Add option to configure poseidon

It exists a default configuration. It can be overwritten by adding
a configuration.yaml. This can again be overwritten by environment
variables.

config/config.go

@@ -0,0 +1,116 @@
+package config
+
+import (
+	"flag"
+	"fmt"
+	"gopkg.in/yaml.v3"
+	"log"
+	"os"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+var (
+	Config = &configuration{
+		Server: server{
+			Address: "127.0.0.1",
+			Port:    3000,
+		},
+		Nomad: nomad{
+			Address: "",
+			Token:   "",
+			TLS:     false,
+		},
+		Logger: logger{
+			Level: "INFO",
+		},
+	}
+)
+
+type server struct {
+	Address string
+	Port    int
+}
+
+type nomad struct {
+	Address string
+	Token   string
+	TLS     bool
+}
+
+type logger struct {
+	Level string
+}
+
+type configuration struct {
+	Server server
+	Nomad  nomad
+	Logger logger
+}
+
+func InitConfig() {
+	content := readConfigFile()
+	Config.mergeYaml(content)
+	Config.mergeEnvironmentVariables()
+}
+
+func readConfigFile() []byte {
+	var configFilePath string
+	flag.StringVar(&configFilePath, "config", "./configuration.yaml", "path of the yaml config file")
+	data, err := os.ReadFile(configFilePath)
+	if err != nil {
+		log.Printf("Using default configuration... (%v)", err)
+	}
+	return data
+}
+
+func (c *configuration) mergeYaml(content []byte) {
+	if err := yaml.Unmarshal(content, c); err != nil {
+		log.Fatalf("Could not parse configuration: %v", err)
+	}
+}
+
+func (c *configuration) mergeEnvironmentVariables() {
+	readFromEnvironment("POSEIDON", reflect.ValueOf(c).Elem())
+}
+
+func readFromEnvironment(prefix string, value reflect.Value) {
+	if !value.CanSet() || !value.CanInterface() {
+		return
+	}
+
+	if value.Kind() != reflect.Struct {
+		content, ok := os.LookupEnv(prefix)
+		if !ok {
+			return
+		}
+		switch value.Kind() {
+		case reflect.String:
+			value.SetString(content)
+		case reflect.Int:
+			integer, err := strconv.Atoi(content)
+			if err != nil {
+				log.Printf("Could not parse environment variable %s with value '%q' as integer", prefix, content)
+				return
+			}
+			value.SetInt(int64(integer))
+		case reflect.Bool:
+			boolean, err := strconv.ParseBool(content)
+			if err != nil {
+				log.Printf("Could not parse environment variable %s with value '%q' as boolean", prefix, content)
+				return
+			}
+			value.SetBool(boolean)
+		default:
+			// ignore this field
+			log.Printf("Setting configuration options of type %s via environment variables is not supported", value.Type().Name())
+		}
+	} else {
+		for i := 0; i < value.NumField(); i++ {
+			fieldName := value.Type().Field(i).Name
+			newPrefix := fmt.Sprintf("%s_%s", prefix, strings.ToUpper(fieldName))
+			readFromEnvironment(newPrefix, value.Field(i))
+		}
+	}
+}

go.mod

@@ -5,4 +5,5 @@ go 1.16
 require (
 	github.com/gorilla/mux v1.8.0
 	github.com/stretchr/testify v1.7.0
+	gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b
 )

go.sum

@@ -9,5 +9,6 @@ github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5Cc
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b h1:h8qDotaEPuJATrMmW04NCwg7v22aHH28wwpauUhK9Oo=
+gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

main.go

@@ -2,7 +2,9 @@ package main
 
 import (
 	"context"
+	"fmt"
 	"gitlab.hpi.de/codeocean/codemoon/poseidon/api"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/config"
 	"log"
 	"net/http"
 	"os"
@@ -11,8 +13,10 @@ import (
 )
 
 func main() {
+	config.InitConfig()
+
 	server := &http.Server{
-		Addr:         "0.0.0.0:3000",
+		Addr:         fmt.Sprintf("%s:%d", config.Config.Server.Address, config.Config.Server.Port),
 		WriteTimeout: time.Second * 15,
 		ReadTimeout:  time.Second * 15,
 		IdleTimeout:  time.Second * 60,

nomad/api.tpl.nomad

@@ -55,6 +55,10 @@ job "${NOMAD_SLUG}" {
         image = "${IMAGE_NAME_ENV}"
       }
 
+      env {
+        POSEIDON_SERVER_ADDRESS = "${POSEIDON_LISTEN_ADDRESS}"
+      }
+
       resources {
         memory = "100" // 100 MB RAM
         cpu    = "100" // 100 MHz