DEV: Make Ruby services thread-safe

A previous refactor of the `Service::Base::Step` class introduced a
non thread-safe behavior. `#call` mutates instance variables at runtime,
and since a step instance is the same for any given service class, this
can sometimes lead to `context` being the wrong one for the running
service.

This patch makes use of `Concurrent::ThreadLocalVar` to fix the issue.

lib/service/base.rb

@@ -129,16 +129,16 @@ module Service
 
     # @!visibility private
     class Step
-      attr_reader :name, :method_name, :class_name, :instance, :context
+      attr_reader :name, :method_name, :class_name
 
       def initialize(name, method_name = name, class_name: nil)
-        @name = name
-        @method_name = method_name
-        @class_name = class_name
+        @name, @method_name, @class_name = name, method_name, class_name
+        @instance = Concurrent::ThreadLocalVar.new
+        @context = Concurrent::ThreadLocalVar.new
       end
 
       def call(instance, context)
-        @instance, @context = instance, context
+        @instance.value, @context.value = instance, context
         context[result_key] = Context.build
         with_runtime { run_step }
       end
@@ -147,6 +147,10 @@ module Service
         "result.#{type}.#{name}"
       end
 
+      def instance = @instance.value
+
+      def context = @context.value
+
       private
 
       def run_step
@@ -252,6 +256,7 @@ module Service
       attr_reader :steps
 
       def initialize(&block)
+        super("")
         @steps = []
         instance_exec(&block)
       end
@@ -268,8 +273,8 @@ module Service
       attr_reader :steps, :keys
 
       def initialize(*keys, &block)
+        super(keys.join(":"))
         @keys = keys
-        @name = keys.join(":")
         @steps = []
         instance_exec(&block)
       end
@@ -308,7 +313,7 @@ module Service
       attr_reader :steps, :exceptions
 
       def initialize(exceptions, &block)
-        @name = "default"
+        super("default")
         @steps = []
         @exceptions = exceptions.presence || [StandardError]
         instance_exec(&block)

spec/lib/service_spec.rb

@@ -122,4 +122,31 @@ RSpec.describe Service do
       end
     end
   end
+
+  describe "Thread safety" do
+    context "when services are run concurrently" do
+      let(:service) do
+        Class.new do
+          include Service::Base
+
+          policy :limit_reached
+          step :update
+
+          def limit_reached
+            $redis.get("my_counter").to_i < 3
+          end
+
+          def update
+            $redis.incr("my_counter")
+          end
+        end
+      end
+
+      it "works properly" do
+        # When not thread-safe, a failed service can hold the wrong context,
+        # which raises an exception
+        expect { 10.times.map { Thread.new { service.call } }.map(&:join) }.not_to raise_error
+      end
+    end
+  end
 end