Fix getOrCreate in a session registry

Your Java service keeps one session per user in a ConcurrentHashMap. The first getOrCreate called get, then put when it found nothing, so two threads could each create a session for the same user. Session ids come from a counter, so they also show how many sessions were ever created.

An assistant wrote assign_session_ids(calls), which replays registry calls with each map operation done in one atomic step. It is in the editor. It has a bug. Find it and fix it.

Input and output:

• calls: the calls in order. user is getOrCreate for that user. user|end closes that user's session.
• Return one number per call.

Rules the tests check:

• Split a call on |. The user is the first part. Return 0 if the user is empty, there are more than 2 parts, or there are 2 parts and the second is not end.
• getOrCreate for a user with a live session returns that session's id.
• getOrCreate for a user with no live session creates one and returns its id. Ids start at 1, and each new session gets the next one.
• end for a user with a live session returns its id and removes it. A later getOrCreate for that user creates a new session.
• end for a user with no live session returns 0.