at_exit hook sometimes freeze forever when a rails application executes test codes

Start by tracing the debug gem's at_exit hook and its behavior after a fork, using the Rails test-code sequence described in the issue as the reproduction context. Check how it waits for non-Ruby child processes and verify that the process finishes successfully without waiting indefinitely.

Your environment

• ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
• rdbg 1.11.0

Describe the bug

This issue is related to https://github.com/ruby/debug/issues/1113.

Since the at_exit hook waits for all child processes other than the Ruby process, it leads to the following problem in the test code of a real Rails application.

1. Load the debug gem
2. The capybara-playwright-driver starts Playwright as a child process
3. The capybara-playwright-driver registers an at_exit hook to terminate Playwright when the test ends
4. During test code execution, when a fork occurs, the debug gem registers the at_exit hook
5. After all test cases have finished, the debug gem's at_exit hook executes first and waits for the child process to terminate, but since the at_exit hook to terminate Playwright does not execute, it continues to wait indefinitely

This can occur normally if the debug gem is required.
If it happens in a CI execution environment, it could result in unnecessary costs by holding computing resources for a long time.

I believe that waiting for all child processes(including not Ruby process) after a fork occurs is a significant issue.

To Reproduce

I described above.

Expected behavior

process finishes successfully.