package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"log"
	"net/http"
	"os"
	"os/signal"
	"syscall"
	"time"
)

var configPath *string = flag.String("c", "config.json", "path to configuration file")
var showVersion *bool = flag.Bool("v", false, "show version")

var version string

// Process contains an alias that will be returned when queried, and a process
// name to look for.
// A process is effectively a substring that is being looked in a cmdline file
// in /proc/ dir on unix-like systems.
type Process struct {
	Alias   string `json:"alias"`
	Process string `json:"process"`
}

// ProcessesState is a map of processes' aliases and its statuses.
type ProcessesState map[string]bool

func GetProcessesState(procs *[]Process) (ps ProcessesState) {
	ps = make(ProcessesState)

	for _, proc := range *procs {
		pids, err := GetProcessPIDs(proc.Process)
		ps[proc.Alias] = err == nil && len(pids) > 0
	}

	return
}

type Configuration struct {
	ListenAddress string    `json:"listen-address"`
	Processes     []Process `json:"processes"`
}

func LoadConfiguration(path string) (conf *Configuration, err error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	conf = &Configuration{}

	if err := json.NewDecoder(f).Decode(conf); err != nil {
		return nil, err
	}

	for i := 0; i < len(conf.Processes); i++ {
		if conf.Processes[i].Process == "" {
			return nil, fmt.Errorf("an empty process field found")
		}

		if conf.Processes[i].Alias == "" {
			conf.Processes[i].Alias = conf.Processes[i].Process
		}
	}

	return conf, nil
}

func main() {
	log.SetFlags(0)
	flag.Parse()

	if *showVersion {
		fmt.Fprintln(os.Stderr, "httpprocprobed ver.", version, "\nCopyright (c) 2021-2024 Alexander \"Arav\" Andreev <me@arav.su>")
		os.Exit(0)
	}

	conf, err := LoadConfiguration(*configPath)
	if err != nil {
		log.Fatalf("Cannot load configuration file: %s\n", err)
	}

	router := http.NewServeMux()
	router.HandleFunc("/processes", func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet {
			w.WriteHeader(http.StatusMethodNotAllowed)
			w.Header().Add("Allow", "GET")
			return
		}

		w.Header().Add("Content-Type", "application/json")
		if err := json.NewEncoder(w).Encode(GetProcessesState(&conf.Processes)); err != nil {
			w.WriteHeader(http.StatusInternalServerError)
			log.Printf("Failed to encode a process list: %s\n", err)
		}
	})

	srv := &http.Server{
		Addr:         conf.ListenAddress,
		Handler:      router,
		ReadTimeout:  5 * time.Second,
		WriteTimeout: 5 * time.Second,
		IdleTimeout:  10 * time.Second,
	}

	go func() {
		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
			log.Fatalf("ListenAndServe: %s\n", err)
		}
	}()
	defer func() {
		srv.SetKeepAlivesEnabled(false)
		if err := srv.Shutdown(context.Background()); err != nil {
			log.Fatalf("%s\n", err)
		}
	}()

	syssignal := make(chan os.Signal, 1)
	signal.Notify(syssignal, os.Interrupt, syscall.SIGINT, syscall.SIGTERM, syscall.SIGHUP)

	for {
		switch <-syssignal {
		case os.Interrupt:
			fallthrough
		case syscall.SIGINT | syscall.SIGTERM:
			return
		case syscall.SIGHUP:
			newconf, err := LoadConfiguration(*configPath)
			if err != nil {
				log.Fatalf("Failed to reload a list of processes from configuration: %s\n", err)
			}
			conf.Processes = newconf.Processes
		}
	}
}