multi: remove debug invoices

Debug invoices are rarely used nowadays, but keep asking for maintenance
every time refactoring in primarily the invoice registry occurs. We have
passed the cost/benefit tipping point, so therefore the debug invoice
concept is removed in this commit.

Previously the debughtlc flag also controlled whether hodl masks were
active. It is safe to remove that additional condition because the hodl
masks are still guarded by the dev build tag.

invoices/invoiceregistry.go

@@ -1,14 +1,11 @@
 package invoices
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"sync"
 	"sync/atomic"
-	"time"
 
-	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lntypes"
@@ -16,18 +13,6 @@ import (
 	"github.com/lightningnetwork/lnd/queue"
 )
 
-var (
-	// DebugPre is the default debug preimage which is inserted into the
-	// invoice registry if the --debughtlc flag is activated on start up.
-	// All nodes initialized with the flag active will immediately settle
-	// any incoming HTLC whose rHash corresponds with the debug
-	// preimage.
-	DebugPre, _ = lntypes.MakePreimage(bytes.Repeat([]byte{1}, 32))
-
-	// DebugHash is the hash of the default preimage.
-	DebugHash = DebugPre.Hash()
-)
-
 var (
 	// ErrInvoiceExpiryTooSoon is returned when an invoice is attempted to be
 	// accepted or settled with not enough blocks remaining.
@@ -73,11 +58,6 @@ type InvoiceRegistry struct {
 	// new single invoice subscriptions are carried.
 	invoiceEvents chan interface{}
 
-	// debugInvoices is a map which stores special "debug" invoices which
-	// should be only created/used when manual tests require an invoice
-	// that *all* nodes are able to fully settle.
-	debugInvoices map[lntypes.Hash]*channeldb.Invoice
-
 	// decodeFinalCltvExpiry is a function used to decode the final expiry
 	// value from the payment request.
 	decodeFinalCltvExpiry func(invoice string) (uint32, error)
@@ -110,7 +90,6 @@ func NewRegistry(cdb *channeldb.DB, decodeFinalCltvExpiry func(invoice string) (
 
 	return &InvoiceRegistry{
 		cdb:                       cdb,
-		debugInvoices:             make(map[lntypes.Hash]*channeldb.Invoice),
 		notificationClients:       make(map[uint32]*InvoiceSubscription),
 		singleNotificationClients: make(map[uint32]*SingleInvoiceSubscription),
 		newSubscriptions:          make(chan *InvoiceSubscription),
@@ -392,32 +371,6 @@ func (i *InvoiceRegistry) deliverSingleBacklogEvents(
 	return nil
 }
 
-// AddDebugInvoice adds a debug invoice for the specified amount, identified
-// by the passed preimage. Once this invoice is added, subsystems within the
-// daemon add/forward HTLCs that are able to obtain the proper preimage
-// required for redemption in the case that we're the final destination.
-func (i *InvoiceRegistry) AddDebugInvoice(amt btcutil.Amount,
-	preimage lntypes.Preimage) {
-
-	paymentHash := preimage.Hash()
-
-	invoice := &channeldb.Invoice{
-		CreationDate: time.Now(),
-		Terms: channeldb.ContractTerm{
-			Value:           lnwire.NewMSatFromSatoshis(amt),
-			PaymentPreimage: preimage,
-		},
-	}
-
-	i.Lock()
-	i.debugInvoices[paymentHash] = invoice
-	i.Unlock()
-
-	log.Debugf("Adding debug invoice %v", newLogClosure(func() string {
-		return spew.Sdump(invoice)
-	}))
-}
-
 // AddInvoice adds a regular invoice for the specified amount, identified by
 // the passed preimage. Additionally, any memo or receipt data provided will
 // also be stored on-disk. Once this invoice is added, subsystems within the
@@ -458,19 +411,8 @@ func (i *InvoiceRegistry) AddInvoice(invoice *channeldb.Invoice,
 //
 // TODO(roasbeef): ignore if settled?
 func (i *InvoiceRegistry) LookupInvoice(rHash lntypes.Hash) (channeldb.Invoice, uint32, error) {
-	// First check the in-memory debug invoice index to see if this is an
-	// existing invoice added for debugging.
-	i.RLock()
-	debugInv, ok := i.debugInvoices[rHash]
-	i.RUnlock()
-
-	// If found, then simply return the invoice directly.
-	if ok {
-		return *debugInv, 0, nil
-	}
-
-	// Otherwise, we'll check the database to see if there's an existing
-	// matching invoice.
+	// We'll check the database to see if there's an existing matching
+	// invoice.
 	invoice, err := i.cdb.LookupInvoice(rHash)
 	if err != nil {
 		return channeldb.Invoice{}, 0, err
@@ -556,18 +498,6 @@ func (i *InvoiceRegistry) NotifyExitHopHtlc(rHash lntypes.Hash,
 		log.Debugf("Invoice(%x): %v, amt=%v, expiry=%v",
 			rHash[:], s, amtPaid, expiry)
 	}
-	// First check the in-memory debug invoice index to see if this is an
-	// existing invoice added for debugging.
-	if invoice, ok := i.debugInvoices[rHash]; ok {
-		debugLog("payment to debug invoice accepted")
-
-		// Debug invoices are never fully settled, so we just settle the
-		// htlc in this case.
-		return &HodlEvent{
-			Hash:     rHash,
-			Preimage: &invoice.Terms.PaymentPreimage,
-		}, nil
-	}
 
 	// If this isn't a debug invoice, then we'll attempt to settle an
 	// invoice matching this rHash on disk (if one exists).