The index is now initialized after the setup phase (chain generation
and txs creation), since it doesn't participate on it at all.
This improves readability and splits setup from what we actually
want to check.
This also adds a check after Sync() to verify the index best block hash
matches the tip, so we know it fully synced before checking the
processed data. This will help catching errors as Sync() could have
aborted prematurely.
As a happy side effect, the SyncWithValidationInterfaceQueue() call at
the end of the test is no longer needed and has been removed.
Adds an outpoint -> txid index, which can be used to find which transactions spent a given output.
We use a composite key with 2 parts (suggested by @romanz): hash(spent outpoint) and tx position, with an empty value.
To find the spending tx for a given outpoint, we do a prefix search (prefix being the hash of the provided outpoint), and for all keys that match this prefix
we load the tx at the position specified in the key and return it, along with the block hash, if does spend the provided outpoint.
To handle reorgs we just erase the keys computed from the removed block.
This index is extremely useful for Lightning and more generally for layer-2 protocols that rely on chains of unpublished transactions.
If enabled, this index will be used by `gettxspendingprevout` when it does not find a spending transaction in the mempool.
