tailfs: initial implementation

Add a WebDAV-based folder sharing mechanism that is exposed to local clients at
100.100.100.100:8080 and to remote peers via a new peerapi endpoint at
/v0/tailfs.

Add the ability to manage folder sharing via the new 'share' CLI sub-command.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailfs/compositefs/mkdir.go

@@ -0,0 +1,39 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package compositefs
+
+import (
+	"context"
+	"os"
+
+	"tailscale.com/tailfs/shared"
+)
+
+// Mkdir implements webdav.Filesystem. The root of this file system is
+// read-only, so any attempts to make directories within the root will fail
+// with os.ErrPermission. Attempts to make directories within one of the child
+// filesystems will be handled by the respective child.
+func (cfs *CompositeFileSystem) Mkdir(ctx context.Context, name string, perm os.FileMode) error {
+	if shared.IsRoot(name) {
+		// root directory already exists, consider this okay
+		return nil
+	}
+
+	pathInfo, err := cfs.pathInfoFor(name)
+	if pathInfo.refersToChild {
+		// children can't be made
+		if pathInfo.child != nil {
+			// since child already exists, consider this okay
+			return nil
+		}
+		// since child doesn't exist, return permission error
+		return os.ErrPermission
+	}
+
+	if err != nil {
+		return err
+	}
+
+	return pathInfo.child.FS.Mkdir(ctx, pathInfo.pathOnChild, perm)
+}