feat(key-wallet): add Signer::extended_public_key

[shumkov]

What

Adds a required extended_public_key(path) -> ExtendedPubKey method to the key_wallet::signer::Signer trait.

Why

The trait exposes sign_ecdsa + public_key, the latter returning only the compressed leaf point. A caller that needs a BIP-32 extended public key at a hardened path — e.g. DashPay contact-xpub derivation under m/9'/coin'/15'/account'/sender/recipient — could not obtain the chain code without the wallet's resident root private key, forcing the seed to be made resident.

This lets a Keychain/hardware-backed signer return the point + chain code + parent fingerprint at a (possibly hardened) path, so the caller can non-hardened-derive descendants offline with no resident seed. It unblocks the dashpay-platform work to delete the attach_wallet_seed workaround (the wallet stays external-signable; the signer answers xpub requests).

Details

• A signer that cannot export an xpub at a hardened path returns an error rather than panicking.
• The two in-repo InMemorySigner test signers derive it from their root xprv (ExtendedPubKey::from_priv); the NoDigestSigner test stubs are unreachable!().
• Pinned by a test asserting the signer's xpub at a hardened path equals the wallet's own derive_extended_public_key.
• Required, not defaulted: the trait's associated Error type is opaque, so a default body cannot construct a Self::Error to return; there are no production Signer impls in this crate.

Test

cargo test -p key-wallet --lib — 497 passed, 0 failed. fmt + clippy clean.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Wallet now supports extended public key derivation and export capabilities, enabling advanced key management scenarios, multi-signature workflows, and improved cross-wallet interoperability.

• Tests

  • Comprehensive tests added to verify extended public key derivation functionality works correctly across various wallet operations and scenarios.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 4f6a4c8d-1359-4a16-9555-020ebb3929e0

📥 Commits

Reviewing files that changed from the base of the PR and between ceee4a9 and b4779fc.

📒 Files selected for processing (3)

• key-wallet/src/signer.rs
• key-wallet/src/wallet/managed_wallet_info/asset_lock_builder.rs
• key-wallet/src/wallet/managed_wallet_info/transaction_building.rs

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📝 Walkthrough

Walkthrough

Adds a new async extended_public_key method to the Signer trait returning a BIP-32 ExtendedPubKey (with chain code and parent fingerprint). The method is implemented in test InMemorySigner stubs in two files via private-key derivation, and NoDigestSigner stubs are updated with unreachable/panic guards. A new Tokio test verifies the signer's output matches the wallet's own derivation.

Changes

Signer::extended_public_key addition

Layer / File(s)	Summary
Signer trait: import and method declaration key-wallet/src/signer.rs	Adds ExtendedPubKey to imports and declares the async extended_public_key(&self, path: &DerivationPath) -> Result<ExtendedPubKey, Self::Error> method on the Signer trait, with docs covering BIP-32 semantics and hardened-path error requirements.
InMemorySigner and NoDigestSigner implementations + test key-wallet/src/wallet/managed_wallet_info/asset_lock_builder.rs, key-wallet/src/wallet/managed_wallet_info/transaction_building.rs	Implements extended_public_key on InMemorySigner in both files by deriving an xpriv at the given path and returning the corresponding ExtendedPubKey. Adds unreachable/panic stubs on NoDigestSigner in both files. Adds a Tokio test asserting the signer's output for a hardened path equals wallet.derive_extended_public_key.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐇 A new key sprouts from the BIP-32 tree,
Chain codes and fingerprints, neat as can be.
The signer now stretches with paths long and deep,
Hardened paths graceful—no panics to weep.
Hop, hop, the wallet and signer agree! 🌿

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 23.08% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'feat(key-wallet): add Signer::extended_public_key' clearly and specifically summarizes the main change—adding a new method to the Signer trait.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/signer-extended-public-key

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[codecov]

Codecov Report

❌ Patch coverage is 68.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.03%. Comparing base (ceee4a9) to head (b4779fc).

Files with missing lines	Patch %	Lines
...c/wallet/managed_wallet_info/asset_lock_builder.rs	0.00%	5 Missing ⚠️
...wallet/managed_wallet_info/transaction_building.rs	85.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #817      +/-   ##
==========================================
+ Coverage   72.82%   73.03%   +0.21%     
==========================================
  Files         323      323              
  Lines       72000    72025      +25     
==========================================
+ Hits        52432    52602     +170     
+ Misses      19568    19423     -145     

Flag	Coverage Δ
core	76.74% <ø> (ø)
ffi	47.35% <ø> (+1.34%)	⬆️
rpc	20.00% <ø> (ø)
spv	90.29% <ø> (+0.11%)	⬆️
wallet	71.83% <68.00%> (+0.03%)	⬆️

Files with missing lines	Coverage Δ
key-wallet/src/signer.rs	100.00% <ø> (ø)
...wallet/managed_wallet_info/transaction_building.rs	88.41% <85.00%> (-0.19%)	⬇️
...c/wallet/managed_wallet_info/asset_lock_builder.rs	81.64% <0.00%> (-1.14%)	⬇️

... and 23 files with indirect coverage changes