A Tale As Old As Time

So, this has been documented many times before.

Even though we all know that ruby’s Timeout.timeout is not threadsafe and Thread#raise can and will interrupt your ensure/rescue code on a Friday at 5 pm, it’s still around in the libraries we use.

Note, I even filtered out some of the results below:

$ grep -r -E "\bTimeout.timeout\b" /Users/joerafaniello/.gem/ruby/2.4.2/gems
ruby_parser-3.10.1/lib/ruby_parser_extras.rb:                                     Timeout.timeout time do
capybara-2.5.0/lib/capybara/server.rb:                                            Timeout.timeout(60) { @server_thread.join(0.1) until responsive? }
dalli-2.7.6/lib/dalli/socket.rb:                                                  Timeout.timeout(options[:socket_timeout]) do
excon-0.59.0/lib/excon/socket.rb:                                                 Timeout.timeout(@data[:read_timeout]) do
excon-0.59.0/lib/excon/unix_socket.rb:                                            Timeout.timeout(@data[:connect_timeout]) do
fog-core-1.45.0/lib/fog/compute/models/server.rb:                                 result = ready? && !ssh_ip_address.nil? && !!Timeout.timeout(sshable_timeout) { ssh("pwd", options) }
hawkular-client-4.1.0/lib/hawkular/operations/operations_api.rb:                  Timeout.timeout(@wait_time) { sleep 0.1 until @ws.open? }
http-0.9.8/lib/http/timeout/global.rb:                                            ::Timeout.timeout(time_left, TimeoutError) do
http-0.9.8/lib/http/timeout/per_operation.rb:                                     ::Timeout.timeout(connect_timeout, TimeoutError) do
httpclient-2.8.3/lib/httpclient/session.rb:                                       ::Timeout.timeout(@send_timeout, SendTimeoutError) do
httpclient-2.8.3/lib/httpclient/session.rb:                                       ::Timeout.timeout(@connect_timeout, ConnectTimeoutError) do
httpclient-2.8.3/lib/httpclient/session.rb:                                       ::Timeout.timeout(@receive_timeout, ReceiveTimeoutError) do
httpclient-2.8.3/lib/httpclient/session.rb:                                       ::Timeout.timeout(@receive_timeout, ReceiveTimeoutError) do
httpclient-2.8.3/lib/httpclient/session.rb:                                       ::Timeout.timeout(@receive_timeout, ReceiveTimeoutError) do
httpclient-2.8.3/lib/httpclient/session.rb:                                       ::Timeout.timeout(@receive_timeout, ReceiveTimeoutError) do
manageiq-network_discovery-0.1.2/lib/manageiq/network_discovery/port_scanner.rb:  Timeout.timeout(ost.timeout) do
manageiq-smartstate-0.1.4/lib/metadata/VmConfig/VmConfig.rb:                      Timeout.timeout(60) do
multipart-post-2.0.0/test/test_composite_io.rb:                                   Timeout.timeout(1) do
net-ping-1.7.8/lib/net/ping/icmp.rb:                                              Timeout.timeout(@timeout){
net-ping-1.7.8/lib/net/ping/udp.rb:                                               Timeout.timeout(@timeout){
nio4r-2.1.0/spec/nio/selector_spec.rb:                                            Timeout.timeout(2) do
nio4r-2.1.0/spec/spec_helper.rb:                                                  sock = Timeout.timeout(1) { TCPSocket.new("localhost", $current_tcp_port) }
open4-1.3.4/lib/open4.rb:                                                         Timeout.timeout(timeout){ yield }
rack-2.0.3/test/spec_utils.rb:                                                    Timeout.timeout(1) do
ruby-dbus-0.13.0/spec/spec_helper.rb:                                             Timeout.timeout(10) do
snmp-1.2.0/lib/snmp/manager.rb:                                                   Timeout.timeout(@timeout) do
vcr-3.0.3/lib/vcr/util/internet_connection.rb:                                    Timeout.timeout(timeout) do
webmock-2.3.2/spec/support/network_connection.rb:                                 Timeout.timeout(timeout) do

$ grep -r -E "\bTimeout::timeout\b" /Users/joerafaniello/.gem/ruby/2.4.2/gems
open4-1.3.4/lib/open4.rb:                                                         Timeout::timeout(timeout) do
 

Those are only gems used by ManageIQ. I’m sure there are many other gems used by other projects that still use Timeout.

Wait a second. We didn’t actually look at the contents of the Timeout.timeout block. That’s the point, it doesn’t matter. If you hit this code at all, there’s a small chance it can Thread#raise at a very inopportune time in code that will never expect it!

You might be thinking, “that’s fine, this why I’m wrapping my code with a begin and a rescue or ensure to clean things up.” It is a regular exception and we would expect it to be rescued while we are inside the begin code. But don’t underestimate Murphy’s law. What if the exception is raised while we’re in the cleanup code, preventing it from completing? To top it off, it will find the weirdest possible place to raise and at the most unfortunate time, such as Friday at 5pm. I’ll repeat the links listed above for convenience because this is exactly what they show: Thread#raise can raise while in your ensure/rescue cleanup code.

Fun times

A “fun” bug recently reminded Beni and I of a “new” and totally awful way Timeout.timeout can ruin your day.

Ironically and hilariously, Beni stumbled upon this when writing a test to workaround the “Timeout can raise during your ensure code”. Ok, so I find it funny. I’m sure Beni didn’t.

Basically, he discovered that his test would sporadically hang when run in the full test suite but not when run in isolation.

After some serious spelunking via rspec bisect and manually minimizing the reproducer by commenting out parts of the remaining tests, he found the hang would occur when another test created a Tempfile. As soon as he put a Tempfile.new('x') in the timeout workaround test, the test would hang “sometimes”.

That’s it, I’m done

We minimized his test case:

require 'tempfile'
require 'timeout'
Tempfile.new("x")

def stuff
  ""
end

20.times do |i|
  begin
    Timeout.timeout(1e-9) do
      loop do
        begin
          "result"
        ensure
        end
      end
    end
  rescue Timeout::Error => e
  end
end

Note: Don’t ask about the stuff method. Without it, I couldn’t get the test to hang sporadically.

We ran it with ruby debug RUBYOPT=-d.

When it worked, it looked like this:

Exception `Timeout::Error' at /Users/joerafaniello/.rubies/ruby-2.3.4/lib/ruby/2.3.0/timeout.rb:112 - execution expired
removing /var/folders/fq/blrz820d3qz7nm7vj8mbtfs40000gq/T/x20170906-57250-s683fs...
done

When it would hang, it looked like this:

Exception `Timeout::Error' at /Users/joerafaniello/.rubies/ruby-2.3.4/lib/ruby/2.3.0/timeout.rb:112 - execution expired
removing /var/folders/fq/blrz820d3qz7nm7vj8mbtfs40000gq/T/x20170906-57351-jfeu6l...

If you’re still reading, you might have noticed that done is missing when the test hangs. The done happens after Tempfile wants to close and unlink the file here.

Therefore, when it hangs, the Tempfile finalizer never completes.

tldr; Timeout.timeout can Thread#raise while in a finalizer for another object! AWESOME.

The final straw

Wait, what’s a finalizer?

There is no guarantee when the ruby garbage collector will garbage collect an object. ObjectSpace.define_finalizer provides a place for you to tell ruby to “run this code when my object is to be garbage collected.” Finalizers are generally used to cleanup things so we don’t cause memory leaks. Interrupting them is probably not something we want to troubleshoot at 5pm on a Friday.

It’s ok, there aren’t many places that define finalizers:

$ grep -r define_finalizer /Users/joerafaniello/.rubies/ruby-2.4.2
lib/ruby/2.4.0/cgi/session.rb:                                         ObjectSpace::define_finalizer(self, Session::callback(@dbprot))
lib/ruby/2.4.0/drb/timeridconv.rb:                                     ObjectSpace.define_finalizer(Object.new) {on_gc}
lib/ruby/2.4.0/tempfile.rb:                                            ObjectSpace.define_finalizer(self, Remover.new(@tmpfile))

$ grep -r define_finalizer /Users/joerafaniello/.gem/ruby/2.4.2/gems
actionview-5.0.6/lib/action_view/template.rb:                          ObjectSpace.define_finalizer(self, Finalizer[method_name, mod])
concurrent-ruby-1.0.5/lib/concurrent/atomic/ruby_thread_local_var.rb:  ObjectSpace.define_finalizer(me, self.class.thread_finalizer(array))
concurrent-ruby-1.0.5/lib/concurrent/atomic/ruby_thread_local_var.rb:  ObjectSpace.define_finalizer(self, self.class.threadlocal_finalizer(@index))
ffi-1.9.18/lib/ffi/autopointer.rb:                                     ObjectSpace.define_finalizer(self, @releaser)
http-cookie-1.0.3/lib/http/cookie_jar/mozilla_store.rb:                ObjectSpace.define_finalizer(self, Callable[@db, :close])
logging-2.2.2/lib/logging.rb:                                          ObjectSpace.define_finalizer self, Logging.method(:shutdown)
tins-1.15.0/lib/tins/thread_local.rb:                                  ObjectSpace.define_finalizer(self, @@cleanup)
vcr-3.0.3/lib/vcr/library_hooks/webmock.rb:                            ObjectSpace.define_finalizer(Thread.current, lambda {
winrm-2.2.3/lib/winrm/shells/base.rb:                                  ObjectSpace.define_finalizer(

It’s a small list but libraries like tempfile, concurrent-ruby, ffi, actionview, etc. are pretty common. Again, this is only in the gems that ManageIQ uses. This list will certainly grow if you searched dependencies of other applications.

The ugly part of this story is that the finalizer and the Thread#raise are both async operations so they can happen at nearly any time. We caught this in a test, but this could happen in development or even production and is really difficult to reason about because the symptoms could be nearly anything.

All in good time

We opened a ruby bug that Tempfile’s finalizer can be interrupted by Timeout’s Thread#raise. It was found that Timeout can raise in any finalizer, not just Tempfile.

This is a reminder to rid your codebase and dependencies of Timeout and perhaps Timeout, which still lives in Ruby’s standard library, will be fixed or replaced.

Finally, there’s a guide on how to configure safe timeouts in various libraries. Please try it out and contribute to it.