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 Gerrit's branch page.

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 - the easiest way to do this is via Gerrit's branch page. The NEW BRANCH button on the top right opens a wizard - fill in the branch name and base revision determined above. If this fails with a permission issue, contact the Discord chat or perfetto-dev mailing list.

Cherry-picking the change(s)

1. If there are no merge conflicts, cherry-picking via Gerrit will be easiest. To attempt this, open your change in Gerrit and use the Cherry pick entry in the overflow menu on the top right, providing the chromium/XXXX branch name (see above).

2. Otherwise, merge the patch locally into a branch tracking origin/chromium/XXXX and upload a Gerrit change 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.
   $ git cl upload    # Remove "Change-Id:" lines from commit message.
   

3. Send the patch 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