added directory listing cache

pkg/fs/dir.go

@@ -3,12 +3,47 @@
 package fs
 
 import (
+	"errors"
 	"io"
 	"os"
+	"sync"
+	"time"
 )
 
+var NoCacheEntry = errors.New("no cache entry")
+
+type DirCacheEntry struct {
+	mtime time.Time
+	fi    []os.FileInfo
+}
+
+type DirCache struct {
+	mu sync.RWMutex
+	m  map[string]*DirCacheEntry
+}
+
+func NewDirCache() *DirCache {
+	return &DirCache{m: make(map[string]*DirCacheEntry)}
+}
+
+func (dc *DirCache) Get(name string) (*DirCacheEntry, error) {
+	dc.mu.RLock()
+	defer dc.mu.RUnlock()
+	if dce, ok := dc.m[name]; ok {
+		return dce, nil
+	}
+	return nil, NoCacheEntry
+}
+
+func (dc *DirCache) Set(name string, dce *DirCacheEntry) {
+	dc.mu.Lock()
+	defer dc.mu.Unlock()
+	dc.m[name] = dce
+}
+
 type Dir struct {
-	f *os.File
+	f  *os.File
+	dc *DirCache
 }
 
 func (f *Dir) Read(_ []byte) (int, error) {
@@ -19,8 +54,23 @@ func (f *Dir) Seek(_ int64, _ int) (int64, error) {
 	return 0, io.EOF
 }
 
-func (f *Dir) Readdir(count int) ([]os.FileInfo, error) {
-	return f.f.Readdir(count)
+func (f *Dir) Readdir(_ int) (fi []os.FileInfo, err error) {
+	i, err := f.Stat()
+	if err != nil {
+		return
+	}
+	name := f.f.Name()
+	dce, err := f.dc.Get(name)
+	if err == NoCacheEntry || i.ModTime().After(dce.mtime) {
+		fi, err = f.f.Readdir(0)
+		if err != nil {
+			return
+		}
+		dce = &DirCacheEntry{i.ModTime(), fi}
+		f.dc.Set(name, dce)
+	}
+	fi = dce.fi
+	return
 }
 
 func (f *Dir) Stat() (os.FileInfo, error) {