electron-chromium-upgrade

name	electron-chromium-upgrade
description	Guide for performing Chromium version upgrades in the Electron project. Use when working on the roller/chromium/main branch to fix patch conflicts during `e sync --3`. Covers the patch application workflow, conflict resolution, analyzing upstream Chromium changes, and proper commit formatting for patch fixes.

Electron Chromium Upgrade: Phase One

Summary

Run e sync --3 repeatedly, fixing patch conflicts as they arise, until it succeeds. Then export patches and commit changes atomically.

Success Criteria

Linter Requirements:

• Code/patch edits: Title must be exactly {CL-Number}: {upstream CL original title} with Ref: {URL} in the body.
• Metadata-only patch updates (hashes/line numbers): Message must be exactly chore: update patches with no body.

Phase One is complete when:

• e sync --3 exits with code 0 (no patch failures)
• All changes are committed per the commit guidelines

Do not stop until these criteria are met.

CRITICAL Do not delete or skip patches unless 100% certain the patch is no longer needed. Complicated conflicts or hard to resolve issues should be presented to the user after you have exhausted all other options. Do not delete the patch just because you can't solve it.

CRITICAL Never use git am --skip and then manually recreate a patch by making a new commit. This destroys the original patch's authorship, commit message, and position in the series. If git am --continue reports "No changes", investigate why — the changes were likely absorbed by a prior conflict resolution's 3-way merge. Present this situation to the user rather than skipping and recreating.

Context

The roller/chromium/main branch is created by automation to update Electron's Chromium dependency SHA. No work has been done to handle breaking changes between the old and new versions.

Key directories:

• Current directory: Electron repo (always run e commands here)
• .. (parent): Chromium repo (where most patches apply)
• patches/: Patch files organized by target
• docs/development/patches.md: Patch system documentation

Pre-flight Checks

Run these once at the start of each upgrade session:

1. Clear rerere cache (if enabled): git rerere clear in both the electron and .. repos. Stale recorded resolutions from a prior attempt can silently apply wrong merges.
2. Ensure pre-commit hooks are installed: Check that .git/hooks/pre-commit exists. If not, run yarn husky to install it. The hook runs lint-staged which handles clang-format for C++ files.

Workflow

1. Run e sync --3 (the --3 flag enables 3-way merge, always required)
2. If succeeds → skip to step 5
3. If patch fails:
   • Identify target repo and patch from error output
   • Analyze failure (see references/patch-analysis.md)
   • Fix conflict in target repo's working directory
   • Run git am --continue in affected repo
   • Repeat until all patches for that repo apply
   • IMPORTANT: Once git am --continue succeeds you MUST run e patches {target} to export fixes
   • Return to step 1
4. When e sync --3 succeeds, run e patches all
5. Read references/phase-one-commit-guidelines.md NOW, then commit changes following those instructions exactly.

Commands Reference

Command	Purpose
e sync --3	Clone deps and apply patches with 3-way merge
git am --continue	Continue after resolving conflict (run in target repo)
e patches {target}	Export commits from target repo to patch files
e patches all	Export all patches from all targets
e patches {target} --commit-updates	Export patches and auto-commit trivial changes
e patches --list-targets	List targets and config paths

Patch System Mental Model

patches/{target}/*.patch  →  [e sync --3]  →  target repo commits
                          ←  [e patches]   ←

When to Edit Patches

Situation	Action
During active git am conflict	Fix in target repo, then git am --continue
Modifying patch outside conflict	Edit .patch file directly
Creating new patch (rare, avoid)	Commit in target repo, then e patches {target}

Fix existing patches 99% of the time rather than creating new ones.

Patch Fixing Rules

1. Preserve authorship: Keep original author in TODO comments (from patch From: field)
2. Never change TODO assignees: TODO(name) must retain original name
3. Update descriptions: If upstream changed (e.g., DCHECK → CHECK_IS_TEST), update patch commit message to reflect current state
4. Never skip-and-recreate a patch: If git am --continue says "No changes — did you forget to use 'git add'?", do NOT run git am --skip and create a replacement commit. The patch's changes were already absorbed by a prior 3-way merge resolution. This means an earlier conflict resolution pulled in too many changes. Present the situation to the user for guidance — the correct fix may require re-doing an earlier resolution more carefully to keep each patch's changes separate.

Electron Chromium Upgrade: Phase Two

Summary

Run e build -k 999 -- --quiet repeatedly, fixing build issues as they arise, until it succeeds. Then run e start --version to validate Electron launches and commit changes atomically.

Run Phase Two immediately after Phase One is complete.

Success Criteria

Linter Requirements:

• Code/patch edits: Title must be exactly {CL-Number}: {upstream CL original title} with Ref: {URL} in the body.
• Metadata-only patch updates (hashes/line numbers): Message must be exactly chore: update patches with no body.

Phase Two is complete when:

• e build -k 999 -- --quiet exits with code 0 (no build failures)
• e start --version has been run to check Electron launches
• All changes are committed per the commit guidelines

Do not stop until these criteria are met. Do not delete code or features, never comment out code in order to take short cut. Make all existing code, logic and intention work.

Context

The roller/chromium/main branch is created by automation to update Electron's Chromium dependency SHA. No work has been done to handle breaking changes between the old and new versions. Chromium APIs frequently are renamed or refactored. In every case the code in Electron must be updated to account for the change in Chromium, strongly avoid making changes to the code in chromium to fix Electrons build.

Key directories:

• Current directory: Electron repo (always run e commands here)
• .. (parent): Chromium repo (do not touch this code to fix build issues, just read it to obtain context)

Workflow

1. Run e build -k 999 -- --quiet (the --quiet flag suppresses per-target status lines, showing only errors and the final result)
2. If succeeds → skip to step 6
3. If build fails:
   • Identify underlying file in "electron" from the compilation error message
   • Analyze failure
   • Fix build issue by adapting Electron's code for the change in Chromium
   • Run e build -t {target_that_failed}.o to build just the failed target we were specifically fixing
     • You can identify the target_that_failed from the failure line in the build log. E.g. FAILED: 2e506007-8d5d-4f38-bdd1-b5cd77999a77 "./obj/electron/chromium_src/chrome/process_singleton_posix.o" CXX obj/electron/chromium_src/chrome/process_singleton_posix.o the target name is obj/electron/chromium_src/chrome/process_singleton_posix.o
   • Read references/phase-two-commit-guidelines.md NOW, then commit changes following those instructions exactly.
   • Return to step 1
4. CRITICAL: After ANY commit (especially patch commits), immediately run git status in the electron repo
   • Look for other modified .patch files that only have index/hunk header changes
   • These are dependent patches affected by your fix
   • Commit them immediately with: git commit -am "chore: update patches"
5. Return to step 1
6. When e build succeeds, run e start --version
7. Check if you have any pending changes in the Chromium repo by running git status
   • If you have changes follow the instructions below in "A. Patch Fixes" to correctly commit those modifications into the appropriate patch file
8. Final commit self-check: run git log --format='%h %B' over the commits this upgrade added (everything since the chore: bump chromium in DEPS commit) and verify that each upstream CL is referenced by exactly one non-fixup commit — full messages, not just titles, since Ref: lines live in commit bodies. If a CL appears in more than one non-fixup commit, consolidate with git commit --fixup + autosquash rebase per references/phase-two-commit-guidelines.md

Commands Reference

Command	Purpose
e build -k 999 -- --quiet	Build Electron, continue on errors, suppress status lines
e build -t {target}.o	Build just one specific target to verify a fix
e start --version	Validate Electron launches after successful build

Two Types of Build Fixes

A. Patch Fixes (for files in chromium_src or patched Chromium files)

When the error is in a file that Electron patches (check with grep -l "filename" patches/chromium/*.patch):

1. Edit the file in the Chromium source tree (e.g., /src/chrome/browser/...)
2. Create a fixup commit targeting the original patch commit:

   cd ..  # to chromium repo
   git add <modified-file>
   git commit --fixup=<original-patch-commit-hash>
   GIT_SEQUENCE_EDITOR=: git rebase --autosquash --autostash -i <commit>^
   

3. Export the updated patch: e patches chromium
4. Commit the updated patch file following references/phase-one-commit-guidelines.md.

To find the original patch commit to fixup: git log --oneline | grep -i "keyword from patch name"

The base commit for rebase is the Chromium commit before patches were applied. Find it by checking the refs/patches/upstream-head ref.

B. Electron Code Fixes (for files in shell/, electron/, etc.)

When the error is in Electron's own source code:

1. Edit files directly in the electron repo
2. Commit directly (no patch export needed)

Critical: Read Before Committing

• Before ANY Phase One commits: Read references/phase-one-commit-guidelines.md
• Before ANY Phase Two commits: Read references/phase-two-commit-guidelines.md

Skill Directory Structure

This skill has additional reference files in references/:

• patch-analysis.md - How to analyze patch failures
• phase-one-commit-guidelines.md - Commit format for Phase One
• phase-two-commit-guidelines.md - Commit format for Phase Two

Read these when referenced in the workflow steps.