Simple file to HTTP healthcheck proxy

main.go

// Serves the resultFile as a response to queries. This is useful for
// assembling a custom healthcheck as a sidecar.
//
// resultFile is of format <status code> SPACE <response text>
//
// You should have a script that does the custom set of healthchecking
// on your apps and write a resulting status file.
//
// Example
//
//   #!/bin/bash
//   while true; do
//     healthy=0
//     if curl localhost:9000 1>/dev/null 2>/dev/null; then
//       healthy=1
//     fi
//     # other checks.
//     if [[ ${healthy} = "1" ]]; then
//       echo "200 ok" > /result
//     else
//       echo "500 unhealthy" > /result
//     fi
//   done
package main

import (
	"flag"
	"fmt"
	"log"
	"net/http"
	"os"
	"strconv"
	"strings"
)

var f = struct {
	bindAddr string
	resultFile string
}{
	bindAddr: ":8080",
	resultFile: "/result",
}

func handle(w http.ResponseWriter, r *http.Request) {
	resultText, err := os.ReadFile(f.resultFile)
	if err != nil {
		log.Fatalf("os.ReadFile(%q) = _, %v", err)
	}
	result := strings.SplitN(string(resultText), " ", 2)
	if len(result) != 2 {
		w.WriteHeader(500)
		err := fmt.Errorf("Invalid resultsText format (%q)", resultText)
		fmt.Fprintf(w, "%v\n", err)
		log.Print(err)
		return
	}
	code, err := strconv.Atoi(result[0])
	if err != nil {
		w.WriteHeader(500)
		err := fmt.Errorf("Invalid resultText %q: invalid status code", resultText)
		fmt.Fprintf(w, "%v\n", err)
		log.Print(err)
		return
	}
	if code < 200 || code >= 600 {
		w.WriteHeader(500)
		err := fmt.Errorf("Invalid resultText %q: invalid status code value", resultText)
		fmt.Fprintf(w, "%v\n", err)
		log.Print(err)
		return
	}
	w.WriteHeader(int(code))
	w.Write([]byte(result[1]))
}

func main() {
	flag.StringVar(&f.bindAddr, "bindAddr", f.bindAddr, "Address to bind healthcheck response from")
	flag.StringVar(&f.resultFile, "resultFile", f.resultFile, `Location of the result file. File should be a single line of the format <status code> SPACE <response>; example: "500 server is unhealthy"`)
	flag.Parse()

	http.HandleFunc("/", handle)
	log.Printf("Listening on %q", f.bindAddr)
	log.Fatal(http.ListenAndServe(f.bindAddr, nil))
}