Merging & patching

If you have a patch to the main branch (e.g. an important bug fix) that needs to be merged into one of the release V8 branches (refs/branch-heads/12.5), read on.

The following examples use a branched 12.3 version of V8. Substitute 12.3 with your version number. Read the documentation on V8’s version numbering for more information.

An associated issue on V8’s issue tracker is mandatory if a patch is merged. This helps with keeping track of merges.

What qualifies a merge candidate? #

• The patch fixes a severe bug (in 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.

The merge process #

The merge process in the V8 tracker is driven by Attributes in the form of:

Merge-Request: 123

once reviewed, this will be adjusted during the review to:

Merge: Approved-123
or
Merge: Rejected-123

After the CL landed, this will be adjusted one more time to:

Merge: Merged-123

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

Use chromiumdash to verify if the relevant CL has Canary coverage.

On top the Releases section should show a Canary.

How to create the merge CL #

Option 1: Using gerrit - Recommended #

1. Open the CL you want to back-merge.
2. Select "Cherry pick" from the extended menu (three vertical dots in the upper right corner).
3. Enter "refs/branch-heads/XX.X" as destination branch (replace XX.X by the proper branch).
4. Modify the commit message:
   1. Prefix the title with "Merged: ".
   2. Remove lines from the footer that correspond to the original CL ("Change-Id", "Reviewed-on", "Reviewed-by", "Commit-Queue", "Cr-Commit-Position"). Definitely keep the "(cherry picked from commit XXX)" line, as this is needed by some tools to relate merges to original CLs.
5. In case of merge conflict, please also go ahead and create the CL. To resolve conflicts (if any) - either using the gerrit UI or you can easily pull the patch locally by using the "download patch" command from the menu (three vertical dots in the upper right corner).
6. Send out for review.

Option 2: Using the automated script #

Let’s assume you’re merging revision af3cf11 to branch 12.2 (please specify full git hashes - abbreviations are used here for simplicity).

https://source.chromium.org/chromium/chromium/src/+/main:v8/tools/release/merge_to_branch_gerrit.py --branch 12.3 -r af3cf11

After landing: Observe the branch waterfall #

If one of the builders is not green after handling your patch, revert the merge immediately. A bot (AutoTagBot) takes 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), cc vahl@ or machenbach@ on the issue. Googlers: please check out the internal site before creating the CL.