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/birthtiming.go

@@ -0,0 +1,83 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tailfsimpl
+
+import (
+	"context"
+	"io/fs"
+	"os"
+	"time"
+
+	"github.com/djherbis/times"
+	"github.com/tailscale/xnet/webdav"
+)
+
+// birthTimingFS extends a webdav.FileSystem to return FileInfos that implement
+// the webdav.BirthTimer interface.
+type birthTimingFS struct {
+	webdav.FileSystem
+}
+
+func (fs *birthTimingFS) Stat(ctx context.Context, name string) (os.FileInfo, error) {
+	fi, err := fs.FileSystem.Stat(ctx, name)
+	if err != nil {
+		return nil, err
+	}
+	return &birthTimingFileInfo{fi}, nil
+}
+
+func (fs *birthTimingFS) OpenFile(ctx context.Context, name string, flag int, perm os.FileMode) (webdav.File, error) {
+	f, err := fs.FileSystem.OpenFile(ctx, name, flag, perm)
+	if err != nil {
+		return nil, err
+	}
+
+	return &birthTimingFile{f}, nil
+}
+
+// birthTimingFileInfo extends an os.FileInfo to implement the BirthTimer
+// interface.
+type birthTimingFileInfo struct {
+	os.FileInfo
+}
+
+func (fi *birthTimingFileInfo) BirthTime(ctx context.Context) (time.Time, error) {
+	if fi.Sys() == nil {
+		return time.Time{}, webdav.ErrNotImplemented
+	}
+
+	if !times.HasBirthTime {
+		return time.Time{}, webdav.ErrNotImplemented
+	}
+
+	return times.Get(fi.FileInfo).BirthTime(), nil
+}
+
+// birthTimingFile extends a webdav.File to return FileInfos that implement the
+// BirthTimer interface.
+type birthTimingFile struct {
+	webdav.File
+}
+
+func (f *birthTimingFile) Stat() (fs.FileInfo, error) {
+	fi, err := f.File.Stat()
+	if err != nil {
+		return nil, err
+	}
+
+	return &birthTimingFileInfo{fi}, nil
+}
+
+func (f *birthTimingFile) Readdir(count int) ([]fs.FileInfo, error) {
+	fis, err := f.File.Readdir(count)
+	if err != nil {
+		return nil, err
+	}
+
+	for i, fi := range fis {
+		fis[i] = &birthTimingFileInfo{fi}
+	}
+
+	return fis, nil
+}