simpleadmin-web/internal/web/avatar.go

package web

import (
	"context"
	"encoding/xml"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"

	"git.zio.sh/cs2/simpleadmin-web/internal/steam"
)

// avatarCache fetches staff members' Steam avatars and keeps them in memory. Pages load them from
// the panel rather than from Steam (the CSP only allows images from 'self', and visitors' browsers
// never contact Steam), and Steam sees about one request per member a day. The profile XML needs no
// Web API key.
type avatarCache struct {
	http    *http.Client
	ttl     time.Duration // how long a fetched avatar is kept
	missTTL time.Duration // how long a failed fetch is remembered before trying again

	mu    sync.Mutex
	items map[string]*avatarEntry
}

type avatarEntry struct {
	done    chan struct{} // closed once the fetch finishes
	img     []byte        // nil if the fetch failed
	ctype   string
	fetched time.Time
}

// Avatars larger than this are refused; Steam's medium avatars are a few KB.
const maxAvatarBytes = 512 << 10

func newAvatarCache(client *http.Client) *avatarCache {
	return &avatarCache{http: client, ttl: 24 * time.Hour, missTTL: time.Hour, items: map[string]*avatarEntry{}}
}

// get returns the avatar for steamid, fetching it if it isn't cached or has gone stale. Concurrent
// requests for the same player share one fetch.
func (c *avatarCache) get(ctx context.Context, steamid string) ([]byte, string, bool) {
	c.mu.Lock()
	e := c.items[steamid]
	if e != nil {
		select {
		case <-e.done:
			ttl := c.ttl
			if e.img == nil {
				ttl = c.missTTL
			}
			if time.Since(e.fetched) > ttl {
				e = nil
			}
		default: // another request is fetching it
		}
	}
	if e == nil {
		e = &avatarEntry{done: make(chan struct{})}
		c.items[steamid] = e
		c.mu.Unlock()
		// Detached from the request, so a client giving up doesn't fail the fetch for everyone.
		fctx, cancel := context.WithTimeout(context.Background(), 15*time.Second)
		e.img, e.ctype, _ = c.fetch(fctx, steamid)
		cancel()
		e.fetched = time.Now()
		close(e.done)
	} else {
		c.mu.Unlock()
	}

	select {
	case <-e.done:
		return e.img, e.ctype, e.img != nil
	case <-ctx.Done():
		return nil, "", false
	}
}

func (c *avatarCache) fetch(ctx context.Context, steamid string) ([]byte, string, error) {
	var profile struct {
		Avatar string `xml:"avatarMedium"`
	}
	body, _, err := c.download(ctx, "https://steamcommunity.com/profiles/"+steamid+"/?xml=1", 1<<20)
	if err != nil {
		return nil, "", err
	}
	if err := xml.Unmarshal(body, &profile); err != nil {
		return nil, "", err
	}
	u, err := url.Parse(strings.TrimSpace(profile.Avatar))
	if err != nil || u.Scheme != "https" || !strings.HasSuffix(u.Hostname(), ".steamstatic.com") {
		return nil, "", fmt.Errorf("unexpected avatar URL %q", profile.Avatar)
	}
	img, ctype, err := c.download(ctx, u.String(), maxAvatarBytes)
	if err != nil {
		return nil, "", err
	}
	if !strings.HasPrefix(ctype, "image/") {
		return nil, "", fmt.Errorf("avatar isn't an image (%s)", ctype)
	}
	return img, ctype, nil
}

func (c *avatarCache) download(ctx context.Context, url string, limit int64) ([]byte, string, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, "", err
	}
	resp, err := c.http.Do(req)
	if err != nil {
		return nil, "", err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, "", fmt.Errorf("%s: %s", url, resp.Status)
	}
	body, err := io.ReadAll(io.LimitReader(resp.Body, limit+1))
	if err != nil {
		return nil, "", err
	}
	if int64(len(body)) > limit {
		return nil, "", errors.New("response too large")
	}
	return body, resp.Header.Get("Content-Type"), nil
}

// avatar serves a staff member's Steam avatar. Anyone else gets a 404, so the panel can't be used
// to fetch arbitrary players' avatars; pages fall back to the initial.
func (s *Server) avatar(w http.ResponseWriter, r *http.Request) {
	steamid := r.PathValue("steamid")
	if !steam.ValidID(steamid) {
		http.NotFound(w, r)
		return
	}
	x, err := s.staffIdx(r.Context())
	if err != nil {
		internal(w, r, err)
		return
	}
	if _, ok := x.bySteam[steamid]; !ok {
		http.NotFound(w, r)
		return
	}
	img, ctype, ok := s.avatars.get(r.Context(), steamid)
	if !ok {
		w.Header().Set("Cache-Control", "public, max-age=3600")
		http.NotFound(w, r)
		return
	}
	w.Header().Set("Content-Type", ctype)
	w.Header().Set("Cache-Control", "public, max-age=86400")
	_, _ = w.Write(img)
}