Sunday, 8 July 2018

Git Bisect Scripting!

So sometimes you want to find out what commit a bug was added, even if you have tried git bisect its a pretty manual thing doesn't seem too cool. Let's just go over the basics for anyone who hasn't used it before.

Bisect basically allows you to do a binary search over a git history to find when a bug was introduced. To start you will need a known bad commit (normally the latest, or a released version) and good commit, probably the commit the feature was added in when it was working all well and good :)

First enter bisect mode
git bisect start [BAD] [GOOD]

you can use all the usual allowed stuff (branch, commit#, HEAD~2, whatever)

Then manually check if the commit is good or bad and report!
git bisect good
git bisect bad 

Also if the project does not build or something you can skip one!
git bisect skip

Then when it tells you what you are looking for you can exit with:
git bisect reset

BORING!


Right lets automate it! You can run any command that you can run on the shell and have it return the state to bisect.

Exit codes
we just exit the script with the following numbers to tell bisect what state the commit is in

  • GOOD - 0
  • BAD - 1
  • SKIP - 125

I'll make a little noddy function to test
module.exports = function add (a, b) {
  return a + b
}

I then add a few commits and break it on the way,

for this example I put my testing script outside the main git repo so that the checkouts won't have a problem.

try {
  const add = require('../bisect_script/')

  const result = add(2, 2)

  if (result == 4) {
    process.exit(0)
  } else {
    process.exit(1)
  }
} catch (err) {
  process.exit(125)
}

Then we can run the script against bisect with
git bisect start HEAD HEAD~5
git bisect run node ../bisect_test_Script/test.js

OUTPUT

cce8f28154071789a33a8b101cd11dc6bae2cf33 is the first bad commit
commit cce8f28154071789a33a8b101cd11dc6bae2cf33
Author: Robert Gill <envman@gmail.com>
Date:   Sun Jul 8 16:48:30 2018 +0100

    more logging: broke it

:100644 100644 e1dbfd4a103f424f065510204f9ae5ff80db1625 5c87270f0517cf407c4d486796899be2d87bc124 M index.js
bisect run success
Just make sure you do git bisect reset after to get back to the starting point.

Code Example (2 repos subject and test script)
https://github.com/envman/bisect_me
https://github.com/envman/bisect_test_script

You may need to update paths in bisect run and the test script to make it work depending on how/where you clone.

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