* The determination of the root set should be made by the privileged

process, so forward the operation.
* Spam the user about GC misconfigurations (NIX-71).
* findRoots: skip all roots that are unreadable - the warnings with
  which we spam the user should be enough.

src/libstore/store-api.hh

@@ -2,6 +2,7 @@
 #define __STOREAPI_H
 
 #include <string>
+#include <map>
 
 #include <boost/shared_ptr.hpp>
 
@@ -33,6 +34,9 @@ struct Substitute
 typedef list<Substitute> Substitutes;
 
 
+typedef std::map<Path, Path> Roots;
+
+
 class StoreAPI 
 {
 public:
@@ -118,6 +122,11 @@ public:
        In either case the permanent root is seen by the collector. */
     virtual void syncWithGC() = 0;
 
+    /* Find the roots of the garbage collector.  Each root is a pair
+       (link, storepath) where `link' is the path of the symlink
+       outside of the Nix store that point to `storePath'.  */
+    virtual Roots findRoots() = 0;
+
 };