Bitcoin Core wallets will record which client last opened and decrypted them, to catch downgrades
Serguey Asael Shinder2 min readBitcoin Core merged pull request #32895, "wallet: Prepare for future upgrades by recording versions of last client to open and decrypt", on 5 October.
The problem it prepares for. When a wallet is upgraded automatically, Bitcoin Core does it in a way that still lets the user open the wallet in an older version. But if the user then loads it into an upgraded version again, the wallet may not repeat the automatic upgrade and can end up holding both upgraded and non-upgraded material. Recording which client last opened the wallet would let a newer version detect that upgrade-downgrade-upgrade sequence and upgrade again. For this to work, the recording must already exist in versions released before any new automatic upgrade.
What the PR does, in three commits.
- Decouples the wallet client version from the node version. All wallet versions must have bit 19 set; the initial value is
0 | (1 << 19). The PR description says the versions only need to be at least 299900, and forcing bit 19 was a simple way to guarantee that. - Adds Wallet Client Features, flags stored in a separate record and distinct from the existing wallet feature flags, because version numbers are less flexible than feature flags. The wallet client version becomes
(1 << 19) + 1. - Adds a last-decrypted features record, because some upgrades need private keys and can only run after the wallet is decrypted.
Why v24.0 is the floor. Wallets already carry a version record with the client version, but older releases did not update it consistently: since v24.0 it is updated whenever the client version differs, while earlier versions updated it only when the client was newer. The PR description concludes that Bitcoin Core will be able to detect a downgrade down to v24.0, and to develop several features that need automatic upgrades in parallel.