The valid results should have a target below the sum of
the selected inputs amounts. Also, it increases the
minimum value for target to make it more realistic.
Instead of using `cost_of_change` for `min_viable_change`
and `change_cost`, and 0 for `change_fee`, use values from
`coin_params`. The previous values don't generate any effects
that is relevant for that context.
I discovered via fuzzing of another coin selection approach that at
extremely high feerates SRD may find input sets that lead to
transactions without change outputs. This is an unintended outcome since
SRD is meant to always produce a transaction with a change output—we use
other algorithms to specifically search for changeless solutions.
The issue occures when the flat allowance of 50,000 ṩ for change is
insufficient to pay for the creation of a change output with a non-dust
amount, at and above 1,613 ṩ/vB. Increasing the change budget by
change_fees makes SRD behave as expected at any feerates.
Uses a min-effective-value heap, so we can remove the least valuable input/s
while the selected weight exceeds the maximum allowed weight.
Co-authored-by: Murch <murch@murch.one>
The simplest scenario where this is useful is on the 'check_max_weight' unit test
already:
We create 1515 UTXOs with 0.033 BTC each, and 1 UTXO with 50 BTC. Then perform
Coin Selection.
As the selection of the 1515 small UTXOs exceeds the max allowed tx size, the
expectation here is to receive a selection result that only contain the big
UTXO (which is not happening for the reasons stated below).
As knapsack returns a result that exceeds the max allowed transaction size, we
fallback to SRD, which selects coins randomly up until the target is met. So
we end up with a selection result with lot more coins than what is needed.
And not hide it inside the `OutputGroup::Insert` method.
This method does not return anything if insertion fails.
We can know before calling `Insert` whether the coin
will be accepted or not.
and also fix spelling in test/lint/lint-locale-dependence.py not caught by the
spelling linter and fix up a paragraph we are touching here in test/README.md.
