Prevent loading both json/ext and json/pure

Fix: #650
Ref: #646

`json_pure` currently doesn't work well at all because `json/ext` is
a default gem, so if you have code requiring `json/pure` and some other
code requiring `json`, you end up with both loaded.

If the `json` and `json_pure` versions match, it's not too bad,
but if they don't match, you might run into issues with private
APIs no longer matching.

Rakefile

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 begin
   require 'rubygems/package_task'
 rescue LoadError

json.gemspec

@@ -45,7 +45,11 @@ spec = Gem::Specification.new do |s|
     "LEGAL",
     "README.md",
     "json.gemspec",
-    *Dir["lib/**/*.rb"],
+    *(
+      Dir["lib/**/*.rb"] -
+      # We keep lib/json/pure/*.rb on purpose for TruffleRuby, but not the entry points.
+      Dir["lib/json/pure.rb"] - Dir["lib/json_pure.rb"]
+    ),
   ]
 
   if java_ext

json_pure.gemspec

@@ -16,36 +16,27 @@ Gem::Specification.new do |s|
 
   s.extra_rdoc_files = ["README.md"]
   s.rdoc_options = ["--title", "JSON implementation for ruby", "--main", "README.md"]
+
+  s.files = [
+    "CHANGES.md",
+    "COPYING",
+    "BSDL",
+    "LEGAL",
+    "README.md",
+    "json.gemspec",
+    *(Dir["lib/**/*.rb"] - Dir["lib/json/ext/**/*.rb"]),
+  ]
+
   s.files = [
     "CHANGES.md",
     "COPYING",
     "BSDL",
     "LEGAL",
     "README.md",
     "json_pure.gemspec",
-    "lib/json.rb",
-    "lib/json/add/bigdecimal.rb",
-    "lib/json/add/complex.rb",
-    "lib/json/add/core.rb",
-    "lib/json/add/date.rb",
-    "lib/json/add/date_time.rb",
-    "lib/json/add/exception.rb",
-    "lib/json/add/ostruct.rb",
-    "lib/json/add/range.rb",
-    "lib/json/add/rational.rb",
-    "lib/json/add/regexp.rb",
-    "lib/json/add/set.rb",
-    "lib/json/add/struct.rb",
-    "lib/json/add/symbol.rb",
-    "lib/json/add/time.rb",
-    "lib/json/common.rb",
-    "lib/json/ext.rb",
-    "lib/json/generic_object.rb",
-    "lib/json/pure.rb",
-    "lib/json/pure/generator.rb",
-    "lib/json/pure/parser.rb",
-    "lib/json/version.rb",
+    *Dir["lib/**/*.rb"],
   ]
+
   s.homepage = "https://ruby.github.io/json"
   s.metadata = {
     'bug_tracker_uri'   => 'https://github.com/ruby/json/issues',

lib/json/common.rb

@@ -1,4 +1,5 @@
 #frozen_string_literal: true
+
 require 'json/version'
 
 module JSON

lib/json/ext.rb

@@ -2,24 +2,25 @@
 
 require 'json/common'
 
-module JSON
-  # This module holds all the modules/classes that implement JSON's
-  # functionality as C extensions.
-  module Ext
-    if RUBY_ENGINE == 'truffleruby'
-      require 'json/ext/parser'
-      require 'json/pure'
-      $DEBUG and warn "Using Ext extension for JSON parser and Pure library for JSON generator."
-      JSON.parser = Parser
-      JSON.generator = JSON::Pure::Generator
-    else
-      require 'json/ext/parser'
-      require 'json/ext/generator'
-      $DEBUG and warn "Using Ext extension for JSON."
-      JSON.parser = Parser
-      JSON.generator = Generator
-    end
+unless defined?(::JSON::JSON_LOADED)
+  $LOADED_FEATURES << File.expand_path("../pure.rb", __FILE__)
+
+  if RUBY_ENGINE == 'truffleruby'
+    require 'json/ext/parser'
+    # We use require_relative to make sure we're loading the same version.
+    # Otherwise if the Gemfile include conflicting versions of `json` and `json_pure`
+    # it may break.
+    require_relative 'pure/generator'
+    $DEBUG and warn "Using Ext extension for JSON parser and Pure library for JSON generator."
+    JSON.parser = JSON::Ext::Parser
+    JSON.generator = JSON::Pure::Generator
+  else
+    require 'json/ext/parser'
+    require 'json/ext/generator'
+    $DEBUG and warn "Using Ext extension for JSON."
+    JSON.parser = JSON::Ext::Parser
+    JSON.generator = JSON::Ext::Generator
   end
 
-  JSON_LOADED = true unless defined?(::JSON::JSON_LOADED)
+  ::JSON::JSON_LOADED = true
 end

lib/json/generic_object.rb

@@ -1,4 +1,5 @@
 # frozen_string_literal: true
+
 begin
   require 'ostruct'
 rescue LoadError

lib/json/pure.rb

@@ -1,16 +1,14 @@
 # frozen_string_literal: true
+
 require 'json/common'
 
-module JSON
-  # This module holds all the modules/classes that implement JSON's
-  # functionality in pure ruby.
-  module Pure
-    require 'json/pure/parser'
-    require 'json/pure/generator'
-    $DEBUG and warn "Using Pure library for JSON."
-    JSON.parser = Parser
-    JSON.generator = Generator
-  end
+unless defined?(::JSON::JSON_LOADED)
+  require 'json/pure/parser'
+  require 'json/pure/generator'
+  $LOADED_FEATURES << File.expand_path("../ext.rb", __FILE__)
+  $DEBUG and warn "Using Pure library for JSON."
+  JSON.parser = JSON::Pure::Parser
+  JSON.generator = JSON::Pure::Generator
 
-  JSON_LOADED = true unless defined?(::JSON::JSON_LOADED)
+  JSON::JSON_LOADED = true
 end

lib/json_pure.rb

@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+require 'json/pure'

test/json/test_helper.rb

@@ -1,23 +1,39 @@
+# frozen_string_literal: true
+
 case ENV['JSON']
 when 'pure'
   $LOAD_PATH.unshift(File.expand_path('../../../lib', __FILE__))
-  $stderr.puts("Testing JSON::Pure")
   require 'json/pure'
+  begin
+    require 'json/ext'
+  rescue LoadError
+    # Ensure `json/ext` can't be loaded after `json/pure`
+  end
+  if [JSON.generator, JSON.parser].map(&:name) != ["JSON::Pure::Generator", "JSON::Pure::Parser"]
+    abort "Expected JSON::Pure to be loaded, got: #{[JSON.generator, JSON.parser]}"
+  end
 when 'ext'
-  $stderr.puts("Testing JSON::Ext")
   $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
   require 'json/ext'
+  require 'json/pure'
+
+  expected = if RUBY_ENGINE == 'truffleruby'
+    ["JSON::Pure::Generator", "JSON::Ext::Parser"]
+  else
+    ["JSON::Ext::Generator", "JSON::Ext::Parser"]
+  end
+
+  if [JSON.generator, JSON.parser].map(&:name) != expected
+    abort "Expected JSON::Ext to be loaded, got: #{[JSON.generator, JSON.parser]}"
+  end
 else
   $LOAD_PATH.unshift(File.expand_path('../../../ext', __FILE__), File.expand_path('../../../lib', __FILE__))
-  $stderr.puts("Testing JSON")
   require 'json'
 end
 
+$stderr.puts("Testing #{JSON.generator} and #{JSON.parser}")
+
 require 'test/unit'
-begin
-  require 'byebug'
-rescue LoadError
-end
 
 if GC.respond_to?(:verify_compaction_references)
   # This method was added in Ruby 3.0.0. Calling it this way asks the GC to