auth lmdbbackend: pdnsutil affected by snapshot merge

[Algirdas-Z]

• [x] This is not a support question, I have read about opensource and will send support questions to the IRC channel, Github Discussions or the mailing list.
• [x] I have read and understood the 'out in the open' support policy

• Program: Authoritative
• Issue type: Bug report

Short description

pdnsutil list-zone gets blocked while lmdb snapshot merge is in progress

Environment

• Operating system: Rocky8
• Software version: Auth 4.8.1
• Software source: PowerDNS repository

Steps to reproduce

1. Have 2 instances synced by lightining stream
2. Change zone on instance1
3. Wait for instance1 to upload snapshot and instance2 to download it
4. On instance2 try to do pdnsutil list-zone $zone (it will hang until snapshot merge completes)

Expected behaviour

Listing or any other read-only operation should not be blocked

Actual behaviour

Can't list zone

Other information

[Habbie]

DNS queries did work, right?

[Algirdas-Z]

Yes, forgot to add. DNS queries works fine, this is only case with pdnsutil