How to check out an old version of Chromium OS

You gotta love the combination of open source projects and git. Getting up and running with a project like Chromium OS is pretty straightforward with the guide available on the Chromium Wiki. But what if, for some reason, you're interested in a version other than the most recent? How do you dig through the combinations of repositories to get it? I recently had to figure this out for a project I'm working on, so I thought I'd pass it along.

First off, you should know that version numbers between Chrome, Chrome OS, Chromium, and Chromium OS do have a certain correlation, but isn't guaranteed to be the same across the repositories for all of them.

Once you've figured out which version of you want, go to https://chromium.googlesource.com/chromiumos/manifest-versions/+/master/paladin/buildspecs/, click on the version number in the list, then select a build number from the next page.

What you see next is a long XML file that tells repo (Google's wrapper around git) what versions of all the Chromium repositories correspond to that build number. Scroll all the way down to the bottom of the page and click on the "txt" link to download it. This will be a Base64-encoded version of the XML file you were just looking at.

Next, decode the text file with a command like this:

base64 -d [manifest_version].txt > [manifest_version].xml

This assumes you're working on a *nix system. A quick Google search will help Windows users. Be sure to replace [manifest_version] as appropriate.

Assuming you've checked out the Chromium OS repositories before using the repo command, run the sync command using the manifest you just downloaded with:

repo sync -m /path/to/[manifest_version].xml

If you haven't checked out the repositories before, follow the instructions on the guide listed above, then replace the repo sync command with the one above. Keep in mind that the sync process takes a VERY long time.

At this point, if you were to try and enter the SDK with the usual cros_sdk and do any commands, they would likely fail because it is too new. Get the appropriate version with:

cros_sdk --replace

This command will also take a long time to complete.

Now you can enter the SDK with cros_sdk. Now you should rebuild all the packages with:

build_packages --nowithdebug --board=${BOARD}

This command will also take a long time to complete.

Finally you can build an image with the usual command (adding any extra flags you need):

build_image --board=${BOARD}

That's it! Good luck working with Chromium OS!