Implement automatic bundle install and caching

* Not yet enabled by default.

Author: eregon

.github/workflows/test.yml

@@ -39,6 +39,7 @@ jobs:
     - uses: ./
       with:
         ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
     - run: ruby -v
 
     - name: build compiler

README.md

@@ -116,8 +116,27 @@ Otherwise, the latest Bundler version is installed (except for Ruby 2.2 and 2.3
 
 This behavior can be customized, see [action.yml](action.yml) for details about the `bundler` input.
 
-### Caching `bundle install`
+### Caching `bundle install` automatically
 
+This action provides a way to automatically run `bundle install` and cache the result:
+```yaml
+    - uses: ruby/setup-ruby@v1
+      with:
+        bundler-cache: true
+```
+
+This caching speeds up installing gems significantly and avoids too many requests to RubyGems.org.  
+It needs a `Gemfile` under the [`working-directory`](#working-directory).  
+The caching works whether there is a `Gemfile.lock` or not.
+If there is a `Gemfile.lock`, `bundle config --local deployment true` is used.
+  
+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.
+
+### Caching `bundle install` manually
+
+You can also cache gems manually,
+but this is not recommended because it is verbose and very difficult to use a correct cache key.
 You can cache the installed gems with these two steps:
 
 ```yaml
@@ -140,7 +159,8 @@ When using `.ruby-version`, replace `${{ matrix.ruby }}` with `${{ hashFiles('.r
 When using `.tool-versions`, replace `${{ matrix.ruby }}` with `${{ hashFiles('.tool-versions') }}`.
 
 This uses the [cache action](https://github.com/actions/cache).
-The code above is a more complete version of the [Ruby - Bundler example](https://github.com/actions/cache/blob/master/examples.md#ruby---bundler).
+The code above is a more complete version of the [Ruby - Bundler example](https://github.com/actions/cache/blob/master/examples.md#ruby---\
+bundler).
 Make sure to include `use-ruby` in the `key` to avoid conflicting with previous caches.
 
 ### Working Directory

action.yml

@@ -13,6 +13,10 @@ inputs:
     description: 'The version of Bundler to install. Either none, 1, 2, latest or Gemfile.lock. The default tries Gemfile.lock and otherwise uses latest.'
     required: false
     default: 'default'
+  bundler-cache:
+    description: 'Run "bundle install", and cache the result automatically. Either true or false.'
+    required: false
+    default: 'false'
   working-directory:
     description: 'The working directory to use for resolving paths for .ruby-version, .tool-versions and Gemfile.lock.'
     required: false

common.js

@@ -1,5 +1,8 @@
 const os = require('os')
 const fs = require('fs')
+const util = require('util')
+const stream = require('stream')
+const crypto = require('crypto')
 const core = require('@actions/core')
 const { performance } = require('perf_hooks')
 
@@ -20,6 +23,14 @@ export function isHeadVersion(rubyVersion) {
   return rubyVersion === 'head' || rubyVersion === 'debug' || rubyVersion === 'mingw' || rubyVersion === 'mswin'
 }
 
+export async function hashFile(file) {
+  // See https://github.com/actions/runner/blob/master/src/Misc/expressionFunc/hashFiles/src/hashFiles.ts
+  const hash = crypto.createHash('sha256')
+  const pipeline = util.promisify(stream.pipeline)
+  await pipeline(fs.createReadStream(file), hash)
+  return hash.digest('hex')
+}
+
 export function getVirtualEnvironmentName() {
   const platform = os.platform()
   if (platform === 'linux') {