Fix potential deadlock in RecursiveReadLocker.

[mpoeter]

Scope & Purpose

Fixes a potential deadlock in RecursiveReadLocker. This could happen if some thread A has acquired a read-lock, then some thread B signals that it wants to acquire a write-lock and now thread A recursively wants to acquire another read-lock. Since our ReadWriteLocks usually prefer writers, the second attempt to acquire the read-lock would block, waiting for the writer to finish while the writer would block, waiting for the first read to finish. To solve this, the RecursiveReadLock does no longer follow the principal that writes have precedence over reads.

Should only be merged after #14104

• 💩 Bugfix (requires CHANGELOG entry)
• 📖 CHANGELOG entry made

Backports:

• No backports required

Testing & Verification

• This PR adds tests that were used to verify all changes:
  • Added new C++ Unit tests