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create_psbt skips full-prevtx requirement for PSBT-backed SegWit v0 inputs

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
74/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
rust
Domain
blockchain

Research direction

Read the PSBT input handling in src/selection.rs, especially the early return in Selection::create_psbt_with_rng and the later non_witness_utxo checks. Add the reproduction test from the issue to the src/selection.rs test module and run cargo test. Done means the test returns MissingFullTxForSegwitV0Input for a PSBT-backed SegWit v0 input with only witness_utxo.

Written by the indexing model from the issue text.

Description

bug

Describe the bug

PsbtParams::mandate_full_tx_for_segwit_v0 defaults to true and is documented as
requiring the full previous transaction (non_witness_utxo) for SegWit v0 inputs.
However, Selection::create_psbt_with_rng returns early for inputs added via
Input::from_psbt_input: it copies the supplied psbt::Input and continues before
reaching the non_witness_utxo checks (src/selection.rs:280-308 on master). A SegWit
v0 input carrying only a witness_utxo therefore produces a PSBT with no
non_witness_utxo, even with default params, so the prevout amount goes unverified
against a full previous transaction.

This issue was found by AI.

To Reproduce

Add to the src/selection.rs test module and run cargo test:

#[test]
fn test_psbt_input_still_requires_full_tx_for_segwit_v0() -> anyhow::Result<()> {
    let secp = Secp256k1::new();
    let (desc, _) = Descriptor::<DescriptorPublicKey>::parse_descriptor(
        &secp,
        &format!("wpkh({TEST_HEX_PK})"),
    )?;
    let prev_txout = TxOut {
        script_pubkey: desc.at_derivation_index(0)?.script_pubkey(),
        value: Amount::from_sat(100_000),
    };
    let input = Input::from_psbt_input(
        OutPoint::null(),
        Sequence::ENABLE_RBF_NO_LOCKTIME,
        bitcoin::psbt::Input {
            witness_utxo: Some(prev_txout),
            ..Default::default()
        },
        100,
        None,
        false,
        None,
    )?;
    let selection = Selection::new(
        vec![input],
        vec![Output::with_script(ScriptBuf::new(), Amount::from_sat(90_000))],
    );

    assert!(matches!(
        selection.create_psbt(PsbtParams::default()),
        Err(CreatePsbtError::MissingFullTxForSegwitV0Input(_))
    ));

    Ok(())
}

On master (b00f35b) this fails: create_psbt returns Ok and the emitted PSBT
input has non_witness_utxo: None.

Expected behavior

The full-previous-transaction requirement for SegWit v0 inputs should apply
consistently, including for inputs imported from an existing PSBT.

Build environment

  • bdk_tx tag/commit: b00f35b (master)

Additional context

The same code path also exists in the open refactors #73 (TxTemplate::build_psbt),
#82 and #85, so a fix should cover those if they land first.

Dominant language
Rust
Stars
13
Forks
17
No merged PRs in 30d

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