_write_read bwr;
    unsigned readbuf[32];

    bwr.write_size = 0;
    bwr.write_consumed = 0;
    bwr.write_buffer = 0;
    
    readbuf[0] = BC_ENTER_LOOPER;
    binder_write(bs, readbuf, sizeof(unsigned));

    for (;;) {
        bwr.read_size = sizeof(readbuf);
        bwr.read_consumed = 0;
        bwr.read_buffer = (unsigned) readbuf;

        res = ioctl(bs->fd, BINDER_WRITE_READ, &bwr);

        if (res < 0) {
            LOGE("binder_loop: ioctl failed (%s)\n", strerror(errno));
            break;
        }

        res = binder_parse(bs, 0, readbuf, bwr.read_consumed, func);
        if (res == 0) {
            LOGE("binder_loop: unexpected reply?!\n");
            break;
        }
        if (res < 0) {
            LOGE("binder_loop: io error %d %s\n", res, strerror(errno));
            break;
        }
    }
}

int svcmgr_handler(struct binder_state *bs,
                   struct binder_txn *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)
{
    struct svcinfo *si;
    uint16_t *s;
    unsigned len;
    void *ptr;

//    LOGI("target=%p code=%d pid=%d uid=%d\n",
//         txn->target, txn->code, txn->sender_pid, txn->sender_euid);

    if (txn->target != svcmgr_handle)
        return -1;

    s = bio_get_string16(msg, &len);

    if ((len != (sizeof(svcmgr_id) / 2)) ||
        memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
        fprintf(stderr,"invalid id %s\n", str8(s));
        return -1;
    }

    switch(txn->code) {
    case SVC_MGR_GET_SERVICE:
    case SVC_MGR_CHECK_SERVICE:
        s = bio_get_string16(msg, &len);
        ptr = do_find_service(bs, s, len);
        if (!ptr)
            break;
        bio_put_ref(reply, ptr);
        return 0;

    case SVC_MGR_ADD_SERVICE:
        s = bio_get_string16(msg, &len);
        ptr = bio_get_ref(msg);
        if (do_add_service(bs, s, len, ptr, txn->sender_euid)) //add a service to svclist
            return -1;
        break;

    case SVC_MGR_LIST_SERVICES: {
        unsigned n = bio_get_uint32(msg);

        si = svclist;
        while ((n-- > 0) && si)
            si = si->next;
        if (si) {
            bio_put_string16(reply, si->name);
            return 0;
        }
        return -1;
    }
    default:
        LOGE("unknown code %d\n", txn->code);
        return -1;
    }

    bio_put_uint32(reply, 0);
    return 0;
}

5. MediaPlayerService等待请求
/system/bin/mediaserver在ProcessState::Self()中打开了binder,其looper又在哪儿呢?

//main_mediaserver.cpp
int main(int argc, char** argv)
{
    sp<ProcessState> proc(ProcessState::self());
    sp<IServiceManager> sm = defaultServiceManager();
    LOGI("ServiceManager: %p", sm.get());
    AudioFlinger::instantiate();
    MediaPlayerService::instantiate();
    CameraService::instantiate();
    AudioPolicyService::instantiate();
    ProcessState::self()->startThreadPool();
    IPCThreadState::self()->joinThreadPool();
}

5.1 startThreadPool

void ProcessState::startThreadPool()
{
    AutoMutex _l(mLock);
    if (!mThreadPoolStarted) {
        mThreadPoolStarted = true;
        spawnPooledThread(true);
    }
}
void ProcessState::spawnPooledThread(bool isMain)
{
    if (mThreadPoolStarted) {
        int32_t s = android_atomic_add(1, &mThreadPoolSeq);
        char buf[32];

        sprintf(buf, "Binder Thread #%d", s);
        LOGV("Spawning new pooled thread, name=%s\n", buf);

        //创建线程池，然后run起来，和java的Thread何其像也。
        sp<Thread> t = new PoolThread(isMain);
        t->run(buf);
    }
}

class PoolThread : public Thread
{
public:
    PoolThread(bool isMain)
        : mIsMain(isMain)
    {
    }
    
protected:
    virtual bool threadLoop()
    {
        IPCThreadState::self()->joinThreadPool(mIsMain);
        return false;
    }
    
    const bool mIsMain;
};

//还没有创建线程
status_t Thread::run(const char* name, int32_t priority, size_t stack)
{
    Mutex::Autolock _l(mLock);

    if (mRunning) {
        // thread already started
        return INVALID_OPERATION;
    }

    // reset status and exitPending to their default value, so we can
    // try again after an error happened (either below, or in readyToRun())
    mStatus = NO_ERROR;
    mExitPending = false;
    mThread = thread_id_t(-1);
    
    // hold a strong reference on ourself
    mHoldSelf = this;

    mRunning = true;

    bool res;
    if (mCanCallJava) {
        //name为android:unnamed_thread
        res = createThreadEtc(_threadLoop,
                this, name, priority, stack, &mThread);
    } else {
        res = androidCreateRawThreadEtc(_threadLoop,
                this, name, priority, stack, &mThread);
    }
    
    
    if (res == false) {
        mStatus = UNKNOWN_ERROR;   // something happened!
        mRunning = false;
        mThread = thread_id_t(-1);
        mHoldSelf.clear();  // "this" may have gone away after this.

        return UNKNOWN_ERROR;
    }
    
    // Do not refer to mStatus here: The thread is already running (may, in fact
    // already have exited with a valid mStatus result). The NO_ERROR indication
    // here merely indicates successfully starting the thread and does not
    // imply successful termination/execution.
    return NO_ERROR;
}

int androidCreateRawThreadEtc(android_thread_func_t entryFunction,
                               void *userData,
                               const char* threadName,
                               int32_t threadPriority,
                               size_t threadStackSize,
                               android_thread_id_t *threadId)
{
    pthread_attr_t attr; 
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);

#ifdef HAVE_ANDROID_OS  /* valgrind is rejecting RT-priority create reqs */
    if (threadPriority != PRIORITY_DEFAULT || threadName != NULL) {
        // We could avoid the trampoline if there was a way to get to the
        // android_thread_id_t (pid) from pthread_t
        thread_data_t* t = new thread_data_t;
        t->priority = threadPriority;
        t->threadName = threadName ? strdup(threadName) : NULL;
        t->entryFunction = entryFunction;
        t->userData = userData;
        entryFunction = (android_thread_func_t)&thread_data_t::trampoline;
        userData = t;            
    }
#endif

    if (threadStackSize) {
        pthread_attr_setstacksize(&attr, threadStackSize);
    }
    
    errno = 0;
    pthread_t thread;
    //终于看到了熟悉的线程创建，入口函数为:_threadLoop
    int result = pthread_create(&thread, &attr,
                    (android_pthread_entry)entryFunction, userData);
    if (result != 0) {
        LOGE("androidCreateRawThreadEtc failed (entry=%p, res=%d, errno=%d)\n"
             "(android threadPriority=%d)",
            entryFunction, result, errno, threadPriority);
        return 0;
    }

    if (threadId != NULL) {
        *threadId = (android_thread_id_t)thread; // XXX: this is not portable
    }
    return 1;
}

 

新开的线程的入口函数为：_threadLoop
 

int Thread::_threadLoop(void* user)
{
    Thread* const self = static_cast<Thread*>(user);
    sp<Thread> strong(self->mHoldSelf);
    wp<Thread> weak(strong);
    self->mHoldSelf.clear();

#if HAVE_ANDROID_OS
    // this is very useful for debugging with gdb
    self->mTid = gettid();
#endif

    bool first = true;

    do {
        bool result;
        if (first) {
            first = false;
            self->mStatus = self->readyToRun();
            result = (self->mStatus == NO_ERROR);

            if (result && !self->mExitPending) {
                // Binder threads (and maybe others) rely on threadLoop
                // running at least once after a successful ::readyToRun()
                // (unless, of course, the thread has already been asked to exit
                // at that point).
                // This is because threads are essentially used like this:
                //   (new ThreadSubclass())->run();
                // The caller therefore does not retain a strong reference to
                // the thread and the thread would simply disappear after the
                // successful ::readyToRun() call instead of entering the
                // threadLoop at least once.
                result = self->threadLoop();  //调用自己的threadLoop
            }
        } else {
            result = self->threadLoop();
        }

        if (result == false || self->mExitPending) {
            self->mExitPending = true;
            self->mLock.lock();
            self->mRunning = false;
            self->mThreadExitedCondition.broadcast();
            self->mLock.unlock();
            break;
        }
        
        // Release our strong reference, to let a chance to the thread
        // to die a peaceful death.
        strong.clear();
        // And immediately, re-acquire a strong reference for the next loop
        strong = weak.promote();
    } while(strong != 0);
    
    return 0;
}

PoolThread::threadLoop

class PoolThread : public Thread
{
public:
    PoolThread(bool isMain)
        : mIsMain(isMain)
    {
    }
    
protected:
    virtual bool threadLoop()
    {   ////mIsMain为true。 而且注意，这是一个新的线程，所以必然会创建一个新的IPCThreadState对象（记得线程本地存储吗？TLS）
        IPCThreadState::self()->joinThreadPool(mIsMain);
        return false;
    }
    
    const bool mIsMain;
};

主线程和工作线程都调用了joinThreadPool

void IPCThreadState::joinThreadPool(bool isMain)
{
    LOG_THREADPOOL("**** THREAD %p (PID %d) IS JOINING THE THREAD POOL\n", (void*)pthread_self(), getpid());

    mOut.writeInt32(isMain ? BC_ENTER_LOOPER : BC_REGISTER_LOOPER);
    
    // This thread may have been spawned by a thread that was in the background
    // scheduling group, so first we will make sure it is in the default/foreground
    // one to avoid performing an initial transaction in the background.
    androidSetThreadSchedulingGroup(mMyThreadId, ANDROID_TGROUP_DEFAULT);
        
    status_t result;
    do {
        int32_t cmd;
        
        // When we've cleared the incoming command queue, process any pending derefs
        if (mIn.dataPosition() >= mIn.dataSize()) {
            size_t numPending = mPendingWeakDerefs.size();
            if (numPending > 0) {
                for (size_t i = 0; i < numPending; i++) {
                    RefBase::weakref_type* refs = mPendingWeakDerefs[i];
                    refs->decWeak(mProcess.get());
                }
                mPendingWeakDerefs.clear();
            }

            numPending = mPendingStrongDerefs.size();
            if (numPending > 0) {
                for (size_t i = 0; i < numPending; i++) {
                    BBinder* obj = mPendingStrongDerefs[i];
                    obj->decStrong(mProcess.get());
                }
                mPendingStrongDerefs.clear();
            }
        }

        // now get the next command to be processed, waiting if necessary
        result = talkWithDriver();
        if (result >= NO_ERROR) {
            size_t IN = mIn.dataAvail();
            if (IN < sizeof(int32_t)) continue;
            cmd = mIn.readInt32();
            IF_LOG_COMMANDS() {
                alog << "Processing top-level Command: "
                    << getReturnString(cmd) << endl;
            }


            result = executeCommand(cmd);
        }
        
        // After executing the command, ensure that the thread is returned to the
        // default cgroup before rejoining the pool.  The driver takes care of
        // restoring the priority, but doesn't do anything with cgroups so we
        // need to take care of that here in userspace.  Note that we do make
        // sure to go in the foreground after executing a transaction, but
        // there are other callbacks into user code that could have changed
        // our group so we want to make absolutely sure it is put back.
        androidSetThreadSchedulingGroup(mMyThreadId, ANDROID_TGROUP_DEFAULT);

        // Let this thread exit the thread pool if it is no longer
        // needed and it is not the main process thread.
        if(result == TIMED_OUT && !isMain) {
            break;
        }
    } while (result != -ECONNREFUSED && result != -EBADF);

    LOG_THREADPOOL("**** THREAD %p (PID %d) IS LEAVING THE THREAD POOL err=%p\n",
        (void*)pthread_self(), getpid(), (void*)result);
    
    mOut.writeInt32(BC_EXIT_LOOPER);
    talkWithDriver(false);
}

有loop了，但是有两个线程都执行了这个啊！这里有两个消息循环？
看看executeCommand

status_t IPCThreadState::executeCommand(int32_t cmd)
{
    BBinder* obj;
    RefBase::weakref_type* refs;
    status_t result = NO_ERROR;
    
    switch (cmd) {
    case BR_ERROR:
        result = mIn.readInt32();
        break;
        
    case BR_OK:
        break;
        
    case BR_ACQUIRE:
        refs = (RefBase::weakref_type*)mIn.readInt32();
        obj = (BBinder*)mIn.readInt32();
        LOG_ASSERT(refs->refBase() == obj,
                   "BR_ACQUIRE: object %p does not match cookie %p (expected %p)",
                   refs, obj, refs->refBase());
        obj->incStrong(mProcess.get());
        IF_LOG_REMOTEREFS() {
            LOG_REMOTEREFS("BR_ACQUIRE from driver on %p", obj);
            obj->printRefs();
        }
        mOut.writeInt32(BC_ACQUIRE_DONE);
        mOut.writeInt32((int32_t)refs);
        mOut.writeInt32((int32_t)obj);
        break;
        
    case BR_RELEASE:
        refs = (RefBase::weakref_type*)mIn.readInt32();
        obj = (BBinder*)mIn.readInt32();
        LOG_ASSERT(refs->refBase() == obj,
                   "BR_RELEASE: object %p does not match cookie %p (expected %p)",
                   refs, obj, refs->refBase());
        IF_LOG_REMOTEREFS() {
            LOG_REMOTEREFS("BR_RELEASE from driver on %p", obj);
            obj->printRefs();
        }
        mPendingStrongDerefs.push(obj);
        break;
        
    case BR_INCREFS:
        refs = (RefBase::weakref_type*)mIn.readInt32();
        obj = (BBinder*)mIn.readInt32();
        refs->incWeak(mProcess.get());
        mOut.writeInt32(BC_INCREFS_DONE);
        mOut.writeInt32((int32_t)refs);
        mOut.writeInt32((int32_t)obj);
        break;
        
    case BR_DECREFS:
        refs = (RefBase::weakref_type*)mIn.readInt32();
        obj = (BBinder*)mIn.readInt32();
        // NOTE: This assertion is not valid, because the object may no
        // longer exist (thus the (BBinder*)cast above resulting in a different
        // memory address).
        //LOG_ASSERT(refs->refBase() == obj,
        //           "BR_DECREFS: object %p does not match cookie %p (expected %p)",
        //           refs, obj, refs->refBase());
        mPendingWeakDerefs.push(refs);
        break;
        
    case BR_ATTEMPT_ACQUIRE:
        refs = (RefBase::weakref_type*)mIn.readInt32();
        obj = (BBinder*)mIn.readInt32();
         
        {
            const bool success = refs->attemptIncStrong(mProcess.get());
            LOG_ASSERT(success && refs->refBase() == obj,
                       "BR_ATTEMPT_ACQUIRE: object %p does not match cookie %p (expected %p)",
                       refs, obj, refs->refBase());
            
            mOut.writeInt32(BC_ACQUIRE_RESULT);
            mOut.writeInt32((int32_t)success);
        }
        break;
    
    case BR_TRANSACTION:
        {
            binder_transaction_data tr;
            result = mIn.read(&tr, sizeof(tr));
            LOG_ASSERT(result == NO_ERROR,
                "Not enough command data for brTRANSACTION");
            if (result != NO_ERROR) break;
            
            Parcel buffer;
            buffer.ipcSetDataReference(
                reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer),
                tr.data_size,
                reinterpret_cast<const size_t*>(tr.data.ptr.offsets),
                tr.offsets_size/sizeof(size_t), freeBuffer, this);
            
            const pid_t origPid = mCallingPid;
            const uid_t origUid = mCallingUid;
            
            mCallingPid = tr.sender_pid;
            mCallingUid = tr.sender_euid;
            
            int curPrio = getpriority(PRIO_PROCESS, mMyThreadId);
            if (gDisableBackgroundScheduling) {
                if (curPrio > ANDROID_PRIORITY_NORMAL) {
                    // We have inherited a reduced priority from the caller, but do not
                    // want to run in that state in this process.  The driver set our
                    // priority already (though not our scheduling class), so bounce
                    // it back to the default before invoking the transaction.
                    setpriority(PRIO_PROCESS, mMyThreadId, ANDROID_PRIORITY_NORMAL);
                }
            } else {
                if (curPrio >= ANDROID_PRIORITY_BACKGROUND) {
                    // We want to use the inherited priority from the caller.
                    // Ensure this thread is in the background scheduling class,
                    // since the driver won't modify scheduling classes for us.
                    // The scheduling group is reset to default by the caller
                    // once this method returns after the transaction is complete.
                    androidSetThreadSchedulingGroup(mMyThreadId,
                                                    ANDROID_TGROUP_BG_NONINTERACT);
                }
            }

            //LOGI(">>>> TRANSACT from pid %d uid %d\n", mCallingPid, mCallingUid);
            
            Parcel reply;
            IF_LOG_TRANSACTIONS() {
                TextOutput::Bundle _b(alog);
                alog << "BR_TRANSACTION thr " << (void*)pthread_self()
                    << " / obj " << tr.target.ptr << " / code "
                    << TypeCode(tr.code) << ": " << indent << buffer
                    << dedent << endl
                    << "Data addr = "
                    << reinterpret_cast<const uint8_t*>(tr.data.ptr.buffer)
                    << ", offsets addr="
                    << reinterpret_cast<const size_t*>(tr.data.ptr.offsets) << endl;
            }
            if (tr.target.ptr) {
                sp<BBinder> b((BBinder*)tr.cookie); //这里用的是BBinder。
                const status_t error = b->transact(tr.code, buffer, &reply, 0);
                if (error < NO_ERROR) reply.setError(error);
                
            } else {
                const status_t error = the_context_object->transact(tr.code, buffer, &reply, 0);
                if (error < NO_ERROR) reply.setError(error);
            }
            
            //LOGI("<<<< TRANSACT from pid %d restore pid %d uid %d\n",
            //     mCallingPid, origPid, origUid);
            
            if ((tr.flags & TF_ONE_WAY) == 0) {
                LOG_ONEWAY("Sending reply to %d!", mCallingPid);
                sendReply(reply, 0);
            } else {
                LOG_ONEWAY("NOT sending reply to %d!", mCallingPid);
            }
            
            mCallingPid = origPid;
            mCallingUid = origUid;

            IF_LOG_TRANSACTIONS() {
                TextOutput::Bundle _b(alog);
                alog << "BC_REPLY thr " << (void*)pthread_self() << " / obj "
                    << tr.target.ptr << ": " << indent << reply << dedent << endl;
            }
            
        }
        break;
    
    case BR_DEAD_BINDER:
        {
            BpBinder *proxy = (BpBinder*)mIn.readInt32();
            proxy->sendObituary();
            mOut.writeInt32(BC_DEAD_BINDER_DONE);
            mOut.writeInt32((int32_t)proxy);
        } break;
        
    case BR_CLEAR_DEATH_NOTIFICATION_DONE:
        {
            BpBinder *proxy = (BpBinder*)mIn.readInt32();
            proxy->getWeakRefs()->decWeak(proxy);
        } break;
        
    case BR_FINISHED:
        result = TIMED_OUT;
        break;
        
    case BR_NOOP:
        break;
        
    case BR_SPAWN_LOOPER:
        mProcess->spawnPooledThread(false);
        break;
        
    default:
        printf("*** BAD COMMAND %d received from Binder driver\n", cmd);
        result = UNKNOWN_ERROR;
        break;
    }

    if (result != NO_ERROR) {
        mLastError = result;
    }
    
    return result;
}

BBinder的transact函数如下：

status_t BBinder::transact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    data.setDataPosition(0);

    status_t err = NO_ERROR;
    switch (code) {
        case PING_TRANSACTION:
            reply->writeInt32(pingBinder());
            break;
        default:
            err = onTransact(code, data, reply, flags);  //调用自己的onTransact,实际调用BnMediaPlayerService::onTransact
            break;
    }

    if (reply != NULL) {
        reply->setDataPosition(0);
    }

    return err;
}

BnMediaPlayerService从BBinder派生，所以会调用到它的onTransact函数，终于水落石出了，让我们看看BnMediaPlayerServcice的onTransact函数。

status_t BnMediaPlayerService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    // BnMediaPlayerService从BBinder和IMediaPlayerService派生
     // 所有IMediaPlayerService提供的函数都通过命令类型来区分

    switch(code) {
        case CREATE_URL: {
            CHECK_INTERFACE(IMediaPlayerService, data, reply);
            pid_t pid = data.readInt32();
            sp<IMediaPlayerClient> client =
                interface_cast<IMediaPlayerClient>(data.readStrongBinder());
            const char* url = data.readCString();

            KeyedVector<String8, String8> headers;
            int32_t numHeaders = data.readInt32();
            for (int i = 0; i < numHeaders; ++i) {
                String8 key = data.readString8();
                String8 value = data.readString8();
                headers.add(key, value);
            }
          //create是一个虚函数，由MediaPlayerService来实现,见MediaPlayerService::create
          //MediaPlayerService.cpp
            sp<IMediaPlayer> player = create(
                    pid, client, url, numHeaders > 0 ? &headers : NULL);

            reply->writeStrongBinder(player->asBinder());
            return NO_ERROR;
        } break;
        ...
   }
}

其实，到这里，我们就明白了。BnXXX的onTransact函数收取命令，然后派发到派生类XXX的对应函数，由他们完成实际的工作。

说明：
    这里有点特殊，startThreadPool和joinThreadPool完后确实有两个线程，主线程和工作线程，而且都在做消息循环。为什么要这么做呢？他们参数isMain都是true。不知道google搞什么。难道是怕一个线程工作量太多，所以搞两个线程来工作？这种解释应该也是合理的。
    网上有人测试过把最后一句屏蔽掉，也能正常工作。但是难道主线程退出了，程序还能不退出吗？这个...管它的，反正知道有两个线程在那处理就行了。