lntest: add bitcoind miner backend

[bhandras]

Summary

This PR extends lntest with a bitcoind-backed miner implementation. Until now, lntest assumed a btcd miner (rpctest harness) even when the chain backend was bitcoind, which made it hard to exercise Bitcoin Core-specific
mining and mempool behavior.

A key motivation for this work is enabling lntest to support mining and validating v3 packages (and other Core policy/package-relay behaviors) in integration tests. Those workflows are most realistically exercised with Bitcoin Core as the miner.

What’s included

• New miner abstraction in lntest/miner with two concrete backends:
  • btcd backend (wrapping existing rpctest.Harness usage)
  • bitcoind backend (spawns a local bitcoind on regtest and implements the miner APIs needed by lntest)
• Harness updates to avoid btcd-only assumptions:
  • Miner lifecycle is routed through the backend interface.
  • Miner RPC calls (e.g. raw tx submission) go through the miner backend.
  • Funding-shim output index handling is made robust across backends.
• Test runner support:
  • Add -minerbackend itest flag so the miner backend can be selected independently from the chain backend.
  • Add a build-tag default for the miner backend.
• CI coverage:
  • Add a CI itest matrix entry that runs with backend=bitcoind and minerbackend=bitcoind to ensure we exercise the new path.

Why this matters for v3 packages

Package relay / v3 transaction workflows are fundamentally Bitcoin Core policy and mining driven. Having lntest able to:

• talk to a Core-backed miner,
• submit transactions via Core,
• mine blocks via Core,

is a prerequisite for writing reliable integration coverage around v3 package construction, acceptance, and mining behavior.

How to run

Locally:

• make itest backend=bitcoind minerbackend=bitcoind

CI will run an additional job (matrix entry) with the same arguments.

Notes

• Default behavior remains unchanged (btcd miner unless explicitly selected).
• make lint passes.

[gemini-code-assist]

Summary of Changes

Hello @bhandras, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the lntest framework by introducing a bitcoind-backed miner implementation. Previously, lntest relied solely on btcd for mining, even when bitcoind was used as the chain backend. This change provides a more realistic testing environment for Bitcoin Core-specific behaviors, such as those related to v3 transaction packages and mempool policies. By abstracting the miner backend, the framework becomes more flexible and robust, enabling comprehensive integration testing for advanced Bitcoin features.

Highlights

• Bitcoind Miner Backend: Introduced a new bitcoind-backed miner implementation for lntest, allowing integration tests to realistically exercise Bitcoin Core-specific mining and mempool behavior, especially for v3 packages.
• Miner Abstraction: A new lntest/miner abstraction was added, supporting both btcd and bitcoind as concrete miner backends, enhancing flexibility and future extensibility.
• Harness Updates: The lntest.Harness was updated to route miner lifecycle and RPC calls through the new MinerBackend interface, removing btcd-only assumptions and standardizing interactions.
• Configurable Miner Backend: Added a -minerbackend flag for integration tests, enabling independent selection of the miner backend from the chain backend, with build-tag defaults for convenience.
• CI Coverage: Expanded the CI matrix to include tests with backend=bitcoind and minerbackend=bitcoind to ensure comprehensive coverage of the new bitcoind miner path.

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Ignored Files

• Ignored by pattern: .github/workflows/** (1)
  • .github/workflows/main.yml

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[gemini-code-assist]

Code Review

This pull request introduces a bitcoind-backed miner implementation for lntest, which is a significant and well-executed enhancement. It allows for more realistic integration testing of Bitcoin Core-specific behaviors. The new miner abstraction is clean, and the refactoring of existing code to use it is thorough. My review includes one suggestion to improve maintainability by reducing code duplication in one of the modified files.