// releaseBufferLocked relinquishes control over a buffer, returning that
// control to the BufferQueue.
//
// Derived classes should override this method to perform any cleanup that
// must take place when a buffer is released back to the BufferQueue.  If
// it is overridden the derived class's implementation must call
// ConsumerBase::releaseBufferLocked.
status_t ConsumerBase::releaseBufferLocked(
        int slot, const sp<GraphicBuffer> graphicBuffer,
        EGLDisplay display, EGLSyncKHR eglFence) {
    ...
    // If consumer no longer tracks this graphicBuffer (we received a new
    // buffer on the same slot), the buffer producer is definitely no longer
    // tracking it.
    if (!stillTracking(slot, graphicBuffer)) {
        return OK;
    }

    status_t err = mConsumer->releaseBuffer(slot, mSlots[slot].mFrameNumber,
            display, eglFence, mSlots[slot].mFence);
    if (err == IGraphicBufferConsumer::STALE_BUFFER_SLOT) {
        freeBufferLocked(slot);
    }
    ...
    return err;
}

bool ConsumerBase::stillTracking(int slot,
        const sp<GraphicBuffer> graphicBuffer) {
    if (slot < 0 || slot >= BufferQueue::NUM_BUFFER_SLOTS) {
        return false;
    }
    return (mSlots[slot].mGraphicBuffer != nullptr &&
            mSlots[slot].mGraphicBuffer->handle == graphicBuffer->handle);
}

• 如果 stillTracking() 返回 false，说明从同一个 slot 中返回了不同的 buffer，就不需要释放。这个机制像是错过了该释放的 GraphicBuffer，所以不释放了。否则调用 BufferQueueConsumer::releaseBuffer() 函数释放该 buffer。
• 如果 BufferQueueConsumer::releaseBuffer() 返回了 STALE_BUFFER_SLOT，表示消费者需要释放 buffer 引用，所以调用了 ConsumerBase::freeBufferLocked()。