closeAndUnregister makes a thread local transaction manager completely dead

[Kaned1as]

After using TransactionManager.closeAndUnregister(db) the thread becomes completely unusable for using any other database.

Approximate reproduction steps:

1. Make a database through Database.connect() in main thread
2. Use this database in a thread pool (threads 1, 2, 3)
3. Use TransactionManager.closeAndUnregister in last task (let's say, thread 1)
4. Now create another database through Database.connect() in main thread
5. Try to use this new database through the thread 1 of thread pool

This is what you get:

java.lang.IllegalStateException: Please call Database.connect() before using this code
	at org.jetbrains.exposed.sql.transactions.NotInitializedManager.currentOrNull(TransactionApi.kt:38) ~[exposed-core-0.37.3.jar!/:na]
	at org.jetbrains.exposed.sql.transactions.TransactionManager$Companion.currentOrNull(TransactionApi.kt:108) ~[exposed-core-0.37.3.jar!/:na]
	at org.jetbrains.exposed.sql.transactions.ThreadLocalTransactionManagerKt$transaction$1.invoke(ThreadLocalTransactionManager.kt:136) ~[exposed-core-0.37.3.jar!/:na]
	at org.jetbrains.exposed.sql.transactions.ThreadLocalTransactionManagerKt.keepAndRestoreTransactionRefAfterRun(ThreadLocalTransactionManager.kt:223) ~[exposed-core-0.37.3.jar!/:na]
	at org.jetbrains.exposed.sql.transactions.ThreadLocalTransactionManagerKt.transaction(ThreadLocalTransactionManager.kt:135) ~[exposed-core-0.37.3.jar!/:na]
	at org.jetbrains.exposed.sql.transactions.ThreadLocalTransactionManagerKt.transaction(ThreadLocalTransactionManager.kt:132) ~[exposed-core-0.37.3.jar!/:na]

[Kaned1as]

I'll provide a bit more context.

In TransactionManager.closeAndUnregister there is following statement:

val manager = registeredDatabases[database]
manager?.let {
    registeredDatabases.remove(database)
    databases.remove(database)
    currentDefaultDatabase.compareAndSet(database, null)
    if (currentThreadManager.get() == it) { // <-------- this one
        currentThreadManager.remove()
    }
}

it calls currentThreadManager.get(). If current thread has this thread local removed (very common situation when there's no current transaction), it will try to re-initialize it. And will fail, because both databases and registeredDatabases are empty now, so it will default to NotInitializedManager, which will stick and destroy any transaction(...) call that you may try in this thread afterwards.

Wrapping it into transaction(...) won't help, it will fail to finish transaction as there won't be any databases/managers left after the closeAndUnregister call.