Why Ansible and Python fork break on macOS High Sierra+ and how to solve

How to avoid the fork fatal error using Ansible and Python

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Environment

macOS High Sierra+

Issue

objc[22868]: +[__NSCFConstantString initialize] may have been in progress in another thread when fork() was called.
objc[22868]: +[__NSCFConstantString initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.

Resolution

• current session only

export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES

• for all future sessions

$ echo "OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES" >> .bash_profile

Save, exit, close terminal, and re-open the terminal. Verify with:

$ env
[...]
OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES

Root Cause

This error is caused by an added security to restrict multithreading in macOS High Sierra and later versions.