Support using an Older Chrome Release

Dockerfile

@@ -5,6 +5,8 @@ FROM --platform=linux/amd64 heroku/heroku:${STACK_VERSION}-build as build
 ARG STACK_VERSION
 ENV STACK="heroku-${STACK_VERSION}"
 
+ARG GOOGLE_CHROME_MILESTONE_OFFSET
+
 # On Heroku-24 and later the default user is not root.
 # Once support for Heroku-22 and older is removed, the `useradd` steps below can be removed.
 USER root
@@ -15,6 +17,9 @@ USER non-root-user
 RUN mkdir -p /tmp/build /tmp/cache /tmp/env
 COPY --chown=non-root-user . /buildpack
 
+# Emulate the platform providing app config vars via env directory
+RUN if [ -n "${GOOGLE_CHROME_MILESTONE_OFFSET}" ]; then echo "${GOOGLE_CHROME_MILESTONE_OFFSET}" > /tmp/env/GOOGLE_CHROME_MILESTONE_OFFSET; fi
+
 # Sanitize the environment seen by the buildpack, to prevent reliance on
 # environment variables that won't be present when it's run by Heroku CI.
 RUN env -i PATH=$PATH HOME=$HOME STACK=$STACK /buildpack/bin/detect /tmp/build

README.md

@@ -25,6 +25,25 @@ by [Google](https://googlechromelabs.github.io/chrome-for-testing/).
 You can control the channel of the release by setting the `GOOGLE_CHROME_CHANNEL`
 config variable to `Stable`, `Beta`, `Dev`, or `Canary`, and then deploy/build the app.
 
+## Using an Older Chrome Release
+
+If the most recent stable Chrome or Chromedriver release has problems, perhaps due to an 
+app dependecy like Selenium not being compatible yet, an older major release can be installed.
+
+You can control how many major versions (milestones) back to install, with 
+`GOOGLE_CHROME_MILESTONE_OFFSET` config variable.
+
+*This version offset strategy does not lock the app to a specific Chrome version. It will still update automatically, but lag behind by the specified offset.*
+
+Set `GOOGLE_CHROME_MILESTONE_OFFSET` to a negative index, such as:
+* `-4` for the previous major stable release
+* `-3` for the current major stable release
+* `-2` for the beta release
+* `-1` for the dev release.
+
+During build, this offset indexes the milestone-sorted data returned from the Chrome for Testing API [`latest-versions-per-milestone.json`](https://googlechromelabs.github.io/chrome-for-testing/latest-versions-per-milestone.json),
+in order to resolve a specific version to install.
+
 ## Migrating from Separate Buildpacks
 
 ### Remove Existing Installations

bin/compile

@@ -46,17 +46,44 @@ if which chrome; then
   error "Chrome is already installed. This buildpack now installs a compatible version of Chrome. Please, remove the other Chrome installation. Chrome may have been installed by another buildpack (such as heroku/heroku-buildpack-google-chrome buildpack), which should no longer be used with this buildpack."
 fi
 
-# Detect requested channel or default to stable
-if [ -f "$ENV_DIR/GOOGLE_CHROME_CHANNEL" ]; then
-  channel=$(cat "$ENV_DIR/GOOGLE_CHROME_CHANNEL")
-  echo "Using env var GOOGLE_CHROME_CHANNEL=$channel" | indent
+# Detect milestone offset or channel-based version selection
+if [ -f "$ENV_DIR/GOOGLE_CHROME_MILESTONE_OFFSET" ]; then
+  milestone_offset=$(cat "$ENV_DIR/GOOGLE_CHROME_MILESTONE_OFFSET")
+  echo "Using env var GOOGLE_CHROME_MILESTONE_OFFSET=$milestone_offset" | indent
+
+  # jq is required to parse Chrome for Testing JSON API response
+  if which jq; then
+    echo "Using existing jq" | indent
+  else
+    echo "Installing jq" | indent
+    JQ_DIR="$INSTALL_DIR/jq"
+    mkdir -p "$JQ_DIR"
+    cd "$JQ_DIR"
+    wget --quiet -O jq https://github.com/jqlang/jq/releases/download/jq-1.7.1/jq-linux-$ARCH
+    chmod +x jq
+    PATH="$JQ_DIR:$PATH"
+    cd -
+  fi
+
+  latest_version_per_milestone="$(curl --silent --show-error --fail --retry 3 --retry-connrefused --connect-timeout 10 "https://googlechromelabs.github.io/chrome-for-testing/latest-versions-per-milestone.json")"
+  VERSION="$(echo $latest_version_per_milestone | jq --raw-output --arg milestone_offset "$milestone_offset" '.milestones | map(.) | sort_by(.milestone)[($milestone_offset | tonumber)] | .version')"
+  echo "Resolved milestone offset $milestone_offset to version $VERSION" | indent
+  INSTALLED_DESCRIPTION="milestone offset $milestone_offset"
+
 else
-  channel=stable
+  # Detect requested channel or default to stable
+  if [ -f "$ENV_DIR/GOOGLE_CHROME_CHANNEL" ]; then
+    channel=$(cat "$ENV_DIR/GOOGLE_CHROME_CHANNEL")
+    echo "Using env var GOOGLE_CHROME_CHANNEL=$channel" | indent
+  else
+    channel=stable
+  fi
+  # The current version endpoint requires ALL CAPS.
+  CHANNEL="$(echo -n "$channel" | awk '{ print toupper($0) }')"
+  VERSION="$(curl --silent --show-error --fail --retry 3 --retry-connrefused --connect-timeout 10 "https://googlechromelabs.github.io/chrome-for-testing/LATEST_RELEASE_$CHANNEL")"
+  echo "Resolved $CHANNEL version $VERSION" | indent
+  INSTALLED_DESCRIPTION="$CHANNEL"
 fi
-# The current version endpoint requires ALL CAPS.
-CHANNEL="$(echo -n "$channel" | awk '{ print toupper($0) }')"
-VERSION="$(curl --silent --show-error --fail --retry 3 --retry-connrefused --connect-timeout 10 "https://googlechromelabs.github.io/chrome-for-testing/LATEST_RELEASE_$CHANNEL")"
-echo "Resolved $CHANNEL version $VERSION" | indent
 
 echo "Downloading Chrome" | indent
 ZIP_URL="https://storage.googleapis.com/chrome-for-testing-public/$VERSION/linux64/chrome-linux64.zip"
@@ -83,4 +110,4 @@ export PATH="$BUILD_DIR/.chrome-for-testing/chrome-linux64:$BUILD_DIR/.chrome-fo
 which chrome | indent
 which chromedriver | indent
 
-echo "Installed Chrome for Testing $CHANNEL version $VERSION" | indent
+echo "Installed Chrome for Testing $INSTALLED_DESCRIPTION version $VERSION" | indent

bin/test.sh

@@ -24,3 +24,11 @@ docker run --rm heroku-buildpack-chrome-for-testing bash -l -c 'chrome --no-sand
 
 # Display a size breakdown of the directories added by the buildpack to the app.
 docker run --rm heroku-buildpack-chrome-for-testing bash -l -c 'du --human-readable --max-depth=1 /app'
+
+
+# Verify that the milestone offset builds too
+docker build --progress=plain --build-arg="STACK_VERSION=${STACK_VERSION}" --build-arg="GOOGLE_CHROME_MILESTONE_OFFSET=-4" -t heroku-buildpack-chrome-for-testing .
+
+# Check the profile.d scripts correctly added the binaries to PATH.
+docker run --rm heroku-buildpack-chrome-for-testing bash -l -c 'chrome --version'
+docker run --rm heroku-buildpack-chrome-for-testing bash -l -c 'chromedriver --version'