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A Bug's Life: What if `select` is Broken After All?

A Bug's Life: What if `select` is Broken After All?

Your project's issue list lies dormant for months, years even. Users seem happy enough, and you've got other priorities. Suddenly, a flurry of users report a crazy-sounding bug. They're seeing a crash, triggered with specific keystrokes, with a common operating system. You can't reproduce it. What in the world?!

You may have heard the debugging tip from the fantastic book Pragmatic Programmer, "select isn't broken". It's always best to assume that bugs are in *your* code. But... what if this one isn't?

In this talk, you'll re-experience this unusual low-level bug, and take away some lessons for the kinds of debugging we need to do all the time as software developers.


Colin Jones

August 23, 2019


  1. A Bug's Life What If select Is Broken After All?

    8th Light, Inc. Colin Jones @trptcolin https://8thlight.com
  2. None
  3. None
  4. Debugging @trptcolin

  5. None
  6. Tip: “Don’t panic"

  7. Tip: “Failing test before fixing code”

  8. Tip: “Don’t assume it - prove it”


  10. Tip: “select isn’t broken”

  11. REPL-y @trptcolin

  12. REPL-y

  13. REPL-y

  14. Addressed pain points • Readline bindings in JLine (v1) were

    lacking • CTRL-C killed the whole REPL • Tab completion didn’t exist
  15. Fun to implement!

  16. Glue code reply nrepl sjacket clojure-complete jline2 leiningen boot

  17. Dependencies!

  18. Glue code reply nrepl sjacket clojure-complete jline2 leiningen boot

  19. Upstream bugs/features

  20. Upstream bugs/features

  21. Upstream bugs/features

  22. Upstream bugs/features

  23. Eventually things were OK

  24. Demotivated “Done”

  25. The Bug @trptcolin

  26. Demotivated “Done”

  27. None
  28. IOException Recurso temporariamente indisponível java.io.FileInputStream.readBytes (FileInputStream.java:-2) java.io.FileInputStream.read (FileInputStream.java:255) java.io.BufferedInputStream.fill (BufferedInputStream.java:246)

    java.io.BufferedInputStream.read (BufferedInputStream.java:265) jline.internal.NonBlockingInputStream.run (NonBlockingInputStream.java:294) java.lang.Thread.run (Thread.java:748) Bye for now!
  29. IOException Recurso te java.io.FileInputStrea java.io.FileInputStrea java.io.BufferedInputS java.io.BufferedInputS jline.internal.NonBloc java.lang.Thread.run (

    Bye for now!
  30. None
  31. None
  32. Tip: “Don’t panic"

  33. Reproduce the issue? vs.

  34. Reproduce the issue!

  35. Reproduce the issue?!?

  36. “Works on my machine”

  37. “Works on my machine” “What is different between our setups?"

  38. Tip: “Don’t panic"

  39. Tip: “select isn’t broken”

  40. Tip: “select isn’t broken”

  41. Tip: “select isn’t broken”

  42. None
  43. None
  44. Tip: “Don’t panic"

  45. None
  46. None
  47. None
  48. None
  49. “Ubuntu has recently pushed what appears to be a bad

    kernel update to Ubuntu 18.04 users.”
  50. “After lots of debugging and some brief discussion on the

    LKML, it appears that this is the result of a bug introduced in Linux on 1 November 2018, and fixed on 9 January 2019.”
  51. None
  52. None
  53. None
  54. “have not applied the more recent https:// github.com/torvalds/linux/commit/ d3736d82e8169768218ee0ef687188759180 91a0

  55. Tip: Use search • Use rare words • Use rare

    combinations of words • Use rare phrases
  56. “ Broadly prior to the introduction of this bug (i.e

    for the last 5 years), the program would echo back console input (once enter is pressed), as expected.”
  57. “After the introduction of this bug, the program immediately terminates

    with "Resource temporarily unavailable".”
  58. “I have attached to this email a very short c

    program, which I believe shows the issue.”
  59. None
  60. None
  61. None
  62. Tip: Write great bug reports • Omit everything that’s non-essential

    • Include everything that is essential
  63. None
  64. “it optimizes fast-path for tty_reopen(), but more importantly it won't

    interrupt ongoing IO on the tty”
  65. None
  66. None


  69. What now?!

  70. What now?! “Not sure if it’s worth trying to build

    / release a workaround - update your kernels if this affects you?”
  71. Best. Comment. Ever.

  72. None
  73. What did we learn? @trptcolin

  74. Tip: “select isn’t broken”

  75. Search engines are your friends!

  76. Ask for help!

  77. We’re stronger together

  78. Tip: “Don’t assume it - prove it”

  79. Tip: “Don’t panic"

  80. Recommended resources • Book by David Thomas & Andrew Hunt:

    The Pragmatic Programmer • Talk by Stuart Halloway: “Debugging with the Scientific Method” • Blog by Julia Evans: “What does debugging a program look like?” @trptcolin
  81. Recommended resources • Leiningen report: https://github.com/technomancy/leiningen/issues/ 2533 • Spigot (Minecraft

    server) forum: https://www.spigotmc.org/threads/ ubuntu-18-04-bad-kernel-update.357871/ • Ubuntu bug report: https://bugs.launchpad.net/ubuntu/+source/ linux/+bug/1813873 • linux-console bug report: https://marc.info/?l=linux- console&m=154883021430618&w=2 About this bug @trptcolin
  82. Thank you! 8th Light, Inc. Colin Jones @trptcolin https://8thlight.com