canon/internal/server/preview.go

package server

import (
	"context"
	"errors"
	"io"
	"net/http"
	"regexp"
	"strconv"
	"sync"
	"time"

	"github.com/cdnutter/canon/internal/providers"
	"github.com/cdnutter/canon/internal/store"
)

// Previews are 30-second clips from Deezer, streamed through Canon so
// visitors never connect to Deezer directly. Deezer's clip URLs expire, so
// they're looked up on demand and cached briefly.

var deezerRefRe = regexp.MustCompile(`^(track|album):[0-9]+$`)

// deezerRef is the Deezer track/album an item can preview, or "".
func deezerRef(it store.Item) string {
	ref := it.Extra["deezer"]
	if it.Provider == "deezer" {
		ref = it.ProviderID
	}
	if !deezerRefRe.MatchString(ref) {
		return ""
	}
	return ref
}

func hasPreviews(category string) bool { return category == "song" || category == "album" }

type previewCache struct {
	mu sync.Mutex
	m  map[string]cachedPreview
}

type cachedPreview struct {
	url     string
	expires time.Time
}

func (c *previewCache) get(ref string) (string, bool) {
	c.mu.Lock()
	defer c.mu.Unlock()
	p, ok := c.m[ref]
	if !ok || time.Now().After(p.expires) {
		return "", false
	}
	return p.url, true
}

func (c *previewCache) put(ref, url string) {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.m == nil || len(c.m) > 2000 {
		c.m = map[string]cachedPreview{}
	}
	c.m[ref] = cachedPreview{url: url, expires: time.Now().Add(10 * time.Minute)}
}

func (c *previewCache) drop(ref string) {
	c.mu.Lock()
	defer c.mu.Unlock()
	delete(c.m, ref)
}

// handleItemPreview plays the preview for a list item. It follows the
// list's visibility, like the list page does.
func (s *Server) handleItemPreview(w http.ResponseWriter, r *http.Request) {
	id, err := strconv.ParseInt(r.PathValue("id"), 10, 64)
	if err != nil {
		http.NotFound(w, r)
		return
	}
	it, err := s.store.ItemByID(r.Context(), id)
	if err != nil {
		http.NotFound(w, r)
		return
	}
	l, err := s.store.ListByID(r.Context(), it.ListID)
	if err != nil || !hasPreviews(l.Category) || !s.site(r.Context()).ShowPreviews {
		http.NotFound(w, r)
		return
	}
	if !l.Visible {
		if _, ok := s.currentUser(r); !ok {
			http.NotFound(w, r)
			return
		}
	}
	ref := deezerRef(it)
	if ref == "" {
		http.NotFound(w, r)
		return
	}
	s.streamPreview(w, r, ref)
}

// handleAdminPreview plays a search result before it's added. It's a plain
// GET behind the session (an <audio> element can't send the API header).
func (s *Server) handleAdminPreview(w http.ResponseWriter, r *http.Request) {
	ref := r.URL.Query().Get("ref")
	if !deezerRefRe.MatchString(ref) {
		http.NotFound(w, r)
		return
	}
	s.streamPreview(w, r, ref)
}

func (s *Server) streamPreview(w http.ResponseWriter, r *http.Request, ref string) {
	for attempt := 0; attempt < 2; attempt++ {
		url, cached := s.previews.get(ref)
		if !cached {
			ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
			var err error
			url, err = s.providers.DeezerPreview(ctx, ref)
			cancel()
			if errors.Is(err, providers.ErrNoPreview) {
				http.Error(w, "No preview available", http.StatusNotFound)
				return
			}
			if err != nil {
				s.log.Warn("preview lookup failed", "ref", ref, "err", err)
				http.Error(w, "Preview unavailable", http.StatusBadGateway)
				return
			}
			s.previews.put(ref, url)
		}
		req, err := http.NewRequestWithContext(r.Context(), http.MethodGet, url, nil)
		if err != nil {
			break
		}
		req.Header.Set("User-Agent", providers.UserAgent)
		// Forward ranges so browsers (Safari especially) can stream and seek.
		if rg := r.Header.Get("Range"); rg != "" {
			req.Header.Set("Range", rg)
		}
		resp, err := s.previewClient.Do(req)
		if err != nil {
			s.log.Warn("preview fetch failed", "ref", ref, "err", err)
			http.Error(w, "Preview unavailable", http.StatusBadGateway)
			return
		}
		if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusPartialContent {
			resp.Body.Close()
			// Probably an expired clip URL; look it up again once.
			s.previews.drop(ref)
			continue
		}
		defer resp.Body.Close()
		h := w.Header()
		for _, k := range []string{"Content-Type", "Content-Length", "Content-Range", "Accept-Ranges"} {
			if v := resp.Header.Get(k); v != "" {
				h.Set(k, v)
			}
		}
		if h.Get("Content-Type") == "" {
			h.Set("Content-Type", "audio/mpeg")
		}
		h.Set("Cache-Control", "private, max-age=600")
		w.WriteHeader(resp.StatusCode)
		io.Copy(w, resp.Body)
		return
	}
	http.Error(w, "Preview unavailable", http.StatusBadGateway)
}

// matchDeezer links a song or album that didn't come from Deezer to its
// Deezer counterpart so it can play a preview. Failures just mean no preview.
func (s *Server) matchDeezer(ctx context.Context, category string, it *store.Item) {
	if !hasPreviews(category) || it.Provider == "deezer" {
		return
	}
	if it.Extra == nil {
		it.Extra = map[string]string{}
	}
	mctx, cancel := context.WithTimeout(ctx, 8*time.Second)
	defer cancel()
	ref, err := s.providers.MatchDeezer(mctx, category, it.Title, it.Subtitle)
	if err != nil {
		s.log.Warn("matching Deezer preview", "title", it.Title, "err", err)
		return
	}
	if ref == "" {
		delete(it.Extra, "deezer")
		return
	}
	it.Extra["deezer"] = ref
}