Run bundle clean when reusing a cache and the lockfile changed

* Fixes https://github.com/ruby/setup-ruby/issues/113

README.md

@@ -136,6 +136,23 @@ This caching speeds up installing gems significantly and avoids too many request
 It needs a `Gemfile` (or `$BUNDLE_GEMFILE` or `gems.rb`) under the [`working-directory`](#working-directory).  
 If there is a `Gemfile.lock` (or `$BUNDLE_GEMFILE.lock` or `gems.locked`), `bundle config --local deployment true` is used.
 
+To use a `Gemfile` which is not at the root or has a different name, set `BUNDLE_GEMFILE` in the `env` at the job level, so it is set for all steps:
+```yaml
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    env:
+      BUNDLE_GEMFILE: subdir/mygemfile
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.6
+        bundler-cache: true
+    - run: bundle exec rake
+```
+Of course you can also use a matrix of gemfiles if you need to test multiple gemfiles.
+
 To perform caching, this action will use `bundle config --local path vendor/bundle`.  
 Therefore, the Bundler `path` should not be changed in your workflow for the cache to work (no `bundle config path`).
 

dist/index.js

@@ -51421,7 +51421,7 @@ async function bundleInstall(gemfile, lockFile, platform, engine, version) {
   const paths = [path]
   const baseKey = await computeBaseKey(platform, engine, version, lockFile)
   const key = `${baseKey}-${await common.hashFile(lockFile)}`
-  // If only Gemfile.lock changes we can reuse part of the cache (but it will keep old gem versions in the cache)
+  // If only Gemfile.lock changes we can reuse part of the cache, and clean old gem versions below
   const restoreKeys = [`${baseKey}-`]
   console.log(`Cache key: ${key}`)
 
@@ -51446,6 +51446,10 @@ async function bundleInstall(gemfile, lockFile, platform, engine, version) {
 
   // @actions/cache only allows to save for non-existing keys
   if (cachedKey !== key) {
+    if (cachedKey) { // existing cache but Gemfile.lock differs, clean old gems
+      await exec.exec('bundle', ['clean'])
+    }
+
     // Error handling from https://github.com/actions/cache/blob/master/src/save.ts
     console.log('Saving cache')
     try {

index.js

@@ -249,7 +249,7 @@ async function bundleInstall(gemfile, lockFile, platform, engine, version) {
   const paths = [path]
   const baseKey = await computeBaseKey(platform, engine, version, lockFile)
   const key = `${baseKey}-${await common.hashFile(lockFile)}`
-  // If only Gemfile.lock changes we can reuse part of the cache (but it will keep old gem versions in the cache)
+  // If only Gemfile.lock changes we can reuse part of the cache, and clean old gem versions below
   const restoreKeys = [`${baseKey}-`]
   console.log(`Cache key: ${key}`)
 
@@ -274,6 +274,10 @@ async function bundleInstall(gemfile, lockFile, platform, engine, version) {
 
   // @actions/cache only allows to save for non-existing keys
   if (cachedKey !== key) {
+    if (cachedKey) { // existing cache but Gemfile.lock differs, clean old gems
+      await exec.exec('bundle', ['clean'])
+    }
+
     // Error handling from https://github.com/actions/cache/blob/master/src/save.ts
     console.log('Saving cache')
     try {