Restructure project

We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout

cmd/poseidon/main.go

@@ -0,0 +1,90 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/api"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/config"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/environment"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/nomad"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/runner"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/logging"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+)
+
+var (
+	gracefulShutdownWait = 15 * time.Second
+	log                  = logging.GetLogger("main")
+)
+
+func runServer(server *http.Server) {
+	log.WithField("address", server.Addr).Info("Starting server")
+	var err error
+	if config.Config.Server.TLS {
+		server.TLSConfig = config.TLSConfig
+		log.
+			WithField("CertFile", config.Config.Server.CertFile).
+			WithField("KeyFile", config.Config.Server.KeyFile).
+			Debug("Using TLS")
+		err = server.ListenAndServeTLS(config.Config.Server.CertFile, config.Config.Server.KeyFile)
+	} else {
+		err = server.ListenAndServe()
+	}
+	if err != nil {
+		if errors.Is(err, http.ErrServerClosed) {
+			log.WithError(err).Info("Server closed")
+		} else {
+			log.WithError(err).Fatal("Error during listening and serving")
+		}
+	}
+}
+
+func initServer() *http.Server {
+	// API initialization
+	nomadAPIClient, err := nomad.NewExecutorAPI(config.Config.NomadAPIURL(), config.Config.Nomad.Namespace)
+	if err != nil {
+		log.WithError(err).WithField("nomad url", config.Config.NomadAPIURL()).Fatal("Error parsing the nomad url")
+	}
+
+	runnerManager := runner.NewNomadRunnerManager(nomadAPIClient, context.Background())
+	environmentManager := environment.NewNomadEnvironmentManager(runnerManager, nomadAPIClient)
+
+	return &http.Server{
+		Addr:         config.Config.PoseidonAPIURL().Host,
+		WriteTimeout: time.Second * 15,
+		ReadTimeout:  time.Second * 15,
+		IdleTimeout:  time.Second * 60,
+		Handler:      api.NewRouter(runnerManager, environmentManager),
+	}
+}
+
+// shutdownOnOSSignal listens for a signal from the operation system
+// When receiving a signal the server shuts down but waits up to 15 seconds to close remaining connections.
+func shutdownOnOSSignal(server *http.Server) {
+	// wait for SIGINT
+	signals := make(chan os.Signal, 1)
+	signal.Notify(signals, syscall.SIGINT, syscall.SIGTERM)
+	<-signals
+	log.Info("Received SIGINT, shutting down ...")
+
+	ctx, cancel := context.WithTimeout(context.Background(), gracefulShutdownWait)
+	defer cancel()
+	if err := server.Shutdown(ctx); err != nil {
+		log.WithError(err).Warn("error shutting server down")
+	}
+}
+
+func main() {
+	if err := config.InitConfig(); err != nil {
+		log.WithError(err).Warn("Could not initialize configuration")
+	}
+	logging.InitializeLogging(config.Config.Logger.Level)
+
+	server := initServer()
+	go runServer(server)
+	shutdownOnOSSignal(server)
+}