// releaseBuffer releases a buffer slot from the consumer back to the
// BufferQueue.  This may be done while the buffer's contents are still
// being accessed.  The fence will signal when the buffer is no longer
// in use. frameNumber is used to indentify the exact buffer returned.
//
// If releaseBuffer returns STALE_BUFFER_SLOT, then the consumer must free
// any references to the just-released buffer that it might have, as if it
// had received a onBuffersReleased() call with a mask set for the released
// buffer.
//
// Note that the dependencies on EGL will be removed once we switch to using
// the Android HW Sync HAL.
status_t BufferQueueConsumer::releaseBuffer(int slot, uint64_t frameNumber,
        const sp<Fence>& releaseFence, EGLDisplay eglDisplay,
        EGLSyncKHR eglFence) {

• 从消费者释放一个 buffer。它有可能在 buffer 内容仍然可以被访问时调用，当 buffer 不再使用时发送 fence 信号。frameNumber 用于标识要释放的 buffer。
• 如果返回了 STALE_BUFFER_SLOT，消费者必须释放所有引用。

    if (slot < 0 || slot >= BufferQueueDefs::NUM_BUFFER_SLOTS ||
            releaseFence == nullptr) {
        BQ_LOGE("releaseBuffer: slot %d out of range or fence %p NULL", slot,
                releaseFence.get());
        return BAD_VALUE;
    }

• 检查 slot 和 releaseFence，如果错误，则返回 BAD_VALUE。

    sp<IProducerListener> listener;
    { // Autolock scope
        std::lock_guard<std::mutex> lock(mCore->mMutex);

        // If the frame number has changed because the buffer has been reallocated,
        // we can ignore this releaseBuffer for the old buffer.
        // Ignore this for the shared buffer where the frame number can easily
        // get out of sync due to the buffer being queued and acquired at the
        // same time.
        if (frameNumber != mSlots[slot].mFrameNumber &&
                !mSlots[slot].mBufferState.isShared()) {
            return STALE_BUFFER_SLOT;
        }

• 如果 frameNumber 与 BufferSlot 的 mFrameNumber 不一致且该 slot 不是共享 buffer，则返回 STALE_BUFFER_SLOT，它表示该 buffer 再分配过，我们可以忽略对旧 buffer 的释放。
• 在共享 buffer，有可能 queue 和 acquire 同时进行导致失去同步，所以对共享 buffer 不检查帧数。

        if (!mSlots[slot].mBufferState.isAcquired()) {
            BQ_LOGE("releaseBuffer: attempted to release buffer slot %d "
                    "but its state was %s", slot,
                    mSlots[slot].mBufferState.string());
            return BAD_VALUE;
        }

• isAcquired() 返回 false，表示所有权不在消费者，返回 BAD_VALUE。

        mSlots[slot].mEglDisplay = eglDisplay;
        mSlots[slot].mEglFence = eglFence;
        mSlots[slot].mFence = releaseFence;
        mSlots[slot].mBufferState.release();
        // After leaving shared buffer mode, the shared buffer will
        // still be around. Mark it as no longer shared if this
        // operation causes it to be free.
        if (!mCore->mSharedBufferMode && mSlots[slot].mBufferState.isFree()) {
            mSlots[slot].mBufferState.mShared = false;
        }

• 设置 BufferSlot 的成员，调用 release() 变更 BufferState 为 FREE。如果关闭了共享 buffer 模式，且 buffer 状态是 FREE，则设置 mShared 为 false。

        // Don't put the shared buffer on the free list.
        if (!mSlots[slot].mBufferState.isShared()) {
            mCore->mActiveBuffers.erase(slot);
            mCore->mFreeBuffers.push_back(slot);
        }

        if (mCore->mBufferReleasedCbEnabled) {
            listener = mCore->mConnectedProducerListener;
        }

        mCore->mDequeueCondition.notify_all();
    } // Autolock scope

    // Call back without lock held
    if (listener != nullptr) {
        listener->onBufferReleased();
    }

    return NO_ERROR;
}

• 如果该 slot 不是共享 buffer，则从 mActiveBuffers 移除，加入到 mFreeBuffers。
• 如果启用了 buffer 释放回调，设置 listener，回调 onBufferReleased()。