* `--min-age' flag in nix-store and nix-collect-garbage to only delete

unreachable paths that haven't been used for N hours.  For instance,
  `nix-collect-garbage --min-age 168' only deletes paths that haven't
  been accessed in the last week.

  This is useful for instance in the build farm where many derivations
  can be shared between consecutive builds, and we wouldn't want a
  garbage collect to throw them all away.  We could of course register
  them as roots, but then we'd to unregister them at some point, which
  would be a pain to manage.  The `--min-age' flag gives us a sort of
  MRU caching scheme.

  BUG: this really shouldn't be in gc.cc since that violates
  mechanism/policy separation.

src/libstore/gc.cc

@@ -2,6 +2,11 @@
 #include "globals.hh"
 
 
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+
 void followLivePaths(Path nePath, PathSet & live)
 {
     /* Just to be sure, canonicalise the path.  It is important to do
@@ -62,16 +67,26 @@ PathSet findLivePaths(const Paths & roots)
 }
 
 
-PathSet findDeadPaths(const PathSet & live)
+PathSet findDeadPaths(const PathSet & live, time_t minAge)
 {
     PathSet dead;
 
     startNest(nest, lvlDebug, "finding dead paths");
 
+    time_t now = time(0);
+
     Strings storeNames = readDirectory(nixStore);
 
     for (Strings::iterator i = storeNames.begin(); i != storeNames.end(); ++i) {
         Path p = canonPath(nixStore + "/" + *i);
+
+        if (minAge > 0) {
+            struct stat st;
+            if (lstat(p.c_str(), &st) != 0)
+                throw SysError(format("obtaining information about `%1%'") % p);
+            if (st.st_atime + minAge >= now) continue;
+        }
+        
         if (live.find(p) == live.end()) {
             debug(format("dead path `%1%'") % p);
             dead.insert(p);