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/compositefs/rename.go

@@ -0,0 +1,49 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package compositefs
+
+import (
+	"context"
+	"os"
+
+	"tailscale.com/tailfs/tailfsimpl/shared"
+)
+
+// Rename implements interface webdav.FileSystem. The root of this file system
+// is read-only, so any attempt to rename a child within the root of this
+// filesystem will fail with os.ErrPermission. Renaming across children is not
+// supported and will fail with os.ErrPermission. Renaming within a child will
+// be handled by the respective child.
+func (cfs *CompositeFileSystem) Rename(ctx context.Context, oldName, newName string) error {
+	if shared.IsRoot(oldName) || shared.IsRoot(newName) {
+		// root directory is read-only
+		return os.ErrPermission
+	}
+
+	oldPathInfo, err := cfs.pathInfoFor(oldName)
+	if oldPathInfo.refersToChild {
+		// children themselves are read-only
+		return os.ErrPermission
+	}
+	if err != nil {
+		return err
+	}
+
+	newPathInfo, err := cfs.pathInfoFor(newName)
+	if newPathInfo.refersToChild {
+		// children themselves are read-only
+		return os.ErrPermission
+	}
+	if err != nil {
+		return err
+	}
+
+	if oldPathInfo.child != newPathInfo.child {
+		// moving a file across children is not permitted
+		return os.ErrPermission
+	}
+
+	// file is moving within the same child, let the child handle it
+	return oldPathInfo.child.FS.Rename(ctx, oldPathInfo.pathOnChild, newPathInfo.pathOnChild)
+}