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_test.go

@@ -0,0 +1,104 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// BirthTime is not supported on Linux, so only run the test on windows and Mac.
+
+//go:build windows || darwin
+
+package tailfsimpl
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/tailscale/xnet/webdav"
+)
+
+func TestBirthTiming(t *testing.T) {
+	ctx := context.Background()
+
+	dir := t.TempDir()
+	fs := &birthTimingFS{webdav.Dir(dir)}
+
+	// create a file
+	filename := "thefile"
+	fullPath := filepath.Join(dir, filename)
+	err := os.WriteFile(fullPath, []byte("hello beautiful world"), 0644)
+	if err != nil {
+		t.Fatalf("writing file failed: %s", err)
+	}
+
+	// wait a little bit
+	time.Sleep(1 * time.Second)
+
+	// append to the file to change its mtime
+	file, err := os.OpenFile(fullPath, os.O_APPEND|os.O_WRONLY, 0644)
+	if err != nil {
+		t.Fatalf("opening file failed: %s", err)
+	}
+	_, err = file.Write([]byte("lookin' good!"))
+	if err != nil {
+		t.Fatalf("appending to file failed: %s", err)
+	}
+	err = file.Close()
+	if err != nil {
+		t.Fatalf("closing file failed: %s", err)
+	}
+
+	checkFileInfo := func(fi os.FileInfo) {
+		if fi.ModTime().IsZero() {
+			t.Fatal("FileInfo should have a non-zero ModTime")
+		}
+		bt, ok := fi.(webdav.BirthTimer)
+		if !ok {
+			t.Fatal("FileInfo should be a BirthTimer")
+		}
+		birthTime, err := bt.BirthTime(ctx)
+		if err != nil {
+			t.Fatalf("BirthTime() failed: %s", err)
+		}
+		if birthTime.IsZero() {
+			t.Fatal("BirthTime() should return a non-zero time")
+		}
+		if !fi.ModTime().After(birthTime) {
+			t.Fatal("ModTime() should be after BirthTime()")
+		}
+	}
+
+	fi, err := fs.Stat(ctx, filename)
+	if err != nil {
+		t.Fatalf("statting file failed: %s", err)
+	}
+	checkFileInfo(fi)
+
+	wfile, err := fs.OpenFile(ctx, filename, os.O_RDONLY, 0)
+	if err != nil {
+		t.Fatalf("opening file failed: %s", err)
+	}
+	defer wfile.Close()
+	fi, err = wfile.Stat()
+	if err != nil {
+		t.Fatalf("statting file failed: %s", err)
+	}
+	if fi == nil {
+		t.Fatal("statting file returned nil FileInfo")
+	}
+	checkFileInfo(fi)
+
+	dfile, err := fs.OpenFile(ctx, ".", os.O_RDONLY, 0)
+	if err != nil {
+		t.Fatalf("opening directory failed: %s", err)
+	}
+	defer dfile.Close()
+	fis, err := dfile.Readdir(0)
+	if err != nil {
+		t.Fatalf("readdir failed: %s", err)
+	}
+	if len(fis) != 1 {
+		t.Fatalf("readdir should have returned 1 file info, but returned %d", 1)
+	}
+	checkFileInfo(fis[0])
+}