* nix-store, nix-instantiate: added an option `--add-root' to

immediately add the result as a permanent GC root.  This is the only
  way to prevent a race with the garbage collector.  For instance, the
  old style

    ln -s $(nix-store -r $(nix-instantiate foo.nix)) \
      /nix/var/nix/gcroots/result

  has two time windows in which the garbage collector can interfere
  (by GC'ing the derivation and the output, respectively).  On the
  other hand,

    nix-store --add-root /nix/var/nix/gcroots/result -r \
      $(nix-instantiate --add-root /nix/var/nix/gcroots/drv \
        foo.nix)

  is safe.

* nix-build: use `--add-root' to prevent GC races.

src/libmain/shared.cc

@@ -31,6 +31,28 @@ void sigintHandler(int signo)
 }
 
 
+Path makeRootName(const Path & gcRoot, int & counter)
+{
+    counter++;
+    if (counter == 1)
+        return gcRoot;
+    else
+        return (format("%1%-%2%") % gcRoot % counter).str();
+}
+
+
+void printGCWarning()
+{
+    static bool warned = false;
+    if (!warned) {
+        printMsg(lvlInfo,
+            "warning: you did not specify `--add-root'; "
+            "the result might be removed by the garbage collector");
+        warned = true;
+    }
+}
+
+
 void setLogType(string lt)
 {
     if (lt == "pretty") logType = ltPretty;
@@ -183,7 +205,7 @@ static void initAndRun(int argc, char * * argv)
 
     /* Automatically clean up the temporary roots file when we
        exit. */
-    RemoveTempRoots removeTempRoots;
+    RemoveTempRoots removeTempRoots; /* unused variable - don't remove */
 
     run(remaining);
 }