Automate Debugging with git bisect
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The document outlines a method for debugging code using Git bisect, emphasizing the importance of automated tests to quickly identify buggy commits within a codebase. It provides a step-by-step guide on setting up tests, running Git bisect, and verifying failures and successes to isolate the source of a bug. Additionally, it highlights using commit messages and diffs for pinpointing bug introductions in a structured manner.
![cbell@CamilleBellConsul/ng.com
18
Star9ng
up
git bisect with
Test
on
the
Middle
git Commit
$ git bisect start
Already on 'master’
$ git bisect bad
$ git bisect good 7a8c508
Bisecting: 1 revision left to test after this (roughly
1 step)
[c546d1f89b5b0c14ab160e227fc83d62fb780e6f] added to_s
$ rspec . --color
..
Finished in 0.00071 seconds
2 examples, 0 failures
$ git bisect good
Bisecting: 0 revisions left to test after this (roughly
0 steps)
[6ca7ee1909bf0b3f7344feee25a5b44a97602e2c] added error
Known
Good
Commit
Known
Bad
Commit
If
the
tests
pass,
Tell
git bisect good
Otherwise
git bisect bad
C
O
M
M
I
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Automate Debugging with git bisect
- 1. cbell@CamilleBellConsul/ng.com 1 Automated Debugging with git bisect TDD Camille Bell Agile Coach / Rails Developer [email protected] Twi6er @agilecamille h;p://www.slideshare.net/Camille_Bell/
- 2. cbell@CamilleBellConsul/ng.com 2 How do you find a bug in a large code base with lots of commits? 1st 2nd 3rd 4th 97th 98th 99th 100th
- 3. cbell@CamilleBellConsul/ng.com 3 Debugging Op9ons • Manually inspect all the code and hope you find the bug !" • Logically eliminate some of the code, inspect all the rest and hope you find the bug ! • Use automa9on to find your bug fast ☺
- 4. cbell@CamilleBellConsul/ng.com 4 Steps to Target the Buggy Code • Create new or use an exis9ng automated bug detector to dis9nguish between bug free code from buggy code (usually an automated test) • Determine the bug commit range – Use git log if needed • Use git bisect – Run the test – Use git diff to narrow bug to code lines
- 5. cbell@CamilleBellConsul/ng.com 5 Automa9ng Bug Detec9on • Perhaps you already have a good automated test, but weren’t running it with every commit. Use that test (and set up a CI server soon). • If not create a new test that should fail (RED) when the bug is present and pass (GREEN) when the bug is fixed. – Write the test in whatever test language makes sense (e.g. RSpec for low level Ruby, Cucumber high level behavior, Jasmine for JavaScript, JUnit for Java, NUnit for C#, etc.)
- 6. cbell@CamilleBellConsul/ng.com 6 Verify the Test Catches the Bug • Run the test on the latest buggy code and watch it fail (RED). • Go back to a known good commit, run the test and watch it pass (GREEN).
- 7. cbell@CamilleBellConsul/ng.com 7 $ rspec . –color F. Failures: 1) Account depositing to account produces the correct balance when 9 dollars is deposited the balance is 10 dollars Failure/Error: @account.balance.should == 1000 expected: 1000 got: 999.9999 (using ==) # ./account_spec.rb:15:in `block (3 levels) in <top (required)>' Finished in 0.00058 seconds 2 examples, 1 failure Failed examples: rspec ./account_spec.rb:12 # Account depositing to account produces the correct balance when 9 dollars is deposited the balance is 10 dollars $ Verifying Failure on Known Bad Commit
- 8. cbell@CamilleBellConsul/ng.com 8 $ rspec . –color .. Finished in 0.00047 seconds 2 examples, 0 failures $ Verifying Success on a Known Good Commit
- 9. cbell@CamilleBellConsul/ng.com 9 Running git bisect • Start with a known bad git repository – $ git bisect start – $ git bisect bad – $ git bisect good <good_commit> • Run your tests • If the test passes – $ git bisect good • If the test fails – $ git bisect bad • Repeat un9l you find the bad commit Use either tag or git commit # If needed checkout a bad git commit #
- 10. cbell@CamilleBellConsul/ng.com 10 !"#"#"#"#" #"#"#" Determine the Bug Commit Range: • The last known commit without the bug • The first commit aeer the bug observed Star9ng Good Commit Star9ng Bad Commit Bug Inserted Within These 8 Commits
- 11. cbell@CamilleBellConsul/ng.com 11 git bisect does a binary search through your commit range. !"#"#"#"#" #"#"#" Good Commit Bad Commit bisect Star9ng Commit bisect begins with the commit halfway between the known good commit and the known bad commit
- 12. cbell@CamilleBellConsul/ng.com 12 Assume the 1st test on bisect failed. !"!"#"#"#" !"!"!" Good Commit Bad Commit bisect Next Commit Then bisect would select the commit between the known good commit and the last bad bisect Bad Commit Can ignore these Commits Bug inserted Within These 4 Commits
- 13. cbell@CamilleBellConsul/ng.com 13 Assume the 2nd test on bisect passed. !"!"#" !"!"!" Good Commit Bad Commit Then bisect would select the commit between the last known good commit and the last bad bisect Bad Commit Can ignore these Commits Bug inserted Within These 2 Commits Can ignore these Commits Good Commit bisect Next Commit
- 14. cbell@CamilleBellConsul/ng.com 14 If the 3rd test on bisect failed. !"!"!" !"!"!" Good Commit Bad Commit Then that commit is where the bug was inserted. Bad Commit Can ignore these Commits Bug inserted Within These 2 Commits Can ignore these Commits Good Commit Final Commit Failed Test
- 15. cbell@CamilleBellConsul/ng.com 15 If the 3rd test on bisect passed. !"!"!"!"!" Good Commit Bad Commit Then the next commit is where the bug was inserted. Bad Commit Can ignore these Commits Bug inserted Within These 2 Commits Can ignore these Commits Good Commit Final Commit Passed Test
- 16. cbell@CamilleBellConsul/ng.com 16 Example Test require_rela9ve 'account' describe Account do before do @star9ng_balance_in_pennies = 100 @account = Account.new(@star9ng_balance_in_pennies) end context "deposi9ng to account produces the correct balance" do it "when 9 dollars is deposited the balance is 10 dollars" do deposit_amount_in_pennies = 900 @account.deposit(deposit_amount_in_pennies) @account.balance.should == 1000 end end end
- 17. cbell@CamilleBellConsul/ng.com 17 Some9mes the Commit Messages from git log Pinpoint the Bug $ git log --pretty="%h - %s" 561bb3a - added withdrawal and deposit messages 6ca7ee1 - added error c546d1f - added to_s 1974592 - added withdrawal 7a8c508 - Initial commit But usually the log only provides a range Known Good Commit Known Bad Commit
- 18. cbell@CamilleBellConsul/ng.com 18 Star9ng up git bisect with Test on the Middle git Commit $ git bisect start Already on 'master’ $ git bisect bad $ git bisect good 7a8c508 Bisecting: 1 revision left to test after this (roughly 1 step) [c546d1f89b5b0c14ab160e227fc83d62fb780e6f] added to_s $ rspec . --color .. Finished in 0.00071 seconds 2 examples, 0 failures $ git bisect good Bisecting: 0 revisions left to test after this (roughly 0 steps) [6ca7ee1909bf0b3f7344feee25a5b44a97602e2c] added error Known Good Commit Known Bad Commit If the tests pass, Tell git bisect good Otherwise git bisect bad C O M M I T
- 19. cbell@CamilleBellConsul/ng.com 19 Test on the final git commit $ rspec . --color F. Failures: 1) Account depositing to account produces the correct balance when 9 dollars is deposited the balance is 10 dollars Failure/Error: @account.balance.should == 1000 expected: 1000 got: 999.9999 (using ==) # ./account_spec.rb:15:in `block (3 levels) in <top (required)>' Finished in 0.00053 seconds 2 examples, 1 failure Failed examples: rspec ./account_spec.rb:12 # Account depositing to account produces the correct balance when 9 dollars is deposited the balance is 10 dollars $
- 20. cbell@CamilleBellConsul/ng.com 20 $ git diff c546d1f 6ca7ee1 diff --git a/account.rb b/account.rb index a979e55..0a572ef 100644 --- a/account.rb +++ b/account.rb @@ -5,7 +5,7 @@ class Account end def deposit(new_deposit) - @balance += new_deposit + @balance += (new_deposit - 0.0001) end def withdrawl(new_withdrawl) $ git diff Targets the Bug Even More Good Commit Just Before Bug Commit Where Bug First Appeared Bug inserted in one or more of the + lines.
- 21. cbell@CamilleBellConsul/ng.com 21 Thank You for Listening Camille Bell Agile Coach / Rails Developer [email protected] Twi6er @agilecamille h;p://www.slideshare.net/Camille_Bell/