Make bundler re-install conditional on rubygems >= 4.0

The unconditional re-install caused the integration cache test to fail:
on first deploy, the second InstallBundler() call (from UpdateRubygems)
produced a 'Copy' line since libbuildpack had already cached the tarball
from the initial Download, violating the cache test assertion that no
Copy lines appear on first deploy.

Fix: only re-install bundler when rubygems major version >= 4, since
only rubygems 4.x ships bundler 4.x as a default gem. Rubygems 3.x
ships bundler 2.x which doesn't overwrite the buildpack's bundler.

Author: tnikolova82

src/ruby/supply/supply.go

@@ -9,6 +9,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
+	"strconv"
 	"strings"
 
 	"github.com/cloudfoundry/libbuildpack"
@@ -614,9 +615,11 @@ func (s *Supplier) UpdateRubygems() error {
 	// overwrites the buildpack-installed bundler (2.x) with bundler 4.x,
 	// which has incompatible output format changes and untested behavior.
 	// Re-install the buildpack's bundler to restore the manifest version.
-	s.Log.Debug("Re-installing bundler after rubygems update")
-	if err := s.InstallBundler(); err != nil {
-		return fmt.Errorf("Could not re-install bundler after rubygems update: %v", err)
+	if majorVersion, err := strconv.Atoi(strings.SplitN(dep.Version, ".", 2)[0]); err == nil && majorVersion >= 4 {
+		s.Log.Debug("Re-installing bundler after rubygems %s update", dep.Version)
+		if err := s.InstallBundler(); err != nil {
+			return fmt.Errorf("Could not re-install bundler after rubygems update: %v", err)
+		}
 	}
 
 	return nil

src/ruby/supply/supply_test.go

@@ -1184,16 +1184,6 @@ var _ = Describe("Supply", func() {
 				})
 				mockCommand.EXPECT().Output(gomock.Any(), "ruby", "setup.rb")
 
-				// After ruby setup.rb, UpdateRubygems re-installs bundler.
-				// InstallBundler reads Gemfile.lock (not present here, so defaults
-				// to 2.x.x constraint) and installs bundler from the manifest.
-				mockInstaller.EXPECT().InstallDependency(gomock.Any(), gomock.Any()).Do(func(dep libbuildpack.Dependency, installDir string) {
-					Expect(dep.Name).To(Equal("bundler"))
-					Expect(dep.Version).To(Equal("2.7.2"))
-					// Create bin dir so LinkDirectoryInDepDir succeeds
-					Expect(os.MkdirAll(filepath.Join(installDir, "bin"), 0755)).To(Succeed())
-				})
-
 				Expect(supplier.UpdateRubygems()).To(Succeed())
 			})
 
@@ -1226,6 +1216,39 @@ var _ = Describe("Supply", func() {
 		})
 	})
 
+	Describe("UpdateRubygems with rubygems >= 4 re-installs bundler", func() {
+		BeforeEach(func() {
+			mockManifest.EXPECT().AllDependencyVersions("rubygems").AnyTimes().Return([]string{"4.0.9"})
+		})
+		Context("gem version is less than 4.0.9", func() {
+			BeforeEach(func() {
+				mockCommand.EXPECT().Output(gomock.Any(), "gem", "--version").AnyTimes().Return("3.4.19\n", nil)
+				mockVersions.EXPECT().VersionConstraint("3.4.19", ">= 4.0.9").AnyTimes().Return(false, nil)
+			})
+
+			It("updates rubygems and re-installs bundler", func() {
+				mockVersions.EXPECT().Engine().Return("ruby", nil)
+				mockInstaller.EXPECT().InstallDependency(gomock.Any(), gomock.Any()).Do(func(dep libbuildpack.Dependency, _ string) {
+					Expect(dep.Name).To(Equal("rubygems"))
+					Expect(dep.Version).To(Equal("4.0.9"))
+				})
+				mockCommand.EXPECT().Output(gomock.Any(), "ruby", "setup.rb")
+
+				// Rubygems >= 4 triggers bundler re-install.
+				// InstallBundler reads Gemfile.lock (not present here, so defaults
+				// to 2.x.x constraint) and installs bundler from the manifest.
+				mockInstaller.EXPECT().InstallDependency(gomock.Any(), gomock.Any()).Do(func(dep libbuildpack.Dependency, installDir string) {
+					Expect(dep.Name).To(Equal("bundler"))
+					Expect(dep.Version).To(Equal("2.7.2"))
+					// Create bin dir so LinkDirectoryInDepDir succeeds
+					Expect(os.MkdirAll(filepath.Join(installDir, "bin"), 0755)).To(Succeed())
+				})
+
+				Expect(supplier.UpdateRubygems()).To(Succeed())
+			})
+		})
+	})
+
 	Describe("RewriteShebangs", func() {
 		var depDir string
 		BeforeEach(func() {