Debugging with Git

Lightning talk on how to use git bisect to debug your code. Presented at
• DDDNorth 2019,
• dotnetsheff June 4th 2019
• UIKonf (remotely) May 18th 2020
• Sky Betting & Gaming Engineering Conference Feb 2023

For more info see here:
https://git-scm.com/docs/git-bisect

Transcript

1. © AND Digital 2023
   Luke Stringer, Tech Principal @ AND Digital
   Debugging with Git
   

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2. © AND Digital 2023
   

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3. © AND Digital 2023
   Given some steps to reproduce…
   Approaches to Debugging
   1. Manually look for the problem using
   • Intuition
   • Print statements & Breakpoints
   • Remote logging
   2. Find when the bug was introduced
   

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4. © AND Digital 2023
   2. Find when the bug was introduced
   How?
   

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5. © AND Digital 2023
   Use your Git history
   

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6. 598fde5
   62d017b
   97b914a
   13f01c7
   40081db
   6f5e527
   eb2c86b
   f89d7b3
   579acfa
   b054ee3
   7aba54b
   

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7. Bug!
   598fde5
   62d017b
   97b914a
   13f01c7
   40081db
   6f5e527
   eb2c86b
   f89d7b3
   579acfa
   b054ee3
   7aba54b
   

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8. Bug!
   598fde5
   62d017b
   97b914a
   13f01c7
   40081db
   6f5e527
   eb2c86b
   f89d7b3
   579acfa
   b054ee3
   7aba54b
   Find the commit where the
   bug was introduced
   • What changed
   • Why it was changed
   • Who changed it
   

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9. © AND Digital 2023
   $ git bisect
   • Automates the process using a binary search.
   • Very ef
   fi
   cient; worst case scenario O(log n)
   • 10 commits takes 4 steps. log2(10)
   • 100 commits takes 7 steps. log2(100)
   • 1000 commits takes 10 steps. log2(10)
   

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10. © AND Digital 2023
    $ git bisect
    • You provide 2 control points:
    • A “good” commit, e.g. bug absent
    • A “bad” commit, e.g. bug present Bad
    Good
    

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11. 598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    © AND Digital 2023
    

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12. 598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    

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13. 598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    

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14. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    

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15. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    $ git bisect good 598fde5

    Good
    

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16. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    $ git bisect good 598fde5

    Bisecting: 4 revisions left 

    to test after this 

    (roughly 2 steps) [6f5e527]
    Good
    

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17. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    $ git bisect good 598fde5

    Bisecting: 4 revisions left 

    to test after this 

    (roughly 2 steps) [6f5e527]
    $ git bisect bad

    Good
    

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18. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    $ git bisect good 598fde5

    Bisecting: 4 revisions left 

    to test after this 

    (roughly 2 steps) [6f5e527]
    $ git bisect bad

    Bisecting: 2 revisions left 

    to test after this 

    (roughly 1 step) [97b914a]
    Good
    

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19. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    $ git bisect good 598fde5

    Bisecting: 4 revisions left 

    to test after this 

    (roughly 2 steps) [6f5e527]
    $ git bisect bad

    Bisecting: 2 revisions left 

    to test after this 

    (roughly 1 step) [97b914a]
    $ git bisect good

    Good
    

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20. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    $ git bisect good 598fde5

    Bisecting: 4 revisions left 

    to test after this 

    (roughly 2 steps) [6f5e527]
    $ git bisect bad

    Bisecting: 2 revisions left 

    to test after this 

    (roughly 1 step) [97b914a]
    $ git bisect good

    Bisecting: 0 revisions left 

    to test after this 

    (roughly 1 step) [40081db]
    Good
    

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21. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    $ git bisect good 598fde5

    Bisecting: 4 revisions left 

    to test after this 

    (roughly 2 steps) [6f5e527]
    $ git bisect bad

    Bisecting: 2 revisions left 

    to test after this 

    (roughly 1 step) [97b914a]
    $ git bisect good

    Bisecting: 0 revisions left 

    to test after this 

    (roughly 1 step) [40081db]
    $ git bisect good

    Good
    

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22. Bad
    598fde5
    62d017b
    97b914a
    13f01c7
    40081db
    6f5e527
    eb2c86b
    f89d7b3
    579acfa
    b054ee3
    7aba54b
    $ git bisect start
    $ git bisect bad
    $ git bisect good 598fde5

    Bisecting: 4 revisions left 

    to test after this 

    (roughly 2 steps) [6f5e527]
    $ git bisect bad

    Bisecting: 2 revisions left 

    to test after this 

    (roughly 1 step) [97b914a]
    $ git bisect good

    Bisecting: 0 revisions left 

    to test after this 

    (roughly 1 step) [40081db]
    $ git bisect good

    6f5e527 is the first bad commit

    commit 6f5e527

    Author: Luke Stringer

    Date: Sat Mar 31

    

    Add user config
    Good
    First Bad
    commit
    

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23. © AND Digital 2023
    For Best Results
    • Have a clear de
    fi
    nition of good & bad
    • You can supply a script to git bisect to
    check if criteria is met
    • Keep commits small (enough) to inspect
    • Write descriptive commit messages to aid
    understanding
    © AND Digital 2023
    

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24. © AND Digital 2023
    🐛
    Luke Stringer, Tech Principal @ AND Digital
    Debugging with Git
    

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