migration algorithm exits without processing all inputs.

[KtorZ]

Context

The migration algorithm is an iterative algorithm, which attempts to construct several coin selections in order to deplete a wallet. An initial "ideal batch size" is calculated beforehand, and then, the algorithm proceed to construct selections from batch of inputs of that ideal size (modulo the last batch).

Constructing a selection also means balancing it. Here, the algorithm will depletes change outputs until reaching an equilibrium and will take care of filtering out dust coins that are below a certain threshold.

There are cases where a given selection could not produce any value (all change output are below the threshold).

Expected Behavior

The algorithm should discard such "no-value" selections, and continue processing next batches until it has traversed all batches.

Current Behavior

The algorithm stops at the first "no-value" selections and returns, with whatever selections has been accumulated this far, possibly leaving many inputs behind.

Resolution

https://github.com/input-output-hk/cardano-coin-selection/blob/cff368d674fe58bc1a12e6564a26ccea21eb5aac/src/library/Cardano/CoinSelection/Algorithm/Migration.hs#L108-L112

The issue is the handling of the Nothing branch ☝️ which should not be pure [] (terminal case of the recursion) but simply migrate, discarding the ongoing selection, but continuing migrating others.