Make tree iterators safe under concurrency

You have a binary tree and an inorder iterator implementation that maintains internal traversal state (e.g., a stack). Consider a multi-threaded program where:

1. Multiple threads may create and use their own iterators over the same tree concurrently.
2. The tree may be either read-only during iteration, or it may be mutated concurrently (inserts/deletes/rotations).

Question

How would you design the iterator and surrounding system to be thread-safe under these scenarios?

In your answer, cover:

• What is safe when the tree is read-only?
• What can go wrong when the tree is mutated concurrently?
• At least two approaches to handle concurrent mutations (e.g., locking, snapshotting/versioning), including trade-offs.