Set BUNDLER_VERSION whenever we know which version to use

* Fixes #845

Author: eregon

.github/workflows/test.yml

@@ -352,6 +352,19 @@ jobs:
         bundler: 2.2.3
     - run: bundle --version | grep -F "Bundler version 2.2.3"
 
+  # https://github.com/ruby/setup-ruby/issues/845
+  testOlderBundlerVersion:
+    name: "Test with an older Bundler version than the one of the default gem"
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v6
+    - uses: ./
+      with:
+        ruby-version: '4.0'
+        bundler: 2.7.2
+        bundler-cache: true
+    - run: bundle --version | grep -F "Bundler version 2.7.2"
+
   testBundlerPre:
     name: "Test with a Bundler pre/rc version"
     runs-on: ubuntu-latest

bundler.js

@@ -162,10 +162,10 @@ export async function bundleInstall(gemfile, lockFile, platform, engine, rubyVer
   }
 
   let envOptions = {}
-  if (bundlerVersion.startsWith('1') && common.isBundler2PlusDefault(engine, rubyVersion)) {
-    // If Bundler 1 is specified on Rubies which ship with Bundler 2+,
-    // we need to specify which Bundler version to use explicitly until the lockfile exists.
-    console.log(`Setting BUNDLER_VERSION=${bundlerVersion} for "bundle config|lock" commands below to ensure Bundler 1 is used`)
+  if (bundlerVersion.match(/^\d+/)) {
+    // If a specific Bundler version is given or determined, we need to specify the version to use explicitly until the lockfile exists.
+    // Otherwise, a newer version of Bundler might be used.
+    console.log(`Setting BUNDLER_VERSION=${bundlerVersion} for "bundle config|lock" commands below to ensure Bundler ${bundlerVersion} is used`)
     envOptions = { env: { ...process.env, BUNDLER_VERSION: bundlerVersion } }
   }