multiprocessing.Process opens a FIFO to the child. This FIFO is not documented the the Process class API and it's purpose is not clear from the documentation. It is a minor documentation bug that the class creates non-transparent resource utilization.

The primary behavioral bug is that incorrect handling of this FIFO creates a resource leak, since the file descriptor is not closed on join(), or even when the parent Process object goes out of scope.

The effect of this bug is that programs generating large numbers of Process objects will hit system resource limits of open file descriptors.

It appears as if the problem is the sentinel FIFO opened by (for example) multiprocessing.popen_fork.Popen._launch(). It registers a finalization class to close the sentinel on garbage collection. Instead, it should be closed in poll() or wait() when the child process is reaped and known to be dead. The sentinel serves no purpose after the child is reaped, and waiting till garbage collection means that programs forking large numbers of processes cannot control file descriptor utilization.