* Allow certain operations to succeed even if we don't have write

permission to the Nix store or database.  E.g., `nix-env -qa' will
  work, but `nix-env -qas' won't (the latter needs DB access).  The
  option `--readonly-mode' forces this mode; otherwise, it's only
  activated when the database cannot be opened.

src/libmain/shared.cc

@@ -160,6 +160,8 @@ static void initAndRun(int argc, char * * argv)
                 throw UsageError(format("`--max-jobs' requires a non-negative integer"));
             maxBuildJobs = n;
         }
+        else if (arg == "--readonly-mode")
+            readOnlyMode = true;
         else remaining.push_back(arg);
     }