/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. If the sync changed `build/siso_revision` (the `gen_siso_revision` hook rewrites it when the new Chromium pins a different siso), commit that file on its own as `chore: update siso revision`. CI builds siso from this file and gn-check fails the roll if it is stale. 6. **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