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_test.go

@@ -0,0 +1,104 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package webdavfs
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"tailscale.com/tailfs/tailfsimpl/shared"
+	"tailscale.com/tstest"
+)
+
+func TestStatCache(t *testing.T) {
+	// Make sure we don't leak goroutines
+	tstest.ResourceCheck(t)
+
+	dir, err := os.MkdirTemp("", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// create file of size 1
+	filename := filepath.Join(dir, "thefile")
+	err = os.WriteFile(filename, []byte("1"), 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	stat := func(name string) (os.FileInfo, error) {
+		return os.Stat(name)
+	}
+	ttl := 1 * time.Second
+	c := newStatCache(ttl)
+
+	// fetch new stat
+	fi, err := c.getOrFetch(filename, stat)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi.Size() != 1 {
+		t.Errorf("got size %d, want 1", fi.Size())
+	}
+	// save original FileInfo as a StaticFileInfo so we can reuse it later
+	// without worrying about the underlying FileInfo changing.
+	originalFI := &shared.StaticFileInfo{
+		Named:      fi.Name(),
+		Sized:      fi.Size(),
+		Moded:      fi.Mode(),
+		ModdedTime: fi.ModTime(),
+		Dir:        fi.IsDir(),
+	}
+
+	// update file to size 2
+	err = os.WriteFile(filename, []byte("12"), 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// fetch stat again, should still be cached
+	fi, err = c.getOrFetch(filename, stat)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi.Size() != 1 {
+		t.Errorf("got size %d, want 1", fi.Size())
+	}
+
+	// wait for cache to expire and refetch stat, size should reflect new size
+	time.Sleep(ttl * 2)
+
+	fi, err = c.getOrFetch(filename, stat)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi.Size() != 2 {
+		t.Errorf("got size %d, want 2", fi.Size())
+	}
+
+	// explicitly set the original FileInfo and make sure it's returned
+	c.set(dir, []fs.FileInfo{originalFI})
+	fi, err = c.getOrFetch(filename, stat)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi.Size() != 1 {
+		t.Errorf("got size %d, want 1", fi.Size())
+	}
+
+	// invalidate the cache and make sure the new size is returned
+	c.invalidate()
+	fi, err = c.getOrFetch(filename, stat)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi.Size() != 2 {
+		t.Errorf("got size %d, want 2", fi.Size())
+	}
+
+	c.stop()
+}