Fix crash caused by RubyGems require gem activation logic running before Bundler can properly register its own monkeypatches

Rakefile

@@ -589,10 +589,13 @@ end
 
 desc "Check RubyGems integration"
 task :check_rubygems_integration do
-  # Bundler monkeypatches RubyGems in some ways that could potentially break gem
-  # activation. Run a non trivial binstub activation, with two different
-  # versions of a dependent gem installed.
+  # Bundler monkeypatches RubyGems in some ways that could potentially break gem activation.
+
+  # Run a non trivial binstub activation, with two different versions of a dependent gem installed.
   sh("ruby -Ilib -S gem install reline:0.3.0 reline:0.3.1 irb && ruby -Ibundler/lib -rbundler -S irb --version")
+
+  # With two psych versions installed, load a gem that depends on pysch and then load rubygems extensions.
+  sh("ruby -Ilib -S gem install psych:5.1.1 psych:5.1.2 rdoc && ruby -Ibundler/lib -rrdoc/task -rbundler/rubygems_ext -e1")
 end
 
 namespace :man do

bundler/lib/bundler/constants.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "rbconfig"
+
 module Bundler
   WINDOWS = RbConfig::CONFIG["host_os"] =~ /(msdos|mswin|djgpp|mingw)/
   FREEBSD = RbConfig::CONFIG["host_os"].to_s.include?("bsd")

bundler/lib/bundler/rubygems_ext.rb

@@ -4,8 +4,6 @@
 
 require "rubygems" unless defined?(Gem)
 
-require "rubygems/specification"
-
 # We can't let `Gem::Source` be autoloaded in the `Gem::Specification#source`
 # redefinition below, so we need to load it upfront. The reason is that if
 # Bundler monkeypatches are loaded before RubyGems activates an executable (for
@@ -17,10 +15,6 @@
 # `Gem::Source` from the redefined `Gem::Specification#source`.
 require "rubygems/source"
 
-require_relative "match_metadata"
-require_relative "force_platform"
-require_relative "match_platform"
-
 # Cherry-pick fixes to `Gem.ruby_version` to be useful for modern Bundler
 # versions and ignore patchlevels
 # (https://github.com/rubygems/rubygems/pull/5472,
@@ -31,7 +25,12 @@
 end
 
 module Gem
+  require "rubygems/specification"
+
   class Specification
+    require_relative "match_metadata"
+    require_relative "match_platform"
+
     include ::Bundler::MatchMetadata
     include ::Bundler::MatchPlatform
 
@@ -148,17 +147,23 @@ def dependencies_to_gemfile(dependencies, group = nil)
 
   module BetterPermissionError
     def data
+      require_relative "shared_helpers"
+
       Bundler::SharedHelpers.filesystem_access(loaded_from, :read) do
         super
       end
     end
   end
 
+  require "rubygems/stub_specification"
+
   class StubSpecification
     prepend BetterPermissionError
   end
 
   class Dependency
+    require_relative "force_platform"
+
     include ::Bundler::ForcePlatform
 
     attr_accessor :source, :groups

bundler/lib/bundler/rubygems_integration.rb

@@ -4,9 +4,7 @@
 
 module Bundler
   class RubygemsIntegration
-    require "monitor"
-
-    EXT_LOCK = Monitor.new
+    autoload :Monitor, "monitor"
 
     def initialize
       @replaced_methods = {}
@@ -173,7 +171,7 @@ def ui=(obj)
     end
 
     def ext_lock
-      EXT_LOCK
+      @ext_lock ||= Monitor.new
     end
 
     def spec_from_gem(path)

bundler/lib/bundler/shared_helpers.rb

@@ -1,15 +1,17 @@
 # frozen_string_literal: true
 
-require "pathname"
-require "rbconfig"
-
 require_relative "version"
-require_relative "constants"
 require_relative "rubygems_integration"
 require_relative "current_ruby"
 
 module Bundler
+  autoload :WINDOWS, File.expand_path("constants", __dir__)
+  autoload :FREEBSD, File.expand_path("constants", __dir__)
+  autoload :NULL, File.expand_path("constants", __dir__)
+
   module SharedHelpers
+    autoload :Pathname, "pathname"
+
     def root
       gemfile = find_gemfile
       raise GemfileNotFound, "Could not locate Gemfile" unless gemfile