Fix bundler: with a minor version installing the wrong version

If passed `bundler: 2.2`, setup-ruby would install bundler 2.3, because
it would generate a constraint that looks like `~> 2.2`. This
unexpectedly installs the latest of the 2.x series, but the expectation
is that it would install the latest of the 2.2.x series.

Fixes #304

Author: Fryguy

.github/workflows/test.yml

@@ -145,6 +145,28 @@ jobs:
         rubygems: 3.2.3
     - run: gem --version | grep -F "3.3.3"
 
+  testMajorBundlerVersion:
+    name: "Test with a major Bundler version"
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ./
+      with:
+        ruby-version: 2.6
+        bundler: 2
+    - run: bundle --version | grep -P "Bundler version 2\.\d+\.\d+"
+
+  testMinorBundlerVersion:
+    name: "Test with a minor Bundler version"
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: ./
+      with:
+        ruby-version: 2.6
+        bundler: 2.2
+    - run: bundle --version | grep -P "Bundler version 2\.2\.\d+"
+
   testExactBundlerVersion:
     name: "Test with an exact Bundler version"
     runs-on: ubuntu-latest

action.yml

@@ -18,7 +18,7 @@ inputs:
       Defaults to 'default'.
   bundler:
     description: |
-      The version of Bundler to install. Either 'none', 'latest', 'Gemfile.lock', or a version number (e.g., 1, 2, 2.1.4).
+      The version of Bundler to install. Either 'none', 'latest', 'Gemfile.lock', or a version number (e.g., 1, 2, 2.1, 2.1.4).
       For 'Gemfile.lock', the version is determined based on the BUNDLED WITH section from the file Gemfile.lock, $BUNDLE_GEMFILE.lock or gems.locked.
       Defaults to 'Gemfile.lock' if the file exists and 'latest' otherwise.
     required: false

bundler.js

@@ -65,8 +65,8 @@ export async function installBundler(bundlerVersionInput, lockFile, platform, ru
     bundlerVersion = '2'
   }
 
-  if (/^\d+/.test(bundlerVersion)) {
-    // OK
+  if (/^\d+(?:\.\d+){0,2}$/.test(bundlerVersion)) {
+    // OK - input is a 1, 2, or 3 part version number
   } else {
     throw new Error(`Cannot parse bundler input: ${bundlerVersion}`)
   }
@@ -95,9 +95,12 @@ export async function installBundler(bundlerVersionInput, lockFile, platform, ru
     console.log(`Using Bundler 1 shipped with ${engine}-${rubyVersion}`)
   } else {
     const gem = path.join(rubyPrefix, 'bin', 'gem')
-    const bundlerVersionConstraint = /^\d+\.\d+\.\d+/.test(bundlerVersion) ? bundlerVersion : `~> ${bundlerVersion}`
     // Workaround for https://github.com/rubygems/rubygems/issues/5245
     const force = (platform.startsWith('windows-') && engine === 'ruby' && floatVersion >= 3.1) ? ['--force'] : []
+
+    const versionParts = [...bundlerVersion.matchAll(/\d+/g)].length
+    const bundlerVersionConstraint = versionParts === 3 ? bundlerVersion : `~> ${bundlerVersion}.0`
+
     await exec.exec(gem, ['install', 'bundler', ...force, '-v', bundlerVersionConstraint])
   }