tailscale: update tailfs file and package names (#11590)

This change updates the tailfs file and package names to their new
naming convention.

Updates #tailscale/corp#16827

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

drive/driveimpl/dirfs/dirfs.go

@@ -0,0 +1,101 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package dirfs provides a webdav.FileSystem that looks like a read-only
+// directory containing only subdirectories.
+package dirfs
+
+import (
+	"slices"
+	"strings"
+	"time"
+
+	"tailscale.com/drive/driveimpl/shared"
+	"tailscale.com/tstime"
+)
+
+// Child is subdirectory of an FS.
+type Child struct {
+	// Name is the name of the child
+	Name string
+
+	// Available is a function indicating whether or not the child is currently
+	// available. Unavailable children are excluded from the FS's directory
+	// listing. Available must be safe for concurrent use.
+	Available func() bool
+}
+
+func (c *Child) isAvailable() bool {
+	if c.Available == nil {
+		return true
+	}
+	return c.Available()
+}
+
+// FS is a read-only webdav.FileSystem that is composed of multiple child
+// folders.
+//
+// When listing the contents of this FileSystem's root directory, children will
+// be ordered in the order they're given to the FS.
+//
+// Children in an FS cannot be added, removed or renamed via operations on the
+// webdav.FileSystem interface like filesystem.Mkdir or filesystem.OpenFile.
+//
+// Any attempts to perform operations on paths inside of children will result
+// in a panic, as these are not expected to be performed on this FS.
+//
+// An FS an optionally have a StaticRoot, which will insert a folder with that
+// StaticRoot into the tree, like this:
+//
+// -- <StaticRoot>
+// ----- <Child>
+// ----- <Child>
+type FS struct {
+	// Children configures the full set of children of this FS.
+	Children []*Child
+
+	// Clock, if given, will cause this FS to use Clock.now() as the current
+	// time.
+	Clock tstime.Clock
+
+	// StaticRoot, if given, will insert the given name as a static root into
+	// every path.
+	StaticRoot string
+}
+
+func (dfs *FS) findChild(name string) (int, *Child) {
+	var child *Child
+	i, found := slices.BinarySearchFunc(dfs.Children, name, func(child *Child, name string) int {
+		return strings.Compare(child.Name, name)
+	})
+	if found {
+		child = dfs.Children[i]
+	}
+	return i, child
+}
+
+// childFor returns the child for the given filename. If the filename refers to
+// a path inside of a child, this will panic.
+func (dfs *FS) childFor(name string) *Child {
+	pathComponents := shared.CleanAndSplit(name)
+	if len(pathComponents) != 1 {
+		panic("dirfs does not permit reaching into child directories")
+	}
+	_, child := dfs.findChild(pathComponents[0])
+	return child
+}
+
+func (dfs *FS) now() time.Time {
+	if dfs.Clock != nil {
+		return dfs.Clock.Now()
+	}
+	return time.Now()
+}
+
+func (dfs *FS) trimStaticRoot(name string) (string, bool) {
+	before, after, found := strings.Cut(name, "/"+dfs.StaticRoot)
+	if !found {
+		return before, false
+	}
+	return after, shared.IsRoot(after)
+}