d8ee8f3cd3 refactor: Make CWalletTx sync state type-safe (Russell Yanofsky)
Pull request description:
Current `CWalletTx` state representation makes it possible to set inconsistent states that won't be handled correctly by wallet sync code or serialized & deserialized back into the same form.
For example, it is possible to call `setConflicted` without setting a conflicting block hash, or `setConfirmed` with no transaction index. And it's possible update individual `m_confirm` and `fInMempool` data fields without setting an overall consistent state that can be serialized and handled correctly.
Fix this without changing behavior by using `std::variant`, instead of an enum and collection of fields, to represent sync state, so state tracking code is safer and more legible.
This is a first step to fixing state tracking bugs https://github.com/bitcoin-core/bitcoin-devwiki/wiki/Wallet-Transaction-Conflict-Tracking, by adding an extra margin of safety that can prevent new bugs from being introduced as existing bugs are fixed.
ACKs for top commit:
laanwj:
re-ACK d8ee8f3cd3
jonatack:
Code review ACK d8ee8f3cd3
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cf24152596