Improve configuration with config file

This adds explanations to the example config file, renames the file
in order to enable yaml syntax highlighting and fixes the parsing of
the flag specifying where to find the configuration file.

config/config.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"crypto/tls"
+	"errors"
 	"flag"
 	"fmt"
 	"gitlab.hpi.de/codeocean/codemoon/poseidon/logging"
@@ -13,6 +14,7 @@ import (
 	"strings"
 )
 
+// Config contains the default configuration of Poseidon.
 var (
 	Config = &configuration{
 		Server: server{
@@ -25,22 +27,25 @@ var (
 		},
 		Nomad: nomad{
 			Address: "",
-			Token:   "",
 			Port:    4646,
+			Token:   "",
 			TLS:     false,
 		},
 		Logger: logger{
 			Level: "INFO",
 		},
 	}
-	log       = logging.GetLogger("config")
-	TLSConfig = &tls.Config{
+	configurationFilePath    = "./configuration.yaml"
+	configurationInitialized = false
+	log                      = logging.GetLogger("config")
+	TLSConfig                = &tls.Config{
 		MinVersion:               tls.VersionTLS13,
 		CurvePreferences:         []tls.CurveID{tls.CurveP521, tls.CurveP384, tls.CurveP256},
 		PreferServerCipherSuites: true,
 	}
 )
 
+// server configures the Poseidon webserver.
 type server struct {
 	Address  string
 	Port     int
@@ -50,27 +55,39 @@ type server struct {
 	KeyFile  string
 }
 
+// nomad configures the used Nomad cluster.
 type nomad struct {
 	Address string
-	Token   string
 	Port    int
+	Token   string
 	TLS     bool
 }
 
+// logger configures the used logger.
 type logger struct {
 	Level string
 }
 
+// configuration contains the complete configuration of Poseidon.
 type configuration struct {
 	Server server
 	Nomad  nomad
 	Logger logger
 }
 
-func InitConfig() {
+// InitConfig merges configuration options from environment variables and
+// a configuration file into the default configuration. Calls of InitConfig
+// after the first call have no effect and return an error. InitConfig
+// should be called directly after starting the program.
+func InitConfig() error {
+	if configurationInitialized {
+		return errors.New("configuration is already initialized")
+	}
+	configurationInitialized = true
 	content := readConfigFile()
 	Config.mergeYaml(content)
 	Config.mergeEnvironmentVariables()
+	return nil
 }
 
 func (c *configuration) NomadAPIURL() *url.URL {
@@ -93,16 +110,21 @@ func parseURL(address string, port int, tls bool) *url.URL {
 }
 
 func readConfigFile() []byte {
-	var configFilePath string
-	flag.StringVar(&configFilePath, "config", "./configuration.yaml", "path of the yaml config file")
-	flag.Parse()
-	data, err := os.ReadFile(configFilePath)
+	parseFlags()
+	data, err := os.ReadFile(configurationFilePath)
 	if err != nil {
 		log.WithError(err).Info("Using default configuration...")
 	}
 	return data
 }
 
+func parseFlags() {
+	if flag.Lookup("config") == nil {
+		flag.StringVar(&configurationFilePath, "config", configurationFilePath, "path of the yaml config file")
+	}
+	flag.Parse()
+}
+
 func (c *configuration) mergeYaml(content []byte) {
 	if err := yaml.Unmarshal(content, c); err != nil {
 		log.WithError(err).Fatal("Could not parse configuration file")
@@ -114,20 +136,20 @@ func (c *configuration) mergeEnvironmentVariables() {
 }
 
 func readFromEnvironment(prefix string, value reflect.Value) {
-	if !value.CanSet() || !value.CanInterface() {
+	logEntry := log.WithField("prefix", prefix)
+	// if value was not derived from a pointer, it is not possible to alter its contents
+	if !value.CanSet() {
+		logEntry.Warn("Cannot overwrite struct field that can not be set")
 		return
 	}
 
 	if value.Kind() != reflect.Struct {
 		content, ok := os.LookupEnv(prefix)
-
-		logEntry := log.
-			WithField("prefix", prefix).
-			WithField("content", content)
-
 		if !ok {
 			return
 		}
+		logEntry = logEntry.WithField("content", content)
+
 		switch value.Kind() {
 		case reflect.String:
 			value.SetString(content)