This is a server hang and not an issue with backup. While concurrent
DDLs in server gets in hanged state, mariabackup waits for DDLs to
finish trying to acquire MDL_BACKUP_BLOCK_DDL.

The server hang is serious in nature and caused by thread pool state
being incorrectly set to thread creation pending state while no creation
is actually pending. Once a thread pool reaches such state no new thread
gets created in the pool.

While it could possibly affect all thread pools in server, the innodb
thread pool is the victim in current bug where IO job gets blocked when
the pool is stuck with much less number of threads than intended.
Available workers are blocked in purge waiting for page lock to be
released by IO write (SX lock) causing a complete deadlock.

The issue is caused by the state variable m_thread_creation_pending
introduced by MDEV-31095: 9e62ab7a. We check and set the variable
early while attempting to create a new thread in pool but fail to reset
it if we exit the flow for other reasons like maximum threads reached
or get into thread creation throttling path.

Fix: The simple fix is to make sure that the state is reset back in case
we don't actually attempt to create the thread.