Monitoring number of open files on Linux in Go

httpd.go

package main

import (
	"expvar"
	"fmt"
	"log/slog"
	"net/http"
	"os"
	"time"

	"golang.org/x/sys/unix"
)

func openFiles() (int, error) {
	entries, err := os.ReadDir("/proc/self/fd")
	if err != nil {
		return 0, err
	}

	return len(entries), nil
}

func maxFiles() (int, error) {
	var limit unix.Rlimit

	if err := unix.Getrlimit(unix.RLIMIT_NOFILE, &limit); err != nil {
		return 0, err
	}

	return int(limit.Max), nil
}

var (
	nFiles = expvar.NewInt("files.count")
	mFiles = expvar.NewInt("files.max")
)

func fileWatcher() {
	t := time.NewTicker(time.Second)
	for {
		n, err := openFiles()
		if err != nil {
			slog.Error("open files", "error", err)
			continue
		}
		nFiles.Set(int64(n))
		<-t.C
	}
}

func main() {
	mf, err := maxFiles()
	if err != nil {
		fmt.Fprintf(os.Stderr, "error: can't get max limit %s", err)
		os.Exit(1)
	}
	mFiles.Set(int64(mf))
	go fileWatcher()

	mux := http.NewServeMux()
	mux.Handle("/debug/vars", expvar.Handler())

	srv := http.Server{
		Addr:    ":8080",
		Handler: mux,
	}

	slog.Info("server starting", "address", srv.Addr)
	if err := srv.ListenAndServe(); err != nil {
		fmt.Fprintf(os.Stderr, "error: %s", err)
		os.Exit(1)
	}
}