multi: Introduce --offline sync mode

This adds the --offline sync mode to the wallet. This mode allows the
wallet to run without connecting to the network through either SPV or
RPC modes. Calls that do not require access to the network can still be
executed.

This is mostly useful for running wallets which are meant only to
generate addresses and sign transactions without actually tracking
balances, such as air-gapped wallets.

Previously, such wallets required running with an unconnected dcrd
instance or in SPV mode with --spvconnect specified to an address that
would not provide network services, but these methods are prone to be
misused and recent changes have broken their correct behavior.

The offline mode is introduced by writing a custom NetworkBackend
implementation that is guaranteed to never attempt any external
connections.

config.go

@@ -133,6 +133,9 @@ type config struct {
 	dial         func(ctx context.Context, network, address string) (net.Conn, error)
 	lookup       func(name string) ([]net.IP, error)
 
+	// Offline mode.
+	Offline bool `long:"offline" description:"Do not sync the wallet"`
+
 	// SPV options
 	SPV        bool     `long:"spv" description:"Sync using simplified payment verification"`
 	SPVConnect []string `long:"spvconnect" description:"SPV sync only with specified peers; disables DNS seeding"`
@@ -869,6 +872,12 @@ func loadConfig(ctx context.Context) (*config, []string, error) {
 		}
 	}
 
+	if cfg.SPV && cfg.Offline {
+		err := errors.E("SPV and Offline mode cannot be specified at the same time")
+		fmt.Fprintln(os.Stderr, err)
+		return loadConfigError(err)
+	}
+
 	if cfg.SPV && cfg.EnableVoting {
 		err := errors.E("SPV voting is not possible: disable --spv or --enablevoting")
 		fmt.Fprintln(os.Stderr, err)

dcrwallet.go

@@ -443,9 +443,12 @@ func run(ctx context.Context) error {
 				}
 			}
 
-			if cfg.SPV {
+			switch {
+			case cfg.Offline:
+				w.SetNetworkBackend(wallet.OfflineNetworkBackend{})
+			case cfg.SPV:
 				spvLoop(ctx, w)
-			} else {
+			default:
 				rpcSyncLoop(ctx, w)
 			}
 		})

sample-dcrwallet.conf

@@ -53,6 +53,9 @@
 ; File containing root certificates to authenticate TLS connections with dcrd
 ; cafile=~/.dcrwallet/dcrd.cert
 
+; When enabled, do not perform any sync with the network, either through RPC or
+; SPV modes. Useful when this is an air-gapped wallet.
+; offline=0
 
 
 ; ------------------------------------------------------------------------------

wallet/network.go

@@ -81,3 +81,41 @@ type Caller interface {
 	// a result (if any), or nil if no result is needed.
 	Call(ctx context.Context, method string, res any, args ...any) error
 }
+
+var errOfflineNetworkBackend = errors.New("operation not supported in offline mode")
+
+// OfflineNetworkBackend is a NetworkBackend that fails every call. It is meant
+// to be used in wallets which will only perform local operations.
+type OfflineNetworkBackend struct{}
+
+func (o OfflineNetworkBackend) Blocks(ctx context.Context, blockHashes []*chainhash.Hash) ([]*wire.MsgBlock, error) {
+	return nil, errOfflineNetworkBackend
+}
+
+func (o OfflineNetworkBackend) CFiltersV2(ctx context.Context, blockHashes []*chainhash.Hash) ([]FilterProof, error) {
+	return nil, errOfflineNetworkBackend
+}
+
+func (o OfflineNetworkBackend) PublishTransactions(ctx context.Context, txs ...*wire.MsgTx) error {
+	return errOfflineNetworkBackend
+}
+
+func (o OfflineNetworkBackend) LoadTxFilter(ctx context.Context, reload bool, addrs []stdaddr.Address, outpoints []wire.OutPoint) error {
+	return errOfflineNetworkBackend
+}
+
+func (o OfflineNetworkBackend) Rescan(ctx context.Context, blocks []chainhash.Hash, save func(block *chainhash.Hash, txs []*wire.MsgTx) error) error {
+	return errOfflineNetworkBackend
+}
+
+func (o OfflineNetworkBackend) StakeDifficulty(ctx context.Context) (dcrutil.Amount, error) {
+	return 0, errOfflineNetworkBackend
+}
+
+func (o OfflineNetworkBackend) Synced(ctx context.Context) (bool, int32) {
+	return true, 0
+}
+
+// Compile time check to ensure OfflineNetworkBackend fulfills the
+// NetworkBackend interface.
+var _ NetworkBackend = OfflineNetworkBackend{}