[C++] Avoid global variables for thread pools and at-fork handlers #33549
Description
Investigation revealed an issue where the global IO thread pool could be constructed before the at-fork handler internal state. The IO thread pool, created on library load, would register an at-fork handler; then, the at-fork handler state would be initialized and clobber the handler registered just before.
Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou
PRs and other links:
Note: This issue was originally created as ARROW-18383. Please see the migration documentation for further details.