thread 'main' panicked at 'already mutably borrowed: BorrowError', libcore/result.rs:945:5
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:380
   3: std::panicking::default_hook
             at libstd/panicking.rs:396
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:576
   5: std::panicking::begin_panic
             at libstd/panicking.rs:537
   6: std::panicking::begin_panic_fmt
             at libstd/panicking.rs:521
   7: rust_begin_unwind
             at libstd/panicking.rs:497
   8: core::panicking::panic_fmt
             at libcore/panicking.rs:71
   9: core::result::unwrap_failed
             at /checkout/src/libcore/macros.rs:23
  10: <core::result::Result<T, E>>::expect
             at /checkout/src/libcore/result.rs:809
  11: <core::cell::RefCell<T>>::borrow
             at /checkout/src/libcore/cell.rs:692
  12: rusty_leveldb::version_set::VersionSet::needs_compaction
             at /home/vi/.cargo/registry/src/github.com-1ecc6299db9ec823/rusty-leveldb-0.2.1/src/version_set.rs:257
  13: rusty_leveldb::db_impl::DB::maybe_do_compaction
             at /home/vi/.cargo/registry/src/github.com-1ecc6299db9ec823/rusty-leveldb-0.2.1/src/db_impl.rs:569
  14: rusty_leveldb::db_impl::DB::get_internal
             at /home/vi/.cargo/registry/src/github.com-1ecc6299db9ec823/rusty-leveldb-0.2.1/src/db_impl.rs:438
  15: rusty_leveldb::db_impl::DB::get
             at /home/vi/.cargo/registry/src/github.com-1ecc6299db9ec823/rusty-leveldb-0.2.1/src/db_impl.rs:456
  16: <dnscache::MyDatabase as dnscache::Database>::get
             at src/main.rs:75
  17: dnscache::details::try_answer_request
             at ./src/details.rs:146
  18: dnscache::details::<impl dnscache::DnsCache<DB, N>>::packet_from_client
             at ./src/details.rs:609
  19: dnscache::details::<impl dnscache::DnsCache<DB, N>>::serve1
             at ./src/details.rs:657
  20: <dnscache::DnsCache<DB, N>>::run_endlessly
             at ./src/lib.rs:173
  21: dnscache::run
             at src/main.rs:118
  22: dnscache::main
             at src/main.rs:124
  23: std::rt::lang_start::{{closure}}
             at /checkout/src/libstd/rt.rs:74
  24: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:479
  25: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  26: std::rt::lang_start_internal
             at libstd/panicking.rs:458
             at libstd/panic.rs:358
             at libstd/rt.rs:58
  27: std::rt::lang_start
             at /checkout/src/libstd/rt.rs:74
  28: main
  29: __libc_start_main
  30: _start