Potential deadlock in batch span processor and periodic metric reader after puma fork child processes

[xuan-cao-swi]

In Rails applications using Puma, Puma forks child processes when running in clustered mode. Components like BatchSpanProcessor and PeriodicMetricReader can restart their threads after forking, thanks to mechanisms like after_fork and reset_on_fork. However, I’ve noticed that the mutexes in these classes are not properly cleaned up (e.g., unlocked or reinitialized) after a fork. Since mutex objects and their states are copied to the child process, a locked mutex in the parent remains locked in the child. Attempting to lock it again in the child can cause a deadlock.

Although I haven’t seen reports of this issue occurring in practice, it’s something developers should be aware of when working with Puma and mutexes