move the compaction bug fix in Circuit to a separate module `RubyWi…

…thUnfixedCompaction`

and move logic to the `Circuit` constructor instead of the `Compiler.`
don't include this fix per default, as this is going to be fixed in Ruby 3.2.7 and 3.3.7.

lib/trailblazer/activity/circuit/ruby_with_unfixed_compaction.rb

@@ -0,0 +1,28 @@
+module Trailblazer
+  class Activity
+    # TODO: we can remove this once we drop Ruby <= 3.3.6.
+    class Circuit
+      # This is a hot fix for Ruby versions that haven't fixed the GC compaction bug:
+      #   https://redmine.ruby-lang.org/issues/20853
+      #   https://bugs.ruby-lang.org/issues/20868
+      #
+      # Affected versions might be: 3.1.x, 3.2.?????????, 3.3.0-3.3.6
+      # You don't need this fix in the following versions: 
+      #
+      # If you experience this bug: https://github.com/trailblazer/trailblazer-activity/issues/60
+      #
+      #   NoMethodError: undefined method `[]' for nil
+      #
+      # you need to do 
+      #
+      #   Trailblazer::Activity::Circuit.include(RubyWithUnfixedCompaction)
+      module RubyWithUnfixedCompaction
+        def initialize(wiring, *args, **options)
+          wiring.compare_by_identity
+
+          super(wiring, *args, **options)
+        end
+      end
+    end
+  end
+end

lib/trailblazer/activity/schema/compiler.rb

@@ -24,7 +24,7 @@ def call(intermediate, implementation, config_merge: {})
 
           # From the intermediate "template" and the actual implementation, compile a {Circuit} instance.
           def schema_components(intermediate, implementation, config)
-            wiring = {}.compare_by_identity # https://ruby-doc.org/3.3.0/Hash.html#class-Hash-label-Modifying+an+Active+Hash+Key
+            wiring = {}
             nodes_attributes  = []
 
             intermediate.wiring.each do |task_ref, outs|

test/activity_test.rb

@@ -88,10 +88,12 @@ def call(*)
   end
 end
 
-
-if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("3.2")
+if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new("3.1")
+  # TODO: we can remove this once we drop Ruby <= 3.3.6.
   class GCBugTest < Minitest::Spec
     it "still finds {.method} tasks after GC compression" do
+      ruby_version = Gem::Version.new(RUBY_VERSION)
+
       intermediate = Activity::Schema::Intermediate.new(
         {
           Activity::Schema::Intermediate::TaskRef(:a) => [Activity::Schema::Intermediate::Out(:success, :b)],
@@ -110,21 +112,39 @@ module Step
         :b => Schema::Implementation::Task(Trailblazer::Activity::End.new(semantic: :success), [], []),
       }
 
-
       activity = Activity.new(Activity::Schema::Intermediate::Compiler.(intermediate, implementation))
 
       assert_invoke activity, seq: %([:create])
 
-  # TODO: add tests for different Rubys
-      GC.verify_compaction_references(expand_heap: true, toward: :empty)
+      if ruby_version >= Gem::Version.new("3.1") && ruby_version < Gem::Version.new("3.2")
+        require "trailblazer/activity/circuit/ruby_with_unfixed_compaction"
+        Trailblazer::Activity::Circuit.prepend(Trailblazer::Activity::Circuit::RubyWithUnfixedCompaction)
+      elsif ruby_version >= Gem::Version.new("3.2.0") && ruby_version <= Gem::Version.new("3.2.6")
+        require "trailblazer/activity/circuit/ruby_with_unfixed_compaction"
+        Trailblazer::Activity::Circuit.prepend(Trailblazer::Activity::Circuit::RubyWithUnfixedCompaction)
+      elsif ruby_version >= Gem::Version.new("3.3.0") && ruby_version <= Gem::Version.new("3.3.6")
+        require "trailblazer/activity/circuit/ruby_with_unfixed_compaction"
+        Trailblazer::Activity::Circuit.prepend(Trailblazer::Activity::Circuit::RubyWithUnfixedCompaction)
+      end
 
-  # Without the fix, this *might* throw the following exception:
-  #
-  # NoMethodError: undefined method `[]' for nil
-  #     /home/nick/projects/trailblazer-activity/lib/trailblazer/activity/circuit.rb:80:in `next_for'
+      activity = Activity.new(Activity::Schema::Intermediate::Compiler.(intermediate, implementation))
+
+      ruby_version_specific_options =
+        if ruby_version >= Gem::Version.new("3.2") # FIXME: future
+          {expand_heap: true, toward: :empty}
+        else
+          {}
+        end
+
+      # Provoke the bug:
+      GC.verify_compaction_references(**ruby_version_specific_options)
+
+      # Without the fix, this *might* throw the following exception:
+      #
+      # NoMethodError: undefined method `[]' for nil
+      #     /home/nick/projects/trailblazer-activity/lib/trailblazer/activity/circuit.rb:80:in `next_for'
 
       assert_invoke activity, seq: %([:create])
     end
-
   end
 end