This is a technical blog post, about how we investigated and solved an error occurring when running Rails tests.

Sometimes, when running a Rails command on macOS, we can encounter a error, promptly followed by a crash:

+[NSCheapMutableString 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.