QA/Platform/Graphics/Guides

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Finding a Regression Window

Installing Mozregression

Instructions for Windows:

  • Download and install Python 2.7
  • Open a Windows command window (Run cmd.exe)
  • Execute this command: 
    pip install -U mozregression
  • Confirm mozregression is installed by executing this command: 
    mozregression --version
  • If you get stuck, see the full instructions here

How Mozregression Works

Mozregression works by downloading Firefox Nightly builds from a range of dates allowing you to easily narrow down the first Nightly which introduced a bug. Once a Nightly is downloaded and installed, Mozregression will start Firefox with a temporary profile and allow you to test your bug. After testing the bug, you give Mozregression a good or bad command and it will split the range in half, downloading the next build. Once the range has been narrowed down to a single day it will give you a pushlog for the build that will identify the changes which may have caused the bug. This should be attached to the bug report.

Using Mozregression

Note: We recommend narrowing down the range manually before using mozregression.

If you don't know the dates, simply run mozregression from a terminal/command window. This will download Firefox Nightly builds starting from January 1, 2009. However, you can save yourself some time and hassle if you give mozregression a narrower range. You may find some of the really old builds will just fail to work, especially on newer platform versions.

If you know the dates, simply run mozgression --good YYYY-MM-DD --bad YYYY-MM-DD where good is the date of the last build you know is good and bad is the date of the first build you know is bad.

Narrowing the Range

Note: We recommend narrowing down the range to six weeks between builds before using mozregression.

  • Start from the date of the build you reproduced the bug with
  • Open the about:support page and make note of the Build ID from the Application Basics section (the build ID follows a YYYYMMDDhhmmss format).
  • Install a build from six weeks before that date from ftp.mozilla.org
  • If that build reproduces the bug, go back another six weeks
  • Repeat this process until you find a build that does not reproduce the bug and make note of its build date
  • Use these dates to run mozregression

For example, if the build ID of a good build is 20150401123456 and the build ID of a bad build is 20150410123456: 

mozregression --good 2015-04-01 --bad 2015-04-10

If Mozregression Fails

If mozregression fails for whatever reason, you'll need to narrow the range manually to a single day:

  • Get the date of the build where you first saw the bug
  • Grab a build that is six weeks older from ftp.mozilla.org
  • Test that build and see if the bug reproduces
  • Repeat this process until you find a build that does not reproduce the bug

Once you've found a range, begin to narrow the range further:

  • Begin by testing the build which represents the midway point.
  • Eg. if the good build is from 2015-04-01 and my bad build is from 2015-05-15, I might test the build from 2015-04-21. The range will be reduced to either 2015-04-21:2015-05-15 or 2015-04-01:2015-04-21, depending on whether or not the bug reproduced.
  • Split the difference again and retest
  • Keep repeating until you've narrowed it down to a single day

Once you find the single day range, get the changeset for each build:

  • Start the Firefox build
  • Open the about:buildconfig page
  • Look at the Source section of this page for an hg.mozilla.org URL
  • Copy the 12-digit ID from the end of that URL (this is the changeset ID for the build)
  • Eg. the changeset of a build built from https://hg.mozilla.org/mozilla-central/rev/4fe763cbe196 is 4fe763cbe196

Once you have both changesets, manually put together the pushlog URL:

Once an offending bug has been identified, that bug should be flagged:

  • mark your bug as blocking the offending bug
  • flag the developer of the offending bug using the needinfo? flag

It may be necessary to bisect the range further but that's outside the scope of this document.

Testing a Try Build

TO BE DETERMINED

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