Branching Perfetto for Chrome milestones

Merging a (set of) Perfetto change(s) to a Chrome milestone release requires creation of a branch in the perfetto repo, cherry-picking of the change(s) to the branch, and updating the DEPS file in Chrome's milestone branch to point to the new perfetto branch's head.

Creating the perfetto branch

1. Determine the branch name: chromium/XXXX, where XXXX is the branch number of the milestone (see Chromium Dash). Example for M87: chromium/4280.

2. Check if the branch already exists: if yes, skip to cherry-picking. To check, you can search for it in https://github.com/google/perfetto/branches.

3. Look up the appropriate base revision for the branch. You should use the revision that Chromium's DEPS of the milestone branch points to (search for perfetto in the file). The DEPS file for branch XXXX is at:

   https://chromium.googlesource.com/chromium/src.git/+/refs/branch-heads/XXXX/DEPS

   Example for M87: DEPS (at time of writing) points to f4cf78e052c9427d8b6c49faf39ddf2a2e236069.

4. Create the branch: Ask a member of perfetto-team to create a chromium/XXXX branch via git push origin 4cf78e05:chromium/4280

Cherry-picking the change(s)

1. Cherry-pick the commit locally and send a pull-request against the branch as usual.

   $ git fetch origin $ git checkout -tb cpick origin/chromium/XXXX $ git cherry-pick -x <commit hash> # Resolve conflicts manually. $ tools/gen_all out/xxx # If necessary. $ gh pr create

2. Send the pull request for review and land it. Note the commit's revision hash.

Updating the DEPS file in Chromium

1. Create, send for review, and land a Chromium patch that edits the top-level DEPS file on the Chromium's milestone branch. You can also combine this step with cherry-picks of any chromium changes. For details, see Chromium's docs. It amounts to:

   $ gclient sync --with_branch_heads $ git fetch $ git checkout -tb perfetto_uprev refs/remotes/branch-heads/XXXX $ ... # Edit DEPS. $ git cl upload