1 files changed, 72 insertions, 0 deletions
diff --git a/activerecord/test/cases/arel/visitors/dispatch_contamination_test.rb b/activerecord/test/cases/arel/visitors/dispatch_contamination_test.rb
new file mode 100644
index 0000000000..a07a1a050a
--- /dev/null
+++ b/activerecord/test/cases/arel/visitors/dispatch_contamination_test.rb
@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+require_relative "../helper"
+require "concurrent"
+
+module Arel
+  module Visitors
+    class DummyVisitor < Visitor
+      def initialize
+        super
+        @barrier = Concurrent::CyclicBarrier.new(2)
+      end
+
+      def visit_Arel_Visitors_DummySuperNode(node)
+        42
+      end
+
+      # This is terrible, but it's the only way to reliably reproduce
+      # the possible race where two threads attempt to correct the
+      # dispatch hash at the same time.
+      def send(*args)
+        super
+      rescue
+        # Both threads try (and fail) to dispatch to the subclass's name
+        @barrier.wait
+        raise
+      ensure
+        # Then one thread successfully completes (updating the dispatch
+        # table in the process) before the other finishes raising its
+        # exception.
+        Thread.current[:delay].wait if Thread.current[:delay]
+      end
+    end
+
+    class DummySuperNode
+    end
+
+    class DummySubNode < DummySuperNode
+    end
+
+    class DispatchContaminationTest < Arel::Spec
+      before do
+        @connection = Table.engine.connection
+        @table = Table.new(:users)
+      end
+
+      it "dispatches properly after failing upwards" do
+        node = Nodes::Union.new(Nodes::True.new, Nodes::False.new)
+        assert_equal "( TRUE UNION FALSE )", node.to_sql
+
+        node.first # from Nodes::Node's Enumerable mixin
+
+        assert_equal "( TRUE UNION FALSE )", node.to_sql
+      end
+
+      it "is threadsafe when implementing superclass fallback" do
+        visitor = DummyVisitor.new
+        main_thread_finished = Concurrent::Event.new
+
+        racing_thread = Thread.new do
+          Thread.current[:delay] = main_thread_finished
+          visitor.accept DummySubNode.new
+        end
+
+        assert_equal 42, visitor.accept(DummySubNode.new)
+        main_thread_finished.set
+
+        assert_equal 42, racing_thread.value
+      end
+    end
+  end
+end