Introduction

If you have a patch to the master branch (e.g. an important bug fix) that needs to be merged into one of the production V8 branches, read on.

For the examples, a branched 2.4 version of V8 will be used. Substitute “2.4” with your version number. See [[Release Process|Release Process]] for more information about version numbers.

An associated issue on Chromium‘s or V8’s issue tracker is mandatory if a patch is merged. This helps with keeping track of merges. You can use a template to create an issue.

What qualifies a merge candidate?

• The patch fixes a severe bug (order of importance)
  1. Security bug
  2. Stability bug
  3. Correctness bug
  4. Performance bug
• The patch does not alter APIs
• The patch does not change behavior present before branch cut (except if the behavior change fixes a bug)

More information can be found on the relevant Chromium page. When in doubt, send an email to v8-dev@googlegroups.com.

Merge process outlined

The merge process in the Chromium and V8 tracker is driven by labels in the form of

Merge-[Status]-[Branch]

The currently important labels for V8 are:

1. Merge-Request-{Branch} initiates the process => This fix should be merged into #.#
2. Merge-Review-{Branch} The merge is not approved yet for #.# e.g. because Canary coverage is missing
3. Merge-Approved-{Branch} => Simply means that the Chrome TPMs have signed off on the merge
4. Merge-Merged-{Branch} => When the merge is done, the Merge-Approved label is swapped with this one. {Branch} is the name/number of the V8 branch e.g. 4.3 for M-43.

Instructions for git using the automated script

How to check if a commit was already merged/reverted/has Canary coverage

Use mergeinfo.py to get all the commits which are connected to the HASH according to Git.

tools/release/mergeinfo.py HASH

If it tells you Is on Canary: No Canary coverage you should not merge yet because the fix was not yet deployed on a Canary build. A good rule of the thumb is to wait at least 3 days after the fix has landed until the merge is conducted.

Step 1: Run the script

Let‘s assume you’re merging revision af3cf11 to branch 2.4 (please specify full git hashes - abbreviations are used here for simplicity).

tools/release/merge_to_branch.py --branch 2.4 af3cf11

Run the script with ‘-h’ to display its help message, which includes more options (e.g. you can specify a file containing your patch, or you can reverse a patch, specify a custom commit message, or resume a merging process you‘ve canceled before). Note that the script will use a temporary checkout of v8 - it won’t touch your work space. You can also merge more than one revision at once, just list them all.

tools/release/merge_to_branch.py --branch 2.4 af3cf11 cf33f1b sf3cf09

In case you are a V8 committer, feel free to use TBR to land the merge if it 1.) has approval and 2.) the merge was clean and no conflicts to resolve.

Step 2: Observe the branch waterfall

If one of the builders is not green after handling your patch, revert the merge immediately. A bot (AutoTagBot) will take care of the correct versioning after a 10 minute wait.

Patching a version used on Canary/Dev

In case you need to patch a Canary/Dev version, which should not happen often follow these instructions:

Step 1: Merge to roll branch

Example version used is 2.4.4.

tools/release/roll_merge.py --branch 5.7.433 af3cf11

Step 2: Make Chromium aware of the fix

Example Chromium branch used is 2978

$ git checkout chromium/2978
$ git merge 5.7.433.1
$ git push

Step 3: The end

Chrome/Chromium should should pick of the change when they build automatically.

FAQ

I get an error during merge that is related to tagging. What should I do?

When two people are merging at the same time a race-condition can happen in the merge scripts. If this is the case, contact machenbach@chromium.org and hablich@chromium.org.

Is there a TL;DR?

1. Create issue on issue tracker
2. Check status of the fix with tools/release/mergeinfo.py
3. Add Merge-Request-{Branch} to the issue
4. Wait until somebody adds Merge-Approved-{Branch}
5. Merge