lnd+cert: recreate TLS files if IPs or DNS changed

This commit makes lnd recreate its TLS certificate if the config's
tlsextradomains or tlsextraips changed. This is useful, since earlier
user would have to manually delete the files to trigger lnd to recreate
them.

To ensure users don't accidentally have their TLS certificate recreated,
we gate it behind a flag --tlsautorefresh that defaults to false.

config.go

@@ -236,10 +236,13 @@ type config struct {
 	ConfigFile   string `short:"C" long:"configfile" description:"Path to configuration file"`
 	DataDir      string `short:"b" long:"datadir" description:"The directory to store lnd's data within"`
 	SyncFreelist bool   `long:"sync-freelist" description:"Whether the databases used within lnd should sync their freelist to disk. This is disabled by default resulting in improved memory performance during operation, but with an increase in startup time."`
+
 	TLSCertPath     string   `long:"tlscertpath" description:"Path to write the TLS certificate for lnd's RPC and REST services"`
 	TLSKeyPath      string   `long:"tlskeypath" description:"Path to write the TLS private key for lnd's RPC and REST services"`
 	TLSExtraIPs     []string `long:"tlsextraip" description:"Adds an extra ip to the generated certificate"`
 	TLSExtraDomains []string `long:"tlsextradomain" description:"Adds an extra domain to the generated certificate"`
+	TLSAutoRefresh  bool     `long:"tlsautorefresh" description:"Re-generate TLS certificate and key if the IPs or domains are changed"`
+
 	NoMacaroons    bool   `long:"no-macaroons" description:"Disable macaroon authentication"`
 	AdminMacPath   string `long:"adminmacaroonpath" description:"Path to write the admin macaroon for lnd's RPC and REST services if it doesn't exist"`
 	ReadMacPath    string `long:"readonlymacaroonpath" description:"Path to write the read-only macaroon for lnd's RPC and REST services if it doesn't exist"`

lnd.go

@@ -717,10 +717,25 @@ func getTLSConfig(tlsCertPath string, tlsKeyPath string, tlsExtraIPs,
 		return nil, nil, "", err
 	}
 
-	// If the certificate expired, delete it and the TLS key and generate a
+	// We check whether the certifcate we have on disk match the IPs and
-	// new pair.
+	// domains specified by the config. If the extra IPs or domains have
-	if time.Now().After(parsedCert.NotAfter) {
+	// changed from when the certificate was created, we will refresh the
-		ltndLog.Info("TLS certificate is expired, generating a new one")
+	// certificate if auto refresh is active.
+	refresh := false
+	if cfg.TLSAutoRefresh {
+		refresh, err = cert.IsOutdated(
+			parsedCert, tlsExtraIPs, tlsExtraDomains,
+		)
+		if err != nil {
+			return nil, nil, "", err
+		}
+	}
+
+	// If the certificate expired or it was outdated, delete it and the TLS
+	// key and generate a new pair.
+	if time.Now().After(parsedCert.NotAfter) || refresh {
+		ltndLog.Info("TLS certificate is expired or outdated, " +
+			"generating a new one")
 
 		err := os.Remove(tlsCertPath)
 		if err != nil {