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
2. © AND Digital 2023
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
4. © AND Digital 2023 2. Find when the bug was introduced How?
5. © AND Digital 2023 Use your Git history
6. 598fde5 62d017b 97b914a 13f01c7 40081db 6f5e527 eb2c86b f89d7b3 579acfa b054ee3 7aba54b
7. Bug! 598fde5 62d017b 97b914a 13f01c7 40081db 6f5e527 eb2c86b f89d7b3 579acfa b054ee3 7aba54b
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
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)
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
11. 598fde5 62d017b 97b914a 13f01c7 40081db 6f5e527 eb2c86b f89d7b3 579acfa b054ee3 7aba54b © AND Digital 2023
12. 598fde5 62d017b 97b914a 13f01c7 40081db 6f5e527 eb2c86b f89d7b3 579acfa b054ee3 7aba54b
13. 598fde5 62d017b 97b914a 13f01c7 40081db 6f5e527 eb2c86b f89d7b3 579acfa b054ee3 7aba54b $ git bisect start
14. Bad 598fde5 62d017b 97b914a 13f01c7 40081db 6f5e527 eb2c86b f89d7b3 579acfa b054ee3 7aba54b $ git bisect start $ git bisect bad
15. Bad 598fde5 62d017b 97b914a 13f01c7 40081db 6f5e527 eb2c86b f89d7b3 579acfa b054ee3 7aba54b $ git bisect start $ git bisect bad $ git bisect good 598fde5
 Good
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
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
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
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
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
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
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
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
24. © AND Digital 2023 🐛 Luke Stringer, Tech Principal @ AND Digital Debugging with Git