Git Bisect

1 month ago 5

I extracted and enhanced this TIL from my April 8th 2020 weeknotes to make it easier to find.

I fixed two bugs in Datasette using git bisect run - a tool which lets you run an automated binary search against a commit log to find the source of a bug.

Since I was figuring out a new tool, I fired up another GitHub issue self-conversation: in issue #716 I document my process of both learning to use git bisect run and using it to find a solution to that particular bug.

It worked great, so I used the same trick on issue #689 as well.

Watching git bisect run churn through 32 revisions in a few seconds and pinpoint the exact moment a bug was introduced is pretty delightful.

The first step is to tell it the range of commits that you want to search in, using git bisect start:

$ git bisect start main 0.34 Bisecting: 32 revisions left to test after this (roughly 5 steps) [dc80e779a2e708b2685fc641df99e6aae9ad6f97] Handle scope path if it is a string

Then you provide a script that will return an error if the bug is present.

Usually you would use pytest or similar for this, but for the bug I was investigating here I wrote this custom script and saved it as check_templates_considered.py:

import asyncio import pathlib from datasette.app import Datasette import httpx async def run_check(): ds = Datasette([]) async with httpx.AsyncClient(app=ds.app()) as client: response = await client.get("http://localhost/") assert 200 == response.status_code assert "Templates considered" in response.text if __name__ == "__main__": loop = asyncio.get_event_loop() loop.run_until_complete(run_check())

This script will fail with an assertion error if Templates considered is not included in the HTML for the homepage.

To run the bisection, use git bisect run <script goes here>:

$ git bisect run python check_templates_considered.py running python check_templates_considered.py Traceback (most recent call last): ... AssertionError Bisecting: 15 revisions left to test after this (roughly 4 steps) [7c6a9c35299f251f9abfb03fd8e85143e4361709] Better tests for prepare_connection() plugin hook, refs #678 running python check_templates_considered.py Traceback (most recent call last): ... AssertionError Bisecting: 7 revisions left to test after this (roughly 3 steps) [0091dfe3e5a3db94af8881038d3f1b8312bb857d] More reliable tie-break ordering for facet results running python check_templates_considered.py Traceback (most recent call last): ... AssertionError Bisecting: 3 revisions left to test after this (roughly 2 steps) [ce12244037b60ba0202c814871218c1dab38d729] Release notes for 0.35 running python check_templates_considered.py Traceback (most recent call last): ... AssertionError Bisecting: 1 revision left to test after this (roughly 1 step) [70b915fb4bc214f9d064179f87671f8a378aa127] Datasette.render_template() method, closes #577 running python check_templates_considered.py Traceback (most recent call last): ... AssertionError Bisecting: 0 revisions left to test after this (roughly 0 steps) [286ed286b68793532c2a38436a08343b45cfbc91] geojson-to-sqlite running python check_templates_considered.py 70b915fb4bc214f9d064179f87671f8a378aa127 is the first bad commit commit 70b915fb4bc214f9d064179f87671f8a378aa127 Author: Simon Willison Date: Tue Feb 4 12:26:17 2020 -0800 Datasette.render_template() method, closes #577 Pull request #664. :040000 040000 def9e31252e056845609de36c66d4320dd0c47f8 da19b7f8c26d50a4c05e5a7f05220b968429725c M datasette bisect run success

The final output shows exactly which commit introduced the bug. In this case it was 70b915fb4bc214f9d064179f87671f8a378aa127 (the "first bad commit").

Created 2022-10-29T12:36:07-07:00, updated 2022-10-31T20:10:45-07:00 · History · Edit

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Git Bisect

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