feat(transcode): add cache pruning and config options

* Added config option to set size of transcode cache and cadence to enforce that sizing via ejection.

* Added cache eject to contrib/config.

* Added error return for CacheEject(). Changed to use WalkDir() instead of Walk().

* Lint fix.

* Added universal lock for cache eject.

* Removed accidentally committed binary.

transcode/transcoder_caching.go

@@ -5,9 +5,12 @@ import (
 	"crypto/md5"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
+	"sort"
 	"sync"
+	"time"
 )
 
 const perm = 0o644
@@ -15,16 +18,21 @@ const perm = 0o644
 type CachingTranscoder struct {
 	cachePath  string
 	transcoder Transcoder
+	limitMB    int
 	locks      keyedMutex
+	cleanLock  sync.RWMutex
 }
 
 var _ Transcoder = (*CachingTranscoder)(nil)
 
-func NewCachingTranscoder(t Transcoder, cachePath string) *CachingTranscoder {
-	return &CachingTranscoder{transcoder: t, cachePath: cachePath}
+func NewCachingTranscoder(t Transcoder, cachePath string, limitMB int) *CachingTranscoder {
+	return &CachingTranscoder{transcoder: t, cachePath: cachePath, limitMB: limitMB}
 }
 
 func (t *CachingTranscoder) Transcode(ctx context.Context, profile Profile, in string, out io.Writer) error {
+	t.cleanLock.RLock()
+	defer t.cleanLock.RUnlock()
+
 	// don't try cache partial transcodes
 	if profile.Seek() > 0 {
 		return t.transcoder.Transcode(ctx, profile, in, out)
@@ -52,6 +60,7 @@ func (t *CachingTranscoder) Transcode(ctx context.Context, profile Profile, in s
 
 	if i, err := cf.Stat(); err == nil && i.Size() > 0 {
 		_, _ = io.Copy(out, cf)
+		_ = os.Chtimes(path, time.Now(), time.Now()) // Touch for LRU cache purposes
 		return nil
 	}
 
@@ -64,6 +73,55 @@ func (t *CachingTranscoder) Transcode(ctx context.Context, profile Profile, in s
 	return nil
 }
 
+func (t *CachingTranscoder) CacheEject() error {
+	t.cleanLock.Lock()
+	defer t.cleanLock.Unlock()
+
+	// Delete LRU cache files that exceed size limit. Use last modified time.
+	type file struct {
+		path string
+		info os.FileInfo
+	}
+
+	var files []file
+	var total int64 = 0
+
+	err := filepath.WalkDir(t.cachePath, func(path string, de fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if !de.IsDir() {
+			info, err := de.Info()
+			if err != nil {
+				return fmt.Errorf("walk cache path for eject: %w", err)
+			}
+			files = append(files, file{path, info})
+			total += info.Size()
+		}
+		return nil
+	})
+
+	if err != nil {
+		return fmt.Errorf("walk cache path for eject: %w", err)
+	}
+
+	sort.Slice(files, func(i, j int) bool {
+		return files[i].info.ModTime().Before(files[j].info.ModTime())
+	})
+
+	for total > int64(t.limitMB)*1024*1024 {
+		curFile := files[0]
+		files = files[1:]
+		total -= curFile.info.Size()
+		err = os.Remove(curFile.path)
+		if err != nil {
+			return fmt.Errorf("remove cache file: %w", err)
+		}
+	}
+
+	return nil
+}
+
 func cacheKey(cmd string, args []string) string {
 	// the cache is invalid whenever transcode command (which includes the
 	// absolute filepath, bit rate args, replay gain args, etc.) changes