mediapipe input policies and synchronization

Question

I have read the document about the mediapipe frameworks concepts. However, there is still something I failed to understand. I have some questions about the input policies and synchronization.

According to the description in Framework Concepts/Synchronization:

If packets with the same timestamp are provided on multiple input streams, they will always be processed together regardless of their arrival order in real time.

which shows how mediapipe deal with the packet with the same timestamp.

However, in Framework Concepts/Calculators, there is another description:

For a calculator with inputs, the framework calls Process() repeatedly whenever at least one input stream has a packet available.

Personally, I think these two properties are a little bit inconsistent. If there are two packet with same timestamp, arriving at different time, the arrival of the first packet will invoke the Process() of the calculator. But the second packet hasn't arrived yet! So how to process these two packets together?

This question really confuse me. I know there must be something that I misunderstood. So could you please tell me how to understand the input policies and sychronization correctly?

What's more, I just wonder how mediapipe keep packets with the same timestamp processed together. Could you please show me the relevant code about this property?