misc(build): give build-tracker a shared git history on PRs

.github/workflows/ci.yml

@@ -44,9 +44,9 @@ jobs:
     - run: yarn i18n:checks
     - run: yarn dogfood-lhci
 
-    # buildtracker runs `git merge-base HEAD origin/master` which needs more history than depth=1. https://github.com/paularmstrong/build-tracker/issues/106
-    - name: Deepen git fetch (for buildtracker)
-      run: git fetch --deepen=100
+    # buildtracker needs history and a common merge commit.
+    - name: Fixup git history (for buildtracker)
+      run: bash $GITHUB_WORKSPACE/lighthouse-core/scripts/git-get-shared-history.sh
     - name: Store in buildtracker
       # TODO(paulirish): Don't allow this to fail the build. https://github.com/paularmstrong/build-tracker/issues/200
       run: yarn bt-cli upload-build || true

lighthouse-core/scripts/git-get-shared-history.sh

@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+
+##
+# @license Copyright 2020 The Lighthouse Authors. All Rights Reserved.
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+# Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
+##
+
+set -euo pipefail
+
+# Overview:
+# - build-tracker relies on a common commit that's shared between HEAD and master.
+# - Lighthouse runs on pull_request, not push, so the checkout is not the branch with shared history, but the result of a merge.
+# - checkout@v2 uses a merge remote (eg. remotes/pull/9605/merge) that often has just a single commit.
+# - This script creates a new branch that matches the current checkout, but does have a shared history.
+
+# See also
+# - https://github.com/paularmstrong/build-tracker/issues/106
+# - https://github.com/paularmstrong/build-tracker/issues/200
+
+# We can always use some more history
+git -c protocol.version=2 fetch --deepen=100
+echo "History is deepened."
+
+if git merge-base HEAD origin/master > /dev/null; then
+  echo "We have a common commit w/ origin/master. Skipping this script…";
+  exit 0
+else
+  echo "We don't have a common commit w/ origin/master. We'll checkout the associated branch, merge master, and then we'll be good"
+fi
+
+# get the human readable remote name
+checkout_name=$(git describe --all --exact-match HEAD)
+
+
+# We only want to keep going if it's a 'remotes/pull/{*}/merge'
+if [[ $checkout_name != remotes/pull/*/merge  ]]; then
+  echo "Don't know how to handle this checkout ($checkout_name). 🤔 Bailing.";
+  exit 0;
+fi
+
+# Extract 9605 from 'remotes/pull/9605/merge'
+pr_num=${checkout_name//[!0-9]/}
+
+lsremote_hash=$(git ls-remote origin "refs/pull/$pr_num/head" | cut -f1)
+
+if [ -z "$lsremote_hash" ]; then
+  echo "ls-remote failed to find this PR's branch. 🤔 Bailing.";
+  exit 0;
+fi
+
+# Checkout our PR branch
+git checkout --progress --force "$lsremote_hash"
+
+# Branch off, for safekeeping
+mergebranch_name="_bt_mergebranch-$pr_num"
+git checkout -b "$mergebranch_name"
+
+
+git merge --no-verify -m "Merge remote-tracking branch 'origin/master' into $mergebranch_name" origin/master
+
+# If there's a diff aginst where we started.. we fucked up
+if git --no-pager diff --color=always --exit-code "$checkout_name" > /dev/null; then
+  echo "No diff, good!"
+else
+  echo "Unexpected difference between $mergebranch_name and $checkout_name. Bailing";
+  exit 0;
+fi
+
+# Lastly, now we should definitely have a merge-base.
+if git merge-base HEAD origin/master > /dev/null; then
+  echo "Merge-base found. Perfect. 👌"
+else
+  echo "No diff, but still no merge-base. Very unexpected. 🤔"
+fi