Fall back to lockfile version when BUNDLE_VERSION is "lockfile"

Bundler treats "lockfile" as a special value for the version setting,
meaning "use the version recorded in BUNDLED WITH". BundlerVersionFinder
was passing the raw string to Gem::Version.new and raising ArgumentError
both from the env var added in #9538 and from .bundle/config.

Author: hsbt

lib/rubygems/bundler_version_finder.rb

@@ -2,15 +2,16 @@
 
 module Gem::BundlerVersionFinder
   def self.bundler_version
-    return if bundle_config_version == "system"
+    bcv = bundle_config_version
+    return if bcv == "system"
 
     v = ENV["BUNDLER_VERSION"]
     v = nil if v&.empty?
 
     v ||= bundle_update_bundler_version
     return if v == true
 
-    v ||= bundle_config_version
+    v ||= bcv unless bcv == "lockfile"
 
     v ||= lockfile_version
     return unless v

test/rubygems/test_gem_bundler_version_finder.rb

@@ -146,6 +146,31 @@ def test_bundler_version_with_empty_bundle_version_env
     end
   end
 
+  def test_bundler_version_with_bundle_version_env_lockfile
+    ENV["BUNDLE_VERSION"] = "lockfile"
+
+    bvf.stub(:lockfile_contents, "\n\nBUNDLED WITH\n   1.1.1.1\n") do
+      assert_equal v("1.1.1.1"), bvf.bundler_version
+    end
+  end
+
+  def test_bundler_version_with_bundle_config_version_lockfile
+    config_content = <<~CONFIG
+      BUNDLE_VERSION: "lockfile"
+    CONFIG
+
+    Tempfile.create("bundle_config") do |f|
+      f.write(config_content)
+      f.flush
+
+      bvf.stub(:bundler_global_config_file, f.path) do
+        bvf.stub(:lockfile_contents, "\n\nBUNDLED WITH\n   1.1.1.1\n") do
+          assert_equal v("1.1.1.1"), bvf.bundler_version
+        end
+      end
+    end
+  end
+
   def test_bundler_version_with_bundle_config_non_existent_file
     bvf.stub(:bundler_global_config_file, "/non/existent/path") do
       assert_nil bvf.bundler_version