funding: enforce BOLT-02 push_msat bound on fundee

[erickcestari]

Reject incoming OpenChannel messages where push_msat exceeds 1000 * funding_satoshis, as required by BOLT-02. The existing RejectPush flag only gates on push_msat > 0 and does not cover the spec bound.

An over-sized push is eventually caught downstream in reservation.go when theirBalance = capacity - fee - push_msat goes negative and ErrFunderBalanceDust is returned. Rejecting it up front produces a clearer, spec-aligned error and avoids the chanacceptor and commitment type negotiation round-trips for a channel we will refuse anyway.

[gemini-code-assist]

Summary of Changes

Hello, 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 enhances the channel funding process by enforcing the BOLT-02 requirement regarding the maximum allowed push amount. By validating this constraint early in the OpenChannel message processing, the system avoids unnecessary downstream computations and provides more descriptive error messages when a peer attempts to open a channel with an invalid configuration.

Highlights

• BOLT-02 Compliance: Implemented a strict check to ensure that incoming OpenChannel messages do not have a push_msat value exceeding 1000 times the funding_satoshis, as mandated by the BOLT-02 specification.
• Improved Error Handling: Added a specific ErrPushAmountTooLarge error to provide clearer, spec-aligned feedback when a channel opening request is rejected due to an invalid push amount.
• Test Coverage: Added a new test case TestFundingManagerPushAmountExceedsCapacity to verify that the funding manager correctly rejects tampered OpenChannel messages that violate the push amount bound.

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[github-actions]

PR Severity: CRITICAL -- Automated classification -- 2 files (excl. tests) -- 23 lines changed (excl. tests) -- funding/manager.go (CRITICAL: funding/) -- lnwallet/errors.go (CRITICAL: lnwallet/) -- funding/manager_test.go excluded (test file) -- No bump rules triggered -- Expert review warranted. <!-- pr-severity-bot -->

[gemini-code-assist]

Code Review

This pull request enforces the BOLT-02 requirement that the push_msat value must be less than or equal to the total funding amount during channel opening. To support this, a new error ErrPushAmountTooLarge was added to the lnwallet package, and a corresponding unit test was implemented in the funding manager to ensure proper rejection of invalid requests. I have no feedback to provide as the existing comments were purely explanatory.

[Abdulkbk]

LGTM ⚡️

Left some non-blocking feedback

[erickcestari]

LGTM ⚡️

Left some non-blocking feedback

Thanks @Abdulkbk for the review! Looking way better now.

[lightninglabs-deploy]

@erickcestari, remember to re-request review from reviewers when ready