mount: fix disappearing cwd problem - fixes #4104

Before this change, the current working directory could disappear
according to the Linux kernel.

This was caused by mount returning different nodes with the same
information in.

This change uses vfs.Node.SetSys to cache the information so we always
return the same node.

cmd/mount2/node.go

@@ -27,11 +27,20 @@ type Node struct {
 var _ fusefs.InodeEmbedder = (*Node)(nil)
 
 // newNode creates a new fusefs.Node from a vfs Node
-func newNode(fsys *FS, node vfs.Node) *Node {
-	return &Node{
-		node: node,
+func newNode(fsys *FS, vfsNode vfs.Node) (node *Node) {
+	// Check the vfsNode to see if it has a fuse Node cached
+	// We must return the same fuse nodes for vfs Nodes
+	node, ok := vfsNode.Sys().(*Node)
+	if ok {
+		return node
+	}
+	node = &Node{
+		node: vfsNode,
 		fsys: fsys,
 	}
+	// Cache the node for later
+	vfsNode.SetSys(node)
+	return node
 }
 
 // String used for pretty printing.
@@ -183,10 +192,7 @@ func (n *Node) Lookup(ctx context.Context, name string, out *fuse.EntryOut) (ino
 	if errno != 0 {
 		return nil, errno
 	}
-	newNode := &Node{
-		node: vfsNode,
-		fsys: n.fsys,
-	}
+	newNode := newNode(n.fsys, vfsNode)
 
 	// FIXME
 	// out.SetEntryTimeout(dt time.Duration)