tailfs: clean up naming and package structure

- Restyles tailfs -> tailFS
- Defines interfaces for main TailFS types
- Moves implemenatation of TailFS into tailfsimpl package

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailfs/tailfsimpl/webdavfs/stat_cache.go

@@ -0,0 +1,71 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package webdavfs
+
+import (
+	"io/fs"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"github.com/jellydator/ttlcache/v3"
+)
+
+// statCache provides a cache for file directory and file metadata. Especially
+// when used from the command-line, mapped WebDAV drives can generate
+// repetitive requests for the same file metadata. This cache helps reduce the
+// number of round-trips to the WebDAV server for such requests.
+type statCache struct {
+	// mu guards the below values.
+	mu    sync.Mutex
+	cache *ttlcache.Cache[string, fs.FileInfo]
+}
+
+func newStatCache(ttl time.Duration) *statCache {
+	cache := ttlcache.New(
+		ttlcache.WithTTL[string, fs.FileInfo](ttl),
+	)
+	go cache.Start()
+	return &statCache{cache: cache}
+}
+
+func (c *statCache) getOrFetch(name string, fetch func(string) (fs.FileInfo, error)) (fs.FileInfo, error) {
+	c.mu.Lock()
+	item := c.cache.Get(name)
+	c.mu.Unlock()
+
+	if item != nil {
+		return item.Value(), nil
+	}
+
+	fi, err := fetch(name)
+	if err == nil {
+		c.mu.Lock()
+		c.cache.Set(name, fi, ttlcache.DefaultTTL)
+		c.mu.Unlock()
+	}
+
+	return fi, err
+}
+
+func (c *statCache) set(parentPath string, infos []fs.FileInfo) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	for _, info := range infos {
+		path := filepath.Join(parentPath, filepath.Base(info.Name()))
+		c.cache.Set(path, info, ttlcache.DefaultTTL)
+	}
+}
+
+func (c *statCache) invalidate() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.cache.DeleteAll()
+}
+
+func (c *statCache) stop() {
+	c.cache.Stop()
+}