backend: fix off-by-one error in accounts scan

CHANGELOG.md

@@ -9,6 +9,7 @@
 - Fetch balances of multiple ETH accounts at the same time, instead of one by one.
 - Fix wrong btc/ltc transaction timestamp during header sync
 - Ethereum bugfix: show all internal transactions that share the same transaction ID
+- Allow up to 6 unused BTC/LTC accounts (previously 5)
 
 ## v4.48.4
 - macOS: fix potential USB communication issue with BitBox02 bootloaders <v1.1.2 and firmwares <v9.23.1

backend/accounts.go

@@ -55,7 +55,7 @@ const hardenedKeystart uint32 = hdkeychain.HardenedKeyStart
 // useful in recovery, so we can scan a fixed number of accounts to discover all funds.  The
 // alternative (or a complement) would be an accounts gap limit, similar to Bitcoin's address gap
 // limit, but simply use a hard limit for simplicity.
-const accountsHardLimit = 5
+const accountsHardLimit = 6
 
 // AccountsList is an accounts.Interface slice which implements a lookup method.
 type AccountsList []accounts.Interface
@@ -1609,11 +1609,12 @@ func (backend *Backend) maybeAddHiddenUnusedAccounts() {
 		// Account scan gap limit:
 		// - Previous account must be used for the next one to be scanned, but:
 		// - The first 5 accounts are always scanned as before we had accounts discovery, the
-		//   BitBoxApp allowed manual creation of 5 accounts, so we need to always scan these.
-		if maxAccount == nil || maxAccount.Used || maxAccountNumber < accountsHardLimit {
+		//   BitBoxApp allowed manual creation of 5 accounts, so we need to always scan these
+		nextAccountNumber := maxAccountNumber + 1
+		if maxAccount == nil || maxAccount.Used || nextAccountNumber < accountsHardLimit {
 			accountCode, err := backend.createAndPersistAccountConfig(
 				coinCode,
-				uint16(maxAccountNumber+1),
+				uint16(nextAccountNumber),
 				true,
 				"",
 				backend.keystore,
@@ -1626,7 +1627,7 @@ func (backend *Backend) maybeAddHiddenUnusedAccounts() {
 			}
 			log.
 				WithField("accountCode", accountCode).
-				WithField("accountNumber", maxAccountNumber+1).
+				WithField("accountNumber", nextAccountNumber).
 				Info("automatically created hidden account")
 			return &accountCode
 		}

backend/accounts_test.go

@@ -268,7 +268,7 @@ func TestNextAccountNumber(t *testing.T) {
 					signing.NewBitcoinConfiguration(
 						signing.ScriptTypeP2WPKH,
 						rootFingerprint2,
-						mustKeypath("m/84'/0'/4'"),
+						mustKeypath("m/84'/0'/5'"),
 						xpub,
 					),
 				},