* After building, scan for actual file system references as

opposed to declared references.  This prunes the reference
  graph, thus allowing better garbage collection and more
  efficient derivate distribution.

src/references.cc

@@ -0,0 +1,98 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <dirent.h>
+#include <fcntl.h>
+
+#include "references.hh"
+#include "hash.hh"
+
+
+static void search(const string & s,
+    Strings & refs, Strings & seen)
+{
+    for (Strings::iterator i = refs.begin();
+         i != refs.end(); )
+    {
+        if (s.find(*i) == string::npos)
+            i++;
+        else {
+            debug(format("found reference to `%1%'") % *i);
+            seen.push_back(*i);
+            i = refs.erase(i);
+        }
+    }
+}
+
+
+void checkPath(const string & path,
+    Strings & refs, Strings & seen)
+{
+    struct stat st;
+    if (lstat(path.c_str(), &st))
+        throw SysError(format("getting attributes of path `%1%'") % path);
+
+    if (S_ISDIR(st.st_mode)) {
+        DIR * dir = opendir(path.c_str());
+
+        struct dirent * dirent;
+        while (errno = 0, dirent = readdir(dir)) {
+            string name = dirent->d_name;
+            if (name == "." || name == "..") continue;
+            search(name, refs, seen);
+            checkPath(path + "/" + name, refs, seen);
+        }
+
+        closedir(dir); /* !!! close on exception */
+    }
+
+    else if (S_ISREG(st.st_mode)) {
+        
+        debug(format("checking `%1%'") % path);
+
+        int fd = open(path.c_str(), O_RDONLY);
+        if (fd == -1) throw SysError(format("opening file `%1%'") % path);
+
+        char * buf = new char[st.st_size];
+
+        if (read(fd, buf, st.st_size) != st.st_size)
+            throw SysError(format("reading file %1%") % path);
+
+        search(string(buf, st.st_size), refs, seen);
+        
+        delete buf; /* !!! autodelete */
+
+        close(fd); /* !!! close on exception */
+    }
+    
+    else if (S_ISLNK(st.st_mode)) {
+        char buf[st.st_size];
+        if (readlink(path.c_str(), buf, st.st_size) != st.st_size)
+            throw SysError(format("reading symbolic link `%1%'") % path);
+        search(string(buf, st.st_size), refs, seen);
+    }
+    
+    else throw Error(format("unknown file type: %1%") % path);
+}
+
+
+Strings filterReferences(const string & path, const Strings & _refs)
+{
+    Strings refs;
+    Strings seen;
+
+    /* For efficiency (and a higher hit rate), just search for the
+       hash part of the file name.  (This assumes that all references
+       have the form `HASH-bla'). */
+    for (Strings::const_iterator i = _refs.begin();
+         i != _refs.end(); i++)
+    {
+        string s = string(baseNameOf(*i), 0, 32);
+        parseHash(s);
+        refs.push_back(s);
+    }
+
+    checkPath(path, refs, seen);
+
+    return seen;
+}