Used preimages not cleared

[h4x3rotab]

Background

There are some used preimages on both Phala and Khala parachains. They were created for democracy proposals and have been consumed by successful execution. However, for some unknown reasons, they are still kept in the storage like shown below:

We have tried to call preimage.unnotePreimage() by the preimage owner account. After calling it, the owner of the preimage was removed, but the entry is still kept in the storage, probably because it always shows "Requested / 1", indicating some reference to the preimage was not removed properly.

Besides that, apparently there are also some undecodable preimages on Khala (shown in the screenshot with "Requested / 4,064,592,897"). It could be caused by the poorly executed runtime migration happened when introducing the preimage pallet. However, none of them is still in used. So we should remove them all.

Deliverables

1. Figure out how to remove the used preimages from the storage and fix the problem causing the problem
2. A plan to remove the undecodable preimages on Khala

Resources

• Build the standalone substrate chain: https://github.com/Phala-Network/phala-blockchain#native-build
• Build the parachain: https://github.com/Phala-Network/khala-parachain
• Parachain runtime config: https://github.com/Phala-Network/khala-parachain/tree/main/runtime
• Phala Polkadot.js console: https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Fphala.api.onfinality.io%2Fpublic-ws#/explorer
• Khala Polkadot.js console: https://polkadot.js.org/apps/?rpc=wss%3A%2F%2Fkhala-rpc.dwellir.com#/