[shubhamkmr04]

• Adds minerFee to BuildRefundTransaction, so a refund can pay an exact fee in sats. 0 keeps the fee rate. CreateRefundTransaction keeps its signature and passes 0.
• Claims and refunds now return an error instead of a transaction with no output or one below dust.

[kaloudis]

Holding this for v13.3.0 rather than the lnd-v0.21.4-beta-zeus.1 release (v13.2.3).

• It's based on lnd-v0.20.1-beta-zeus.1, so against .1 it conflicts well beyond mobile/swaps.go.
• mobile: build swap claims and refunds without broadcasting them #10 (for Swaps: native claim and refund broadcasts are hardcoded to mempool.space zeus#4939) refactors the same code. It adds BuildReverseClaimTransaction and BuildRefundTransaction, which return the signed transaction so the app can store and broadcast it through the user's explorer. The broadcastTx helper here keeps the hardcoded mempool.space broadcast that mobile: build swap claims and refunds without broadcasting them #10 takes out of the app's path.
• Adding minerFee to CreateRefundTransaction changes its signature, which breaks the current Zeus bridge unless feat: add fee rate / miner fee toggle to refund swap view zeus#4065 ships with it.

Once #10 is in, the simplest route for the exact refund fee is a minerFee parameter on BuildRefundTransaction. CreateRefundTransaction can then stay as it is for older app builds. Exact fees for reverse claims are already in the .1 series.

[kaloudis]

@shubhamkmr04 could you rebase this?

[shubhamkmr04]

Rebased onto lnd-v0.21.4-beta-zeus.1. #10 already covered the helper refactor, so this is now one commit: minerFee on both refund functions, with a shared swapFee helper and a test.

[kaloudis]

One bug: an exact minerFee at or above the lockup value produces an unbroadcastable transaction with a nil error. Details inline.

[kaloudis]

With a sats fee, nothing checks minerFee against the lockup value, and the Results from boltz.ConstructTransaction are discarded on line 234.

In boltz-client (pkg/boltz/transaction.go:134-140, same in v2.9.0 and v2.12.5), when the value is less than the fee, getOutValues only records value less than fee in Results and skips the output. constructBtcTransaction then builds a transaction with no outputs, and for a non-cooperative refund it signs the input and returns err == nil. When the fee equals the value, the check passes and a 0-sat output is added.

Reproduced on this branch with the existing newRefundFixture (100000 sat lockup), non-cooperative:

minerFee	err	outputs
100000	nil	1 output, value 0
150000	nil	none

Neither transaction relays (bad-txns-vout-empty / dust), and CreateRefundTransaction passes the mempool.space rejection back with no hint of the cause. Before this PR the refund only took a fee rate, so this needed an absurd rate.

Suggested fix:

1. Keep the results (refundTransaction, results, err := ...) and return results[id].Err when it is set.
2. Reject a minerFee that leaves the output below dust (330 sat for P2TR, 294 for P2WPKH), since Results has no error for the equal-value case.

A test calling BuildRefundTransaction with minerFee of 100000 and 150000 against the fixture, expecting an error, would cover both cases.

[shubhamkmr04]

Fixed in 08c2b5f. Both builders now return the error ConstructTransaction records in results, and reject a transaction with no output or one below dust (mempool.IsDust at the default relay fee, so 330/294). Tests cover 99800, 100000 and 150000 against the fixture.

[kaloudis]

Same issue here: results from ConstructTransaction (line 169) is discarded. If Boltz refuses the cooperative claim, boltz-client falls back to a non-cooperative claim, which can return the same empty-output or 0-value transaction with a nil error when minerFee is at or above the lockup value.

[shubhamkmr04]

Fixed in 08c2b5f with the same check. A test claims against a host that refuses to co-sign, with minerFee of 100000 and 150000, and expects an error.

[kaloudis]

This adds minerFee to CreateRefundTransaction's signature, but the doc comment says this wrapper is kept for app builds that predate BuildRefundTransaction. The current Zeus app still calls it with the old 13 arguments:

• ios/LncMobile/LncModule.mm:343 (LndmobileCreateRefundTransaction(..., feeRate, timeoutBlockHeight, ...))
• android/app/src/main/java/com/zeus/lnc-rn/LncModule.kt:269 (Lndmobile.createRefundTransaction(...))

Both stop compiling once this build is bundled, unless they are updated at the same time. CreateReverseClaimTransaction keeps its signature, so the refund wrapper should do the same: drop the new parameter here and pass 0 for minerFee to BuildRefundTransaction, which keeps the fee-rate behaviour. Only BuildRefundTransaction needs the new argument.