fix(context): do not modify stack array directly when attach and detach

api/lib/opentelemetry/context.rb

@@ -41,8 +41,9 @@ def current
       # @return [Object] A token to be used when detaching
       def attach(context)
         s = stack
-        s.push(context)
-        s.size
+        new_stack = s + [context]
+        Thread.current[STACK_KEY] = new_stack
+        new_stack.size
       end
 
       # Restores the previous Context associated with the current Fiber.
@@ -57,7 +58,7 @@ def detach(token)
         calls_matched = (token == s.size)
         OpenTelemetry.handle_error(exception: DetachError.new('calls to detach should match corresponding calls to attach.')) unless calls_matched
 
-        s.pop
+        Thread.current[STACK_KEY] = s[...-1] || []
         calls_matched
       end
 

api/test/opentelemetry/context_test.rb

@@ -331,5 +331,60 @@
         _(Context.current[bar_key]).must_be_nil
       end
     end
+
+    describe 'when current thread local variable get copied to new thread' do
+      # NOTE: this is the similar behavior of ActionController::Live as identified in open-telemetry/opentelemetry-ruby-contrib#772
+      it 'attach and detach in child thread will not affect parent thread' do
+        OpenTelemetry::TestHelpers.with_test_logger do |log_stream|
+          ctx = new_context
+
+          t1 = Thread.current
+          locals = t1.keys.map { |key| [key, t1[key]] }
+
+          done_attach_new_context = false
+
+          token1 = Context.attach(ctx)
+          t = Thread.new do
+            t2 = Thread.current
+            locals.each { |k, v| t2[k] = v }
+
+            Context.attach(ctx)
+            done_attach_new_context = true
+          end
+
+          while !done_attach_new_context; end
+
+          Context.detach(token1)
+
+          _(log_stream.string).wont_match(/OpenTelemetry error: calls to detach should match corresponding calls to attach/)
+        end
+      end
+
+      it 'clear in child thread will not affect parent thread' do
+        OpenTelemetry::TestHelpers.with_test_logger do |log_stream|
+          ctx = new_context
+
+          t1 = Thread.current
+          locals = t1.keys.map { |key| [key, t1[key]] }
+
+          done_clear_context = false
+
+          token1 = Context.attach(ctx)
+          t = Thread.new do
+            t2 = Thread.current
+            locals.each { |k, v| t2[k] = v }
+
+            Context.clear
+            done_clear_context = true
+          end
+
+          while !done_clear_context; end
+
+          Context.detach(token1)
+
+          _(log_stream.string).wont_match(/OpenTelemetry error: calls to detach should match corresponding calls to attach/)
+        end
+      end
+    end
   end
 end