1. A Toolbelt of
a Seasoned Bug Hunter
Damir Zekić

3. (["foo")

4. Baaaad bugs Not so bad bugs

5. Silent bugs Loud bugs

6. 2016:
a debugging odyssey

8. Step 1: Reproduce

9. Step 1: Reproduce
SUCCESS

10. Step 2: What introduced it?

11. git bisect

12. 13 37 c0 ff ee c0 de a0

13. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"

14. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"
no bug 7 commits ago
"good"

15. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"
no bug 7 commits ago
"good"

16. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"
no bug 7 commits ago
"good"

17. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"
no bug 7 commits ago
"good"

18. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"
no bug 7 commits ago
"good"

19. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"
no bug 7 commits ago
"good"

20. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"
no bug 7 commits ago
"good"

21. 13 37 c0 ff ee c0 de a0
bug is present here
"bad"
no bug 7 commits ago
"good"

22. 13 37 c0 ff ee c0 de a0
cli-ff-hanger

24. Every commit must be stable

25. But, which
commit was "good"?!

26. 13 37 c0 ff ee c0 de a0
no bug 7 commits ago
"good"

27. $ ./find_slow.rb

29. Step 2: What introduced it?
UNSURE

30. Brainstorming:
Is it a memory leak?

31. A Problem:
Ruby doesn't release
memory back to OS

32. Brainstorming:
Is it a memory leak?UNSURE

33. Assume it is
and check heap dumps

35. require 'objspace'
ObjectSpace.trace_object_allocations_start
class Foo
def bar
puts "Hello, world!"
end
end
foo = Foo.new
foo.bar
file = File.open("/tmp/heap.dump", 'w')
ObjectSpace.dump_all(output: file)
file.close

36. $ wc -l /tmp/heap.dump
33297 /tmp/heap.dump

37. {"address":"0x7f9fa7076ea0", "type":"STRING", "class":"0x7f9fa70cfe10", "embedded":true, "bytesize":13, "value":"Hello,
world!", "encoding":"UTF-8", "file":"(irb)", "line":5, "method":"bar", "generation":13, "memsize":40, "flags":
{"wb_protected":true}}
{"address":"0x7f9fa7076ec8", "type":"IMEMO", "class":"0x7f9fa70dc2f0", "memsize":40, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7076ef0", "type":"IMEMO", "references":["0x7f9fa70dd4e8"], "memsize":40, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7076f18", "type":"STRING", "class":"0x7f9fa70cfe10", "frozen":true, "embedded":true, "fstring":true,
"bytesize":5, "value":"abort", "encoding":"US-ASCII", "memsize":40, "flags":{"wb_protected":true, "old":true,
"uncollectible":true, "marked":true}}
{"address":"0x7f9fa7076f40", "type":"STRING", "class":"0x7f9fa70cfe10", "frozen":true, "bytesize":59, "capacity":127,
"value":"/Users/damir/.rbenv/versions/2.4.1/lib/ruby/x86_64-darwin16", "memsize":168, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7076f68", "type":"IMEMO", "class":"0x7f9fa70dc2f0", "memsize":40, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7076f90", "type":"IMEMO", "references":["0x7f9fa70dd4e8"], "memsize":40, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7076fb8", "type":"IMEMO", "class":"0x7f9fa70dc2f0", "memsize":40, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7076fe0", "type":"IMEMO", "references":["0x7f9fa70dd4e8"], "memsize":40, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7077008", "type":"IMEMO", "class":"0x7f9fa70dc2f0", "memsize":40, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7077030", "type":"IMEMO", "references":["0x7f9fa70dd4e8"], "memsize":40, "flags":{"wb_protected":true,
"old":true, "uncollectible":true, "marked":true}}
{"address":"0x7f9fa7077058", "type":"STRING", "class":"0x7f9fa70cfe10", "frozen":true, "embedded":true, "fstring":true,
"bytesize":5, "value":"spawn", "encoding":"US-ASCII", "memsize":40, "flags":{"wb_protected":true, "old":true,
"uncollectible":true, "marked":true}}

38. {"address":"0x7f9fa7076ea0",
"type":"STRING",
"class":"0x7f9fa70cfe10",
"embedded":true,
"bytesize":13,
"value":"Hello, world!",
"encoding":"UTF-8",
"file":"(irb)", "line":5,
"method":"bar",
"generation":13,
"memsize":40,
"flags":{"wb_protected":true}}

39. $ gem install heapy

40. $ ./analyze_heapy_diffs.rb

41. Class DSL objects
survive longest

42. !

43. Brainstorming:
Let's trace a call stack

44. Flamegraphs

49. Brainstorming:
Let's trace a call stackSUCCESS

50. Internal framework creates controllers
Classes created by routing don't get destroyed
Router removes methods from all controllers

51. Now let's fix it!