Try to find the Bundler version from Gemfile.lock, otherwise install latest

Author: eregon

README.md

@@ -105,9 +105,11 @@ jobs:
 
 ### Bundler
 
-Currently, Bundler is guaranteed to be installed for all versions.
-If the Ruby ships with Bundler (Ruby >= 2.6), that version is used.
-Otherwise (Ruby < 2.6), Bundler 1 is installed when that Ruby was built.
+By default, if there is a `Gemfile.lock` file with a `BUNDLED WITH` section,
+the latest version of Bundler with the same major version will be installed.
+Otherwise, the latest Bundler version is installed (except for Ruby 2.2 where only Bundler 1 is supported).
+
+This behavior can be customized, see [action.yml](action.yml) for details about the `bundler` input.
 
 ### Caching `bundle install`
 

action.yml

@@ -9,6 +9,10 @@ inputs:
     description: 'Engine and version to use, see the syntax in the README. Reads from .ruby-version if unset.'
     required: false
     default: 'default'
+  bundler:
+    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'
 outputs:
   ruby-prefix:
     description: 'The prefix of the installed ruby'

dist/index.js

@@ -25,7 +25,7 @@ export async function run() {
 
     setupPath(ruby, newPathEntries)
 
-    await installBundler(platform, rubyPrefix)
+    await installBundler(platform, rubyPrefix, engine, version)
 
     core.setOutput('ruby-prefix', rubyPrefix)
   } catch (error) {
@@ -111,11 +111,55 @@ function setupPath(ruby, newPathEntries) {
     core.exportVariable('PATH', [...newPathEntries, ...cleanPath].join(path.delimiter))
 }
 
-async function installBundler(platform, rubyPrefix) {
-  const bundle_exe = platform === 'windows-latest' ? 'bundle.cmd' : 'bundle'
-  // Install Bundler if not already part of the stdlib
-  if (!fs.existsSync(path.join(rubyPrefix, 'bin', bundle_exe))) {
-    await exec.exec(path.join(rubyPrefix, 'bin', 'gem'), ['install', 'bundler', '-v', '~> 1', '--no-document'])
+function readBundledWithFromGemfileLock() {
+  if (fs.existsSync('Gemfile.lock')) {
+    const contents = fs.readFileSync('Gemfile.lock', 'utf8')
+    const lines = contents.split(/\r?\n/)
+    const bundledWithLine = lines.findIndex(line => /^BUNDLED WITH$/.test(line.trim()))
+    if (bundledWithLine !== -1) {
+      const nextLine = lines[bundledWithLine+1]
+      if (nextLine && /^\d+/.test(nextLine.trim())) {
+        const bundlerVersion = nextLine.trim()
+        const majorVersion = bundlerVersion.match(/^\d+/)[0]
+        console.log(`Using Bundler ${majorVersion} from Gemfile.lock BUNDLED WITH ${bundlerVersion}`)
+        return majorVersion
+      }
+    }
+  }
+  return null
+}
+
+async function installBundler(platform, rubyPrefix, engine, rubyVersion) {
+  var bundlerVersion = core.getInput('bundler')
+  if (bundlerVersion === 'none') {
+    return
+  }
+
+  if (bundlerVersion === 'default' || bundlerVersion === 'Gemfile.lock') {
+    bundlerVersion = readBundledWithFromGemfileLock()
+    if (!bundlerVersion) {
+      bundlerVersion = 'latest'
+    }
+  }
+
+  let versionArray
+  if (rubyVersion.startsWith('2.2')) {
+    console.log('Bundler 2 requires Ruby 2.3+, using Bundler 1 on Ruby 2.2')
+    versionArray = ['-v', '~> 1']
+  } else if (/^\d+/.test(bundlerVersion)) {
+    versionArray = ['-v', `~> ${bundlerVersion}`]
+  } else if (bundlerVersion === 'latest') {
+    versionArray = []
+  } else {
+    throw new Error(`Cannot parse bundler input: ${bundlerVersion}`)
+  }
+
+  if (engine === 'rubinius') {
+    console.log(`Rubinius only supports the version of Bundler shipped with it`)
+  } else if (bundlerVersion === '1' && engine === 'truffleruby') {
+    console.log(`Using the Bundler version shipped with ${engine}`)
+  } else {
+    await exec.exec(path.join(rubyPrefix, 'bin', 'gem'), ['install', 'bundler', ...versionArray, '--no-document'])
   }
 }