minstrel/internal/catalogue/art.go

package catalogue

import (
	"context"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"sync"
	"time"

	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
)

// NoArtRetry is how long "no source has art for this" is believed before
// the sources are asked again.
const NoArtRetry = 7 * 24 * time.Hour

// fetchTimeout bounds one look-up across every source.
const fetchTimeout = 30 * time.Second

// ErrNoArt: no enabled source has an image, or the sources could not be
// reached. The caller shows its placeholder.
var ErrNoArt = errors.New("catalogue: no art")

var mbidPattern = regexp.MustCompile(`^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$`)

// ValidMBID reports whether s is shaped like a MusicBrainz id. It is the
// guard that keeps a request path out of the cache directory's parents.
func ValidMBID(s string) bool { return mbidPattern.MatchString(s) }

// Art serves images for artists and release groups that are not in the
// library (search results, Discover suggestions, wanted albums), from the
// enabled cover-art sources. Each image is fetched once and kept
// under the data directory, so the browser only ever talks to Minstrel.
type Art struct {
	dir           string
	artistSources func() []coverart.ArtistArtProvider
	albumSources  func() []coverart.AlbumCoverProvider
	now           func() time.Time

	mu       sync.Mutex
	inflight map[string]*artCall
}

type artCall struct {
	done chan struct{}
	body []byte
	err  error
}

// ArtSources lists the image sources to ask, in order. The cover-art
// settings service is one: it answers with the sources an admin enabled.
type ArtSources interface {
	EnabledArtistProviders() []coverart.ArtistArtProvider
	EnabledAlbumProviders() []coverart.AlbumCoverProvider
}

// NewArt keeps images under dataDir/catalogue-art and asks the given
// sources. Nil sources ask nothing, so every image is the placeholder.
func NewArt(dataDir string, sources ArtSources) *Art {
	a := &Art{dir: filepath.Join(dataDir, "catalogue-art"), now: time.Now, inflight: map[string]*artCall{}}
	if sources != nil {
		a.artistSources = sources.EnabledArtistProviders
		a.albumSources = sources.EnabledAlbumProviders
	}
	return a
}

// Artist returns an image of the artist: the first enabled artist source
// with one wins, its thumbnail preferred over its backdrop.
func (a *Art) Artist(ctx context.Context, mbid, name string) ([]byte, error) {
	return a.cached(ctx, "artist", mbid, func(ctx context.Context) ([]byte, error) {
		if a.artistSources == nil {
			return nil, coverart.ErrNotFound
		}
		ref := coverart.ArtistRef{MBID: mbid, Name: name}
		return firstOf(a.artistSources(), func(p coverart.ArtistArtProvider) ([]byte, error) {
			thumb, fanart, err := p.FetchArtistArt(ctx, ref)
			if err == nil && len(thumb) == 0 {
				thumb = fanart
			}
			return thumb, err
		})
	})
}

// ReleaseGroup returns a cover for the release group: a source that can
// answer by group (the Cover Art Archive) is asked by its id, the rest by
// artist and title.
func (a *Art) ReleaseGroup(ctx context.Context, mbid, artist, title string) ([]byte, error) {
	return a.cached(ctx, "release-group", mbid, func(ctx context.Context) ([]byte, error) {
		if a.albumSources == nil {
			return nil, coverart.ErrNotFound
		}
		byName := coverart.AlbumRef{ArtistName: artist, AlbumTitle: title}
		return firstOf(a.albumSources(), func(p coverart.AlbumCoverProvider) ([]byte, error) {
			if g, ok := p.(coverart.ReleaseGroupCoverProvider); ok {
				return g.FetchReleaseGroupCover(ctx, mbid)
			}
			if artist == "" || title == "" {
				return nil, coverart.ErrNotFound
			}
			return p.FetchAlbumCover(ctx, byName)
		})
	})
}

// firstOf asks each source in turn. It reports ErrNotFound only when every
// source said so; if any could not be reached, the miss is transient.
func firstOf[P any](sources []P, ask func(P) ([]byte, error)) ([]byte, error) {
	transient := false
	for _, p := range sources {
		body, err := ask(p)
		if err == nil && len(body) > 0 {
			return body, nil
		}
		if err != nil && !errors.Is(err, coverart.ErrNotFound) {
			transient = true
		}
	}
	if transient {
		return nil, coverart.ErrTransient
	}
	return nil, coverart.ErrNotFound
}

// cached serves kind/mbid from disk, or fetches it once however many
// requests ask at the same moment. A definite "nobody has one" is written
// down and believed for NoArtRetry; an unreachable source is not, so the
// next request asks again.
func (a *Art) cached(ctx context.Context, kind, mbid string, fetch func(context.Context) ([]byte, error)) ([]byte, error) {
	if !ValidMBID(mbid) {
		return nil, ErrNoArt
	}
	dir := filepath.Join(a.dir, kind)
	imgPath := filepath.Join(dir, mbid)
	nonePath := imgPath + ".none"

	if body, err := os.ReadFile(imgPath); err == nil && len(body) > 0 {
		return body, nil
	}
	if fi, err := os.Stat(nonePath); err == nil && a.now().Sub(fi.ModTime()) < NoArtRetry {
		return nil, ErrNoArt
	}

	key := kind + "/" + mbid
	a.mu.Lock()
	if call, ok := a.inflight[key]; ok {
		a.mu.Unlock()
		select {
		case <-call.done:
			return call.body, call.err
		case <-ctx.Done():
			return nil, ctx.Err()
		}
	}
	call := &artCall{done: make(chan struct{})}
	a.inflight[key] = call
	a.mu.Unlock()

	defer func() {
		a.mu.Lock()
		delete(a.inflight, key)
		a.mu.Unlock()
		close(call.done)
	}()

	// The fetch outlives a browser that navigates away: a second request is
	// likely, and the image is worth keeping.
	fctx, cancel := context.WithTimeout(context.WithoutCancel(ctx), fetchTimeout)
	defer cancel()
	body, err := fetch(fctx)
	switch {
	case err == nil && len(body) > 0:
		if werr := writeFile(dir, imgPath, body); werr != nil {
			// Serving it uncached is still better than a placeholder.
			call.body = body
			return body, nil
		}
		_ = os.Remove(nonePath)
		call.body = body
		return body, nil
	case err == nil || errors.Is(err, coverart.ErrNotFound):
		_ = writeFile(dir, nonePath, nil)
	}
	call.err = ErrNoArt
	return nil, ErrNoArt
}

func writeFile(dir, path string, body []byte) error {
	if err := os.MkdirAll(dir, 0o755); err != nil {
		return fmt.Errorf("catalogue art: %w", err)
	}
	tmp, err := os.CreateTemp(dir, ".part-*")
	if err != nil {
		return fmt.Errorf("catalogue art: %w", err)
	}
	if _, err := tmp.Write(body); err != nil {
		_ = tmp.Close()
		_ = os.Remove(tmp.Name())
		return fmt.Errorf("catalogue art: %w", err)
	}
	if err := tmp.Close(); err != nil {
		_ = os.Remove(tmp.Name())
		return fmt.Errorf("catalogue art: %w", err)
	}
	if err := os.Rename(tmp.Name(), path); err != nil {
		_ = os.Remove(tmp.Name())
		return fmt.Errorf("catalogue art: %w", err)
	}
	return nil
}