lnconfig: Support utilizing Environment Variables in lnd.conf for `…

…rpcuser` and `rpcpass` fields.

- Added `extractEnvVariable` function in `config.go` that returns The value of the specified environment variable, or the default value if provided, or the original input string if no matching variable is found or set.- Called `extractEnvVariable` function in `parseRPCParams` function in `config.go` file where `rpcuser` and `rpcpass` fields are being parsed.

config.go

@@ -1777,6 +1777,12 @@ func parseRPCParams(cConfig *lncfg.Chain, nodeConfig interface{},
 	var daemonName, confDir, confFile, confFileBase string
 	switch conf := nodeConfig.(type) {
 	case *lncfg.Btcd:
+		// Resolves environment variable references in RPCUser and RPCPass fields.
+		conf.RPCUser = extractEnvVariable(conf.RPCUser)
+		conf.RPCPass = extractEnvVariable(conf.RPCPass)
+
+		fmt.Println("conf.RPCUser:", conf.RPCUser)
+
 		// If both RPCUser and RPCPass are set, we assume those
 		// credentials are good to use.
 		if conf.RPCUser != "" && conf.RPCPass != "" {
@@ -1822,6 +1828,12 @@ func parseRPCParams(cConfig *lncfg.Chain, nodeConfig interface{},
 		confFile = conf.ConfigPath
 		confFileBase = BitcoinChainName
 
+		// Resolves environment variable references in RPCUser and RPCPass fields.
+		conf.RPCUser = extractEnvVariable(conf.RPCUser)
+		conf.RPCPass = extractEnvVariable(conf.RPCPass)
+
+		fmt.Println("conf.RPCUser:", conf.RPCUser)
+
 		// Check that cookie and credentials don't contradict each
 		// other.
 		if (conf.RPCUser != "" || conf.RPCPass != "") &&
@@ -1917,6 +1929,52 @@ func parseRPCParams(cConfig *lncfg.Chain, nodeConfig interface{},
 	return nil
 }
 
+// extractEnvVariable extracts the value of an environment variable from a string.
+// It supports the following formats:
+// 1) $ENVIRONMENT_VARIABLE
+// 2) ${ENVIRONMENT_VARIABLE}
+// 3) ${ENVIRONMENT_VARIABLE:-DEFAULT_VALUE}
+//
+// Standard environment variable naming conventions:
+// - ENVIRONMENT_VARIABLE contains letters, digits, and underscores, and does not start with a digit.
+// - DEFAULT_VALUE follows the rule that it can contain any characters except whitespace.
+//
+// Parameters:
+// - value: The input string containing references to environment variables.
+//
+// Returns:
+//   - string: The value of the specified environment variable, or the default value
+//     if provided, or the original input string if no matching variable is found or set.
+func extractEnvVariable(value string) string {
+	re := regexp.MustCompile(`^(?:\${([a-zA-Z_][a-zA-Z0-9_]*)?(:-(\S+))?})|(?:\$([a-zA-Z_][a-zA-Z0-9_]*))$`)
+
+	matches := re.FindStringSubmatch(value)
+
+	if len(matches) > 0 {
+		// envVariable, Group 1 corresponding to this format: ${ENVIRONMENT_VARIABLE}
+		// or ${ENVIRONMENT_VARIABLE:-DEFAULT_VALUE}
+		envVariable := matches[1]
+		// envVariable, Group 4 corresponding to this format: $ENVIRONMENT_VARIABLE
+		if matches[4] != "" {
+			envVariable = matches[4]
+		}
+
+		// defaultValue, Group 3 corrosponding to this format: ${ENVIRONMENT_VARIABLE:-DEFAULT_VALUE}
+		defaultValue := matches[3]
+
+		envValue := os.Getenv(envVariable)
+
+		// Use the default value if the environment variable is not set
+		if envValue == "" && defaultValue != "" {
+			envValue = defaultValue
+		}
+
+		return envValue
+	}
+
+	return value
+}
+
 // extractBtcdRPCParams attempts to extract the RPC credentials for an existing
 // btcd instance. The passed path is expected to be the location of btcd's
 // application data directory on the target system.
@@ -1960,7 +2018,11 @@ func extractBtcdRPCParams(btcdConfigPath string) (string, string, error) {
 		return "", "", fmt.Errorf("unable to find rpcuser in config")
 	}
 
-	return string(userSubmatches[1]), string(passSubmatches[1]), nil
+	// Resolves environment variable references in RPCUser and RPCPass fields.
+	rpcUser := extractEnvVariable(string(userSubmatches[1]))
+	rpcPass := extractEnvVariable(string(passSubmatches[1]))
+
+	return rpcUser, rpcPass, nil
 }
 
 // extractBitcoindRPCParams attempts to extract the RPC credentials for an
@@ -2085,7 +2147,11 @@ func extractBitcoindRPCParams(networkName, bitcoindDataDir, bitcoindConfigPath,
 			"in config")
 	}
 
-	return string(userSubmatches[1]), string(passSubmatches[1]),
+	// Resolves environment variable references in RPCUser and RPCPass fields.
+	rpcUser := extractEnvVariable(string(userSubmatches[1]))
+	rpcPass := extractEnvVariable(string(passSubmatches[1]))
+
+	return rpcUser, rpcPass,
 		zmqBlockHost, zmqTxHost, nil
 }