* Reorder imports to follow convention
Place the imports from `std` at the top.
* Add transaction errors for double spends
Add a variant for each pool. They represent a double spend inside a
transaction.
* Add `check::spend_conflicts` implementation
Checks if a transaction has spend conflicts, i.e., if a transaction
spends a UTXO more than once or if it reveals a nullifier more than
once.
* Reject transactions with internal spend conflicts
The transaction verifier should reject transactions that spend the same
transparent UTXO or that reveal the same nullifier.
* Add transparent spend consensus rule
Add it to the documentation to help with understanding and auditing it.
Co-authored-by: teor <teor@riseup.net>
* Use different nullifiers by default
Don't use the same nullifier twice when mocking a
`sprout::JoinSplitData` because it will lead to an invalid transaction.
* Test transactions with repeated spend outpoints
Since that represents a spend conflict, they should be rejected.
* Test duplicate nullifiers in joinsplit
Check if a mock transaction with a joinsplit that reveals the same
nullifier twice is rejected.
* Test duplicate nullifiers across joinsplits
Check if a duplicate nullifier in two different joinsplits in the same
transaction is rejected.
* Test V4 transaction with duplicate Sapling spend
Check if a V4 transaction that has a duplicate Sapling spend is
rejected.
* Test V5 transaction with duplicate Sapling spend
Check if a V5 transaction that has a duplicate Sapling spend is
rejected.
* Test V5 transaction with duplicate Orchard actions
Check if a V5 transaction that has duplicate Orchard actions is rejected
by the transaction verifier.
Co-authored-by: teor <teor@riseup.net>
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