Keep controller start idempotent

Assisted-By: devx/3236e566-7538-432e-a30a-2bdf37265ed4

Author: samuel-williams-shopify

lib/async/container/controller.rb

@@ -181,16 +181,21 @@ def stop(graceful = @graceful_stop)
 			# This is equivalent to a blue-green deployment.
 			def start(restart: false)
 				old_container = nil
+				new_container = nil
 
 				@guard.synchronize do
-					if @container && restart
-						@notify&.restarting!
-						
-						Console.info(self, "Restarting container...")
+					if @container
+						if restart
+							@notify&.restarting!
+							
+							Console.info(self, "Restarting container...")
+						else
+							return @container
+						end
 					else
 						Console.info(self, "Starting container...")
 					end
-					
+						
 					container = self.create_container
 
 					begin
@@ -217,6 +222,7 @@ def start(restart: false)
 					# The following swap should be atomic:
 					old_container = @container
 					@container = container
+					new_container = container
 					container = nil
 
 					@notify&.ready!(size: @container.size, status: "Running with #{@container.size} children.")
@@ -234,6 +240,8 @@ def start(restart: false)
 					Console.info(self, "Stopping old container...")
 					old_container.stop(@graceful_stop)
 				end
+				
+				return new_container
 			end
 
 			# Reload the existing container. Children instances will be reloaded using `SIGHUP`.

test/async/container/controller.rb

@@ -155,6 +155,27 @@ def controller.setup(container)
 			controller.stop
 		end
 
+		it "does not restart an already running container" do
+			count = 0
+			
+			controller.define_singleton_method(:setup) do |container|
+				count += 1
+				
+				container.spawn do |instance|
+					instance.ready!
+					sleep
+				end
+			end
+			
+			first = controller.start
+			second = controller.start
+			
+			expect(second).to be == first
+			expect(count).to be == 1
+			
+			controller.stop(false)
+		end
+		
 		it "propagates exceptions" do
 			def controller.setup(container)
 				raise "Boom!"