* A simple hack to fix NIX-18: the garbage collector cannot run when

the disk is full (because to delete something from the Nix store, we
  need a Berkeley DB transaction, which takes up disk space).  Under
  normal operation, we make sure that there exists a file
  /nix/var/nix/db/reserved of 1 MB.  When running the garbage
  collector, we delete that file before we open the Berkeley DB
  environment.

src/nix-store/main.cc

@@ -686,7 +686,7 @@ void run(Strings args)
     if (!op) throw UsageError("no operation specified");
 
     if (op != opDump && op != opRestore) /* !!! hack */
-        openDB();
+        openDB(op != opGC);
 
     op(opFlags, opArgs);
 }