multi: log warnings if deprecated config options are used

This commit adds warning logs if the user uses deprecated config
options, paving the way for future removal.

config.go

@@ -813,6 +813,9 @@ func LoadConfig(interceptor signal.Interceptor) (*Config, error) {
 		ltndLog.Warnf("%v", configFileError)
 	}
 
+	// Finally, log warnings for deprecated config options if they are set.
+	logWarningsForDeprecation(*cleanCfg)
+
 	return cleanCfg, nil
 }
 
@@ -2112,12 +2115,20 @@ func checkEstimateMode(estimateMode string) error {
 		bitcoindEstimateModes[:])
 }
 
-// configToFlatMap converts the given config struct into a flat map of key/value
+// configToFlatMap converts the given config struct into a flat map of
-// pairs using the dot notation we are used to from the config file or command
+// key/value pairs using the dot notation we are used to from the config file
-// line flags.
+// or command line flags. It also returns a map containing deprecated config
-func configToFlatMap(cfg Config) (map[string]string, error) {
+// options.
+func configToFlatMap(cfg Config) (map[string]string,
+	map[string]struct{}, error) {
+
 	result := make(map[string]string)
 
+	// deprecated stores a map of deprecated options found in the config
+	// that are set by the users. A config option is considered as
+	// deprecated if it has a `hidden` flag.
+	deprecated := make(map[string]struct{})
+
 	// redact is the helper function that redacts sensitive values like
 	// passwords.
 	redact := func(key, value string) string {
@@ -2159,6 +2170,8 @@ func configToFlatMap(cfg Config) (map[string]string, error) {
 			longName := fieldType.Tag.Get("long")
 			namespace := fieldType.Tag.Get("namespace")
 			group := fieldType.Tag.Get("group")
+			hidden := fieldType.Tag.Get("hidden")
+
 			switch {
 			// We have a long name defined, this is a config value.
 			case longName != "":
@@ -2172,6 +2185,11 @@ func configToFlatMap(cfg Config) (map[string]string, error) {
 					"%v", field.Interface(),
 				))
 
+				// If there's a hidden flag, it's deprecated.
+				if hidden == "true" && !field.IsZero() {
+					deprecated[key] = struct{}{}
+				}
+
 			// We have no long name but a namespace, this is a
 			// nested struct.
 			case longName == "" && namespace != "":
@@ -2202,5 +2220,18 @@ func configToFlatMap(cfg Config) (map[string]string, error) {
 	// Turn the whole config struct into a flat map.
 	printConfig(reflect.ValueOf(cfg), "")
 
-	return result, nil
+	return result, deprecated, nil
+}
+
+// logWarningsForDeprecation logs a warning if a deprecated config option is
+// set.
+func logWarningsForDeprecation(cfg Config) {
+	_, deprecated, err := configToFlatMap(cfg)
+	if err != nil {
+		ltndLog.Errorf("Convert configs to map: %v", err)
+	}
+
+	for k := range deprecated {
+		ltndLog.Warnf("Config '%s' is deprecated, please remove it", k)
+	}
 }

config_test.go

@@ -24,9 +24,15 @@ func TestConfigToFlatMap(t *testing.T) {
 	cfg.DB.Etcd.Pass = testPassword
 	cfg.DB.Postgres.Dsn = testPassword
 
-	result, err := configToFlatMap(cfg)
+	// Set a deprecated field.
+	cfg.Bitcoin.Active = true
+
+	result, deprecated, err := configToFlatMap(cfg)
 	require.NoError(t, err)
 
+	// Check that the deprecated option has been parsed out.
+	require.Contains(t, deprecated, "bitcoin.active")
+
 	// Pick a couple of random values to check.
 	require.Equal(t, DefaultLndDir, result["lnddir"])
 	require.Equal(

lncfg/chain.go

@@ -10,7 +10,7 @@ import (
 //
 //nolint:lll
 type Chain struct {
-	Active   bool   `long:"active" description:"DEPRECATED: If the chain should be active or not. This field is now ignored since only the Bitcoin chain is supported"`
+	Active   bool   `long:"active" description:"DEPRECATED: If the chain should be active or not. This field is now ignored since only the Bitcoin chain is supported" hidden:"true"`
 	ChainDir string `long:"chaindir" description:"The directory to store the chain's data within."`
 
 	Node string `long:"node" description:"The blockchain interface to use." choice:"btcd" choice:"bitcoind" choice:"neutrino" choice:"nochainbackend"`

rpcserver.go

@@ -3054,7 +3054,7 @@ func (r *rpcServer) GetInfo(_ context.Context,
 func (r *rpcServer) GetDebugInfo(_ context.Context,
 	_ *lnrpc.GetDebugInfoRequest) (*lnrpc.GetDebugInfoResponse, error) {
 
-	flatConfig, err := configToFlatMap(*r.cfg)
+	flatConfig, _, err := configToFlatMap(*r.cfg)
 	if err != nil {
 		return nil, fmt.Errorf("error converting config to flat map: "+
 			"%w", err)