Make Socket#transmit and #close idempotent.

lib/async/cable/socket.rb

@@ -65,12 +65,16 @@ def run(parent: Async::Task.current)
 			# @parameter data [Object] The data to transmit, which will be encoded by the coder.
 			def transmit(data)
 				# Console.info(self, "Transmitting data:", data, task: Async::Task.current?)
+				return if @output.closed?
 				@output.push(@coder.encode(data))
+			rescue ClosedQueueError
+				# Closed concurrently between the check and the push.
 			end
 
 			# Close the outbound queue, causing the drain task to terminate once all pending messages have been sent.
 			def close
-				@output.close
+				# Console.info(self, "Closing socket.", task: Async::Task.current?)
+				@output.close unless @output.closed?
 			end
 
 			# This can be called from the work pool, off the event loop.

test/async/cable/socket.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require "async/cable/socket"
+require "action_cable"
+
+describe Async::Cable::Socket do
+	let(:socket) {subject.new({}, nil, ::ActionCable::Server::Base.new)}
+
+	it "transmit returns nil after close" do
+		socket.close
+		expect(socket.transmit({type: "ping"})).to be_nil
+	end
+
+	it "close is idempotent" do
+		socket.close
+		socket.close
+	end
+end