How to view the call stack when an object is thrown in Ruby

Question

The warden gem uses throw and catch functions to handle control flow. This makes it super hard to debug authentication systems, especially in devise where the gem throws the :warden symbol and the warden gem catches it.

I my program, I have a function that (indirectly) throws a :warden symbol, but I don't know what is the exact call site of thethrow function. Is there a way to find it?

It seems that we cannot use the TracePoint class because it only supports raise events.

Answer 1

I was able to get something working using TracePoint and the fact that throw is a c language routine by trapping c_call:

TracePoint.new(:c_call) do |trace|
  if trace.method_id == :throw
    p [trace.path, trace.lineno]
  end
end

This will only get you where the call to throw was actually made, not a full stack trace of everything called up to that point, though you could play around with catching :call as well, and put something together that captures more information. As a quick example:

TracePoint.new(:call, :c_call) do |trace|
  if trace.event == :call || trace.method_id == :throw
    p [trace.method_id, trace.path, trace.lineno]
  end

  trace.disable if trace.method_id == :throw
end

Full example:

# might_throw_cont.rb
def might_throw_second
  throw :warden if rand(100) < 10

  might_throw_third
end

def might_throw_third
  throw :warden if rand(100) < 10

  might_throw_final
end

# example.rb
require './might_throw_cont'

def might_throw_first
  throw :warden if rand(100) < 10

  might_throw_second
end

def might_throw_final
  throw :warden if rand(100) < 10

  will_throw
end

def will_throw
  throw :warden
end

TracePoint.new(:call, :c_call) do |trace|
  if trace.event == :call || trace.method_id == :throw
    p [trace.method_id, trace.path, trace.lineno]
  end

  trace.disable if trace.method_id == :throw
end.enable do
  catch :warden do
    might_throw_first
  end

  puts "done"
end

Obviously, going to be hard to tell which method actually threw the symbol in this example. But running the example, a couple times I'll be able to see in the output (2 example runs):

# run 1
[:might_throw_first, "example.rb", 3]
[:might_throw_second, "/Users/simplelime/Documents/Ruby/might_throw_cont.rb", 1]
[:might_throw_third, "/Users/simplelime/Documents/Ruby/might_throw_cont.rb", 7]
[:might_throw_final, "example.rb", 9]
[:will_throw, "example.rb", 15]
[:throw, "example.rb", 16] # < only line you'll see if you trace only :c_call
done

# run 2
[:might_throw_first, "example.rb", 3]
[:throw, "example.rb", 4] # < only line you'll see if you trace only :c_call
done