Ruby Debugger Internals

How does a Ruby debugger look and work on the inside? Is it difficult to write a debugger? Hear the full story of supporting Ruby 2.0 straight from the maintainer of the RubyMine debugger. In this session we'll expose debugger internals, including different APIs used by debuggers; review how they evolved; and look at what it takes to keep the performance of a debugged application at a reasonable level. We'll also talk about alternative implementations including JRuby and Rubinius.

Transcript

1. Ruby Debugger Internals Dennis Ushakov
4. Two questions • Where are we? • What’s going on?
5. Speed
6. Measurement • Small sample program • 42M iterations • No IO operations • require ‘benchmark’
7. Sample Program MAGIC_NUMBER = 42_000_000
 
 def goto_rails_conf
 "RailsConf 2015"
 end
 
 def our_action
 MAGIC_NUMBER.times { goto_rails_conf }
 end
8. Sample Program MAGIC_NUMBER = 42_000_000
 
 def goto_rails_conf
 "RailsConf 2015"
 end
 
 def our_action
 MAGIC_NUMBER.times { goto_rails_conf }
 end 4.97s
9. Ruby 1.0: set_trace_func set_trace_func proc { |event,file,line,id,binding,classname|
 }
10. Event Types • line • call & return • c-call & c-return • class & end • raise • b-call & b-return — Ruby 2.0 • thread-begin & thread-end — Ruby 2.0
11. Binding • Kernel.binding • execution context (variables, methods)
12. Sample Output call sample.rb:7 our_action Object
 line sample.rb:8 our_action Object
 c-call sample.rb:8 times Integer
 line sample.rb:8 our_action Object
 call sample.rb:3 goto_rails_conf Object
 line sample.rb:4 goto_rails_conf Object
 return sample.rb:5 goto_rails_conf Object
 c-return sample.rb:8 times Integer
 return sample.rb:9 our_action Object
13. Ruby 1.0: set_trace_func MAGIC_NUMBER = 42_000_000
 
 def goto_rails_conf
 "RailsConf 2015"
 end
 
 def our_action
 MAGIC_NUMBER.times { goto_rails_conf }
 end 120.14s
14. set_trace_func set_trace_func proc { 
 |event,file,line,id,binding,classname| 
 }
15. Binding • Walk the stack • Store the environment
17. Ruby 1.8.3: rb_add_event_hook 
 void
 rb_add_event_hook(
 rb_event_hook_func_t func,
 rb_event_flag_t events,
 VALUE data
 );
18. Ruby 1.8.3: rb_add_event_hook static void
 debug_event_hook(rb_event_flag_t event, VALUE data, VALUE self, ID mid, VALUE klass)
 {
 } 10.55s
19. event_hook gem • If there’s a C API there should be a wrapper gem for it • Well, no
20. Being lazy
21. Ruby 2.0: moar API • TracePoint • debug_inspector
22. Ruby 2.0: TracePoint trace = TracePoint.new(*events) do |tp|
 end
 
 trace.enable
23. Ruby 2.0: TracePoint trace = TracePoint.new(*events) do |tp|
 end
 
 trace.enable 29.44s
24. Ruby 2.0: TracePoint trace = TracePoint.new(*events) do |tp| tp.binding
 end
 
 trace.enable 115.09s
25. Ruby 2.0: debug_inspector VALUE rb_debug_inspector_open(
 rb_debug_inspector_func_t func,
 void *data
 );
26. debug_inspector gem • If there’s a C API there should be a wrapper gem for it that’s obsolete • Well, no
27. debug_inspector gem RubyVM::DebugInspector.open do |dc|
 locations = dc.backtrace_locations
 locations.size.times do |i|
 p dc.frame_binding(i)
 p dc.frame_iseq(i)
 p dc.frame_class(i)
 end
 end
28. Performance summary • run 4.97s • set_trace_func 120.14s • rb_add_event_hook 10.55s • TracePoint 29.44s
29. Usages • set_trace_func • ruby -r debug.rb • rb_add_event_hook • ruby-debug-base/debugger • rcov
30. Usages • TracePoint + DebugInspector • debase • byebug • binding_of_caller
31. Under the hood • Everyone is using rb_add_event_hook
32. Being lazy, pt 2
33. Rubinius • No need for explicit file & line checks • Breakpoints are attached to the instruction
34. Rubinius: set_breakpoint @breakpoints.each { |b|
 if b.source == script.file_path
 exec, ip = script.compiled_code.locate_line(b.pos)
 if exec
 exec.set_breakpoint ip, b
 end
 end
 }
35. Rubinius: performance • Run 3.16s • Full debugger enabled 3.18s
36. en_Dal dennis.ushakov@jetbrains.com denofevil
37. JRuby • org.jruby.runtime.EventHook • public boolean isInterestedInEvent(RubyEvent event) • public void eventHandler(
 ThreadContext tCtx, 
 String event, 
 String file, int line, 
 String methodName, 
 IRubyObject klass
 )