feat(selector): allow multiple change sources with ChangeDescriptor#18
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Previously, the `change_descriptor` field only accepted a definite descriptor to derive the change output script pubkey. This worked for most cases but failed for scenarios - such as silent payments - where a definite descriptor is not available. Now `change_descriptor` is `change_script` and can take a ScriptSource, which can be a descriptor or the script itself. Also, `change_weights` field is added as a field of SelectorParams, to externalize the calculus of the satisfaction weight from the API. This allowed the remove of the `to_cs_change_weigths` method. `From<(ScriptSource, Amount)>` for Output is added for conveniency.
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ApproachNACK There is a problem with the current approach:
The caller is responsible for keeping these in sync, but nothing enforces that. If |
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… SelectorParams 8189b03 refactor!: inline ChangePolicy fields into SelectorParams (志宇) ffd1211 feat: add constructor methods for ChangeScript and ChangePolicy (志宇) b9e7b42 refactor!: eliminate redundant satisfaction weight in SelectorParams (志宇) Pull request description: Fixes #40 Fixes #42 ## Summary This PR fixes two issues with `SelectorParams`: **1. Redundant satisfaction weight (invalid representation)** `SelectorParams` previously had two independent fields that could both specify the change output's satisfaction weight: - `change_script: ScriptSource` — when this is the `Descriptor` variant, the satisfaction weight is derivable from the descriptor via `max_weight_to_satisfy`. - `change_policy: ChangePolicy` — also encodes the satisfaction weight (the spend cost of the change output). Nothing enforced that these agreed with each other, so callers could silently provide inconsistent values. Fixed by introducing **`ChangeScript`**, which bundles a raw script with an optional `satisfaction_weight`, or holds a `DefiniteDescriptor` from which it is derived automatically. `DrainWeights` is now computed internally from `ChangeScript` — there is a single source of truth. `ChangeScript::Descriptor` also accepts optional `satisfaction_assets` — when provided, the satisfaction weight is computed via `Plan` for a tighter estimate (useful for multisig/complex descriptors). Otherwise it falls back to `max_weight_to_satisfy`. The raw `bdk_coin_select::ChangePolicy` is also replaced with a **`ChangePolicy` enum** (`NoDust`, `NoDustLeastWaste`) that is converted to the `bdk_coin_select` type internally. This is `Debug + Clone + PartialEq + Eq`. Both `ChangeScript` and `ChangePolicy` have constructor methods to reduce boilerplate: - `ChangeScript::from_descriptor(desc)`, `from_descriptor_with_assets(desc, assets)`, `from_script(script, weight)` - `ChangePolicy::no_dust()`, `no_dust_least_waste(rate)`, with a builder-style `.min_value(amt)` **2. Hacky `AbsoluteFee` implementation** `FeeStrategy::AbsoluteFee` was implemented by smuggling the fee into `value_sum` and setting the feerate to zero. This meant the coin selector had no awareness of the fee, which interacted poorly with RBF logic (the zero feerate bypasses the original tx's feerate floor). Removed in favor of a direct `target_feerate: FeeRate` field. ### Design rationale The satisfaction weight is a physical property of the script — it describes how much it costs to spend a particular output. It is not a policy choice. When `change_script` is a descriptor, the satisfaction weight is literally derived from it. The fact that it *affects* coin selection doesn't make it *part of* the change policy, any more than the dust threshold is (and dust is already derived from the script). For the raw script case (e.g. silent payments), the satisfaction weight can't be derived and must be provided externally — but it's still a property of the script, not a policy decision. Bundling it into `ChangeScript::Script` makes this explicit. ## Context * #18 (comment) * #32 (comment) ACKs for top commit: aagbotemi: ACK 8189b03 ValuedMammal: ACK 8189b03 Tree-SHA512: c3bd9af598ec55c60ab04656d7907ec3882bd50bbd0783add94057bd2603bc1d0cbe134a3a2d601988e73c57d4c83c91538060e4091e3a150005489c293d9b21
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Description
Previously, the
change_descriptorfield only accepted a definite descriptor to derive the change output script pubkey. This worked for most cases but failed for scenarios - such as silent payments - where a definite descriptor is not yet available.To address this, introduce the
ChangeDescriptorenum, which supports:This makes change output generation flexible enough to handle both standard and exceptional workflows.
Fixes #17
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cargo +nigthly fmtbefore committingNew Features: