[rubygems/rubygems] Fix missing required rubygems version when using old APIs

A while ago, we fixed resolution when using old dependency endpoints to
also consider metadata dependencies, by requesting the full gemspec from
the marsahaled index, which includes this information as opposed to
these old APIs. This has made resolution slower, but correct, but also
introduced the issue that some old marshaled gemspecs don't include the
`required_rubygems_version` field because they were created with a
RubyGems version that predates its addition.

Use a default value in this case.

https://github.com/rubygems/rubygems/commit/5dc94afcc0

Co-authored-by: Ilya Dudarenko <i.dudarenko@tinkoff.ru>

lib/bundler/endpoint_specification.rb

@@ -26,8 +26,11 @@ module Bundler
       @required_ruby_version ||= _remote_specification.required_ruby_version
     end
 
+    # A fallback is included because the original version of the specification
+    # API didn't include that field, so some marshalled specs in the index have it
+    # set to +nil+.
     def required_rubygems_version
-      @required_rubygems_version ||= _remote_specification.required_rubygems_version
+      @required_rubygems_version ||= _remote_specification.required_rubygems_version || Gem::Requirement.default
     end
 
     def fetch_platform

spec/bundler/install/gemfile/sources_spec.rb

@@ -1496,4 +1496,45 @@ RSpec.describe "bundle install with gems on multiple sources" do
       L
     end
   end
+
+  context "when default source uses the old API and includes old gems with nil required_rubygems_version" do
+    before do
+      build_repo4 do
+        build_gem "pdf-writer", "1.1.8"
+      end
+
+      path = "#{gem_repo4}/#{Gem::MARSHAL_SPEC_DIR}/pdf-writer-1.1.8.gemspec.rz"
+      spec = Marshal.load(Bundler.rubygems.inflate(File.binread(path)))
+      spec.instance_variable_set(:@required_rubygems_version, nil)
+      File.open(path, "wb") do |f|
+        f.write Gem.deflate(Marshal.dump(spec))
+      end
+
+      gemfile <<~G
+        source "https://localgemserver.test"
+
+        gem "pdf-writer", "= 1.1.8"
+      G
+    end
+
+    it "handles that fine" do
+      bundle "install --verbose", :artifice => "endpoint", :env => { "BUNDLER_SPEC_GEM_REPO" => gem_repo4.to_s }
+
+      expect(lockfile).to eq <<~L
+        GEM
+          remote: https://localgemserver.test/
+          specs:
+            pdf-writer (1.1.8)
+
+        PLATFORMS
+          #{specific_local_platform}
+
+        DEPENDENCIES
+          pdf-writer (= 1.1.8)
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      L
+    end
+  end
 end