Multi-threading issues; inconsistent locking

https://github.com/apache/incubator-ponymail-foal/blob/bb4a6642506f0bb2c5a2a67570433536c3b22f66/tools/import-mbox.py#L154

According to [1], list.pop() is thread-safe, so the lock should not be needed.
However the documentation does not say that len(list) is thread-safe.
If list.pop() does need to be protected by a lock, then surely [2] should be similarly protected?

The FAQ [1] does say that incrementing an integer is not atomic, so the various global counts might not be incremented properly.

[1] https://docs.python.org/3/faq/library.html#what-kinds-of-global-value-mutation-are-thread-safe
[2] https://github.com/apache/incubator-ponymail-foal/blob/bb4a6642506f0bb2c5a2a67570433536c3b22f66/tools/import-mbox.py#L131

Read tools/import-mbox.py around lines 131 and 154, then review the linked Python thread-safety FAQ. Trace the global counters and list operations across the worker threads to determine whether the accesses use a consistent locking policy. Done means the locking and counter behavior is documented by the relevant tests or verification, with no remaining ambiguity between the two cited locations.

Written by the indexing model from the issue text.