fix: Use a hybrid strategy to auto-requiring appmap modules

First check if the corresponding framework library is defined. If not, use TracePoint
to watch the code getting loaded, and
require the appmap module when its
corresponding framework library is loaded.

Author: kgilpin

lib/appmap.rb

@@ -112,19 +112,57 @@ def detect_metadata
   end
 end
 
-if Gem.loaded_specs['rails']
-  require 'active_support'
-  require 'active_support/core_ext'
-  require 'rails'
-  require 'appmap/railtie' 
-end
+lambda do
+  Initializer = Struct.new(:class_name, :module_name, :gem_module_name)
+
+  INITIALIZERS = {
+    'Rails::Railtie' => Initializer.new('AppMap::Railtie', 'appmap/railtie', 'railtie'),
+    'RSpec' => Initializer.new('AppMap::RSpec', 'appmap/rspec', 'rspec-core'),
+    'Minitest::Unit::TestCase' => Initializer.new('AppMap::Minitest', 'appmap/minitest', 'minitest')
+  }
+
+  print_msg = if defined?(::Rails) && Rails.logger
+    Rails.logger
+  elsif ENV['DEBUG'] == 'true'
+    Object.method(:warn)
+  else
+    ->(msg) {}
+  end
 
-if Gem.loaded_specs['rspec-core']
-  require 'appmap/rspec'
-end
+  TracePoint.new(:class) do |tp|
+    cls_name = tp.self.name
+    initializers = INITIALIZERS.delete(cls_name)
+    if initializers
+      initializers = [ initializers ] unless initializers.is_a?(Array)
+      next if Object.const_defined?(initializers.first.class_name)
+
+      gem_module_name = initializers.first.gem_module_name
+
+      print_msg.call AppMap::Util.color(<<~LOAD_MSG, :magenta)
+      When 'appmap' was loaded, '#{gem_module_name}' had not been loaded yet. Now '#{gem_module_name}' has
+      just been loaded, so the following AppMap modules will be automatically required:
+
+      #{initializers.map(&:module_name).join("\n")}
+
+      To suppress this message, ensure '#{gem_module_name}' appears before 'appmap' in your Gemfile.
+      LOAD_MSG
+      initializers.each do |init|
+        require init.module_name
+      end
+    end
+  end.enable
+
+  if defined?(::Rails::Railtie)
+    require 'appmap/railtie'
+  end
 
-if Gem.loaded_specs['minitest']
-  require 'appmap/minitest'
-end
+  if defined?(::RSpec)
+    require 'appmap/rspec'
+  end
+  
+  if defined?(::Minitest)
+    require 'appmap/minitest'
+  end
 
-AppMap.initialize_configuration if ENV['APPMAP'] == 'true'
+  AppMap.initialize_configuration
+end.call if ENV['APPMAP'] == 'true'