The function of the _ in_progress variable under the Hooks class in the CI framework

I am using CI3.0.4 version question: do not understand the function of the _ in_progress variable; prevent endless loops? 1. Under what circumstances does an infinite loop occur? 2. Why is it not set in the first line of the _ run_hooks function? 3. What if the _ in_progress variable is not set for is_call (data? This... the CI 3.0.4 I used

Problem:
Do not understand the function of the _ in_progress variable; prevent endless loops?
1. Under what circumstances does an infinite loop occur?
2. Why is it not set in the first line of the _ run_hooks function?
3. What if the _ in_progress variable is not set for is_call (data?

I am puzzled by the role and use of this variable. Thank you for your explanation.

Reply content:

My CI 3.0.4

Problem:
Do not understand the function of the _ in_progress variable; prevent endless loops?
1. Under what circumstances does an infinite loop occur?
2. Why is it not set in the first line of the _ run_hooks function?
3. What if the _ in_progress variable is not set for is_call (data?

I am puzzled by the role and use of this variable. Thank you for your explanation.

Prevents endless loops caused by mutual calls between hooks