Sync only to the upstream branch and do not cherry-pick

Upstream sometimes uses merge commits which make it very difficult to
maintain a cherry-pick based sync workflow. This change modifies the way
syncing works so that it simply does a hard reset of the branch to the
filtered repository. This should be more foolproof, with the downside
that `main` is no longer up to date with upstream. Instead a new
`upstream` branch is always up-to-date.

Fixes #4861.

Author: mrobinson

.github/sync/UPSTREAM_COMMIT

@@ -12,28 +12,17 @@
 #    repository with the filtered contents into a directory specified by the
 #    argument passed to this script. The filtered contents are determined
 #    by the configuration in `.github/sync/webrender.paths`.
-# 3. Cherry-pick the new commits into the repository in the current working
-#    directory. The commits applied from the filtered repository are determined
-#    by choosing every commit after the hash found in the file
-#    `.github/sync/UPSTREAM_COMMIT`
+# 3. Reset the `upstream` branch in the target repository to the new filtered
+#    version.
 #
 # Note that this script relies on the idea that filtering `gecko-dev` the same
-# way more than once will result in the same commit hashes.
-#
-# If at some point, `webrender.paths` is modified and the commit hashes change,
-# then a single manual filter will have to happen in order to translate the
-# hash in the original filtered repository to the new one. The procedure for this
-# is roughly:
-#
-# 1. Run `git-filter-repo` locally and note the new hash of the latest
-#    commit included from upstream.
-# 2. Replace the contents `UPSTREAM_COMMIT` with that hash and commit
-#    it together with your changes to `webrender.paths`.
+# way more than once will result in the same commit hashes, otherwise multiple
+# copies of the version control history will be included in the repository.
 #
 # [1]: <https://github.com/mozilla/gecko-dev/> mirrored from
 #      <https://hg.mozilla.org/mozilla-central>
 # [2]: <https://github.com/mozilla/gecko-dev/>
-set -eu
+set -eux
 
 root_dir=$(pwd)
 cache_dir=$root_dir/_cache
@@ -89,15 +78,9 @@ cd "$root_dir"
 git remote add filtered-upstream "$filtered"
 git fetch filtered-upstream
 
-hash_file=".github/sync/UPSTREAM_COMMIT"
-hash=`cat $hash_file`
-number_of_commits=`git log $hash..filtered-upstream/master --pretty=oneline | wc -l`
+step "Resetting 'upstream' branch to filtered repository HEAD"
+git switch -c upstream
+git reset --hard filtered-upstream/master
 
-if [ $number_of_commits != '0' ]; then
-    step "Applying $number_of_commits new commits"
-    git -c user.email="$git_email" -c user.name="$git_name" cherry-pick $hash..filtered-upstream/master
-    git rev-parse filtered-upstream/master > "$hash_file"
-    git -c user.email="$git_email" -c user.name="$git_name" commit "$hash_file" -m "Syncing to upstream (`cat $hash_file`)"
-else
-    step "No new commits. Doing nothing."
-fi
+step Pushing new 'upstream'
+git push -f origin upstream

.github/sync/sync.sh

@@ -19,6 +19,4 @@ jobs:
           path: _cache/upstream
           key: upstream
       - name: Run synchronization script
-        run:  ./.github/sync/sync.sh _filtered
-      - name: Pushing new `main`
-        run: git push origin main
+        run: ./.github/sync/sync.sh _filtered

.github/workflows/sync-upstream.yml

@@ -2,17 +2,23 @@
 
 [![Version](https://img.shields.io/crates/v/webrender.svg)](https://crates.io/crates/webrender)
 
-WebRender is a GPU-based 2D rendering engine written in [Rust](https://www.rust-lang.org/). [Firefox](https://www.mozilla.org/firefox), the research web browser [Servo](https://github.com/servo/servo), and other GUI frameworks draw with it. It currently uses the OpenGL API internally.
-
-Note that the canonical home for this code is in gfx/wr folder of the
-mozilla-central repository at https://hg.mozilla.org/mozilla-central. The
-Github repository at https://github.com/servo/webrender should be considered
+WebRender is a GPU-based 2D rendering engine written in
+[Rust](https://www.rust-lang.org/). [Firefox](https://www.mozilla.org/firefox),
+the research web browser [Servo](https://github.com/servo/servo), and other GUI
+frameworks draw with it. It currently uses the OpenGL API internally.
+
+**Note that the upstream home for this code is in gfx/wr folder of the
+mozilla-central repository at https://hg.mozilla.org/mozilla-central.** The
+GitHub repository at https://github.com/servo/webrender should be considered
 a downstream mirror, although it contains additional metadata (such as Github
-wiki pages) that do not exist in mozilla-central. Pull requests against the
-Github repository are still being accepted, although once reviewed, they will
-be landed on mozilla-central first and then mirrored back. If you are familiar
-with the mozilla-central contribution workflow, filing bugs in
-[Bugzilla](https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&component=Graphics%3A%20WebRender)
+wiki pages) that do not exist in mozilla-central.
+
+Generally speaking, improvements to WebRender should be submitted upstream
+in Gecko, but those relevant to the Servo project or those unsuitable for
+upstream may be accepted here.
+
+If you are familiar with the mozilla-central contribution workflow, filing bugs
+in [Bugzilla](https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&component=Graphics%3A%20WebRender)
 and submitting patches there would be preferred.
 
 ## Update as a Dependency