# 
# patch "roster3.cc"
#  from [f0fb9e38f87eb501f7fe93ff34a60fd8aa9b3aa3]
#    to [876a0b829edfdc1ade90f093b151668537eca086]
# 
# patch "roster3.hh"
#  from [e48dd575719a83620200e980d527640c38efa31d]
#    to [a297b3264ae4c6c133ddb15b29c3f8a7f016365e]
# 
========================================================================
--- roster3.cc	f0fb9e38f87eb501f7fe93ff34a60fd8aa9b3aa3
+++ roster3.cc	876a0b829edfdc1ade90f093b151668537eca086
@@ -207,9 +207,7 @@
 roster_t::clear_attr(split_path const & pth,
                      attr_name const & name)
 {
-  node_id nid = lookup(pth);
-  node_t & n = node(nid);
-  safe_erase(n.attrs, name);
+  set_attr(pth, name, std::make_pair(false, attr_val()));
 }
 
 void
@@ -217,9 +215,18 @@
                    attr_name const & name,
                    attr_val const & val)
 {
+  set_attr(pth, name, std::make_pair(true, val));
+}
+
+void
+roster_t::set_attr(split_path const & pth,
+                   attr_name const & name,
+                   std::pair<bool, attr_val> const & val)
+{
+  I(val.first || val.second().empty());
   node_id nid = lookup(pth);
   node_t & n = node(nid);
-  std::map<attr_key, attr_value>::iterator i = n.attrs.find(name);
+  std::map<attr_key, std::pair<bool, attr_value> >::iterator i = n.attrs.find(name);
   I(i != n.attrs.end());
   I(i->second != val);
   i->second = val;
========================================================================
--- roster3.hh	e48dd575719a83620200e980d527640c38efa31d
+++ roster3.hh	a297b3264ae4c6c133ddb15b29c3f8a7f016365e
@@ -44,7 +44,10 @@
   // this is null iff this is a root dir
   path_component name;
   file_id content;
-  std::map<attr_key, attr_value> attrs;
+  // (true, "val") or (false, "") are both valid attr values (for proper
+  // merging, we have to widen the attr_value type to include a first-class
+  // "undefined" value).
+  std::map<attr_key, std::pair<bool, attr_value> > attrs;
 };
 
 typedef std::map<path_component, node_id> dir_map;
@@ -75,6 +78,9 @@
   void set_attr(split_path const & pth,
                 attr_name const & name,
                 attr_val const & val);
+  void set_attr(split_path const & pth,
+                attr_name const & name,
+                std::pair<bool, attr_val> const & val);
   std::map<node_id, node_t> const & all_nodes() const
   {
     return nodes;