invoices: add SkipAmountCheck field to invoiceUpdateCtx type

This commit introduces the `SkipAmountCheck` field to the
`invoiceUpdateCtx` type. This field serves as a flag to determine
whether to bypass the amount verification during the invoice settlement
process. It is set based on the client's input, allowing the invoice to
be settled even if the HTLC amount is less than the stated invoice
amount.

Author: ffranr

invoices/update.go

@@ -23,6 +23,10 @@ type invoiceUpdateCtx struct {
 	mpp                  *record.MPP
 	amp                  *record.AMP
 	metadata             []byte
+
+	// SkipAmountCheck is a flag that indicates whether the amount check
+	// should be skipped during the invoice settlement process.
+	SkipAmountCheck bool
 }
 
 // invoiceRef returns an identifier that can be used to lookup or update the
@@ -189,13 +193,13 @@ func updateMpp(ctx *invoiceUpdateCtx, inv *Invoice) (*InvoiceUpdateDesc,
 	}
 
 	// Don't accept zero-valued sets.
-	if ctx.mpp.TotalMsat() == 0 {
+	if !ctx.SkipAmountCheck && ctx.mpp.TotalMsat() == 0 {
 		return nil, ctx.failRes(ResultHtlcSetTotalTooLow), nil
 	}
 
 	// Check that the total amt of the htlc set is high enough. In case this
 	// is a zero-valued invoice, it will always be enough.
-	if ctx.mpp.TotalMsat() < inv.Terms.Value {
+	if !ctx.SkipAmountCheck && ctx.mpp.TotalMsat() < inv.Terms.Value {
 		return nil, ctx.failRes(ResultHtlcSetTotalTooLow), nil
 	}
 
@@ -204,7 +208,7 @@ func updateMpp(ctx *invoiceUpdateCtx, inv *Invoice) (*InvoiceUpdateDesc,
 	// Check whether total amt matches other htlcs in the set.
 	var newSetTotal lnwire.MilliSatoshi
 	for _, htlc := range htlcSet {
-		if ctx.mpp.TotalMsat() != htlc.MppTotalAmt {
+		if !ctx.SkipAmountCheck && ctx.mpp.TotalMsat() != htlc.MppTotalAmt { //nolint:lll
 			return nil, ctx.failRes(ResultHtlcSetTotalMismatch), nil
 		}
 
@@ -239,7 +243,7 @@ func updateMpp(ctx *invoiceUpdateCtx, inv *Invoice) (*InvoiceUpdateDesc,
 
 	// If the invoice cannot be settled yet, only record the htlc.
 	setComplete := newSetTotal >= ctx.mpp.TotalMsat()
-	if !setComplete {
+	if !ctx.SkipAmountCheck && !setComplete {
 		return &update, ctx.acceptRes(resultPartialAccepted), nil
 	}