Avoid draining pool in gardener cleanup

lib/async/pool/controller.rb

@@ -260,7 +260,13 @@ def start_gardener
 					end
 				ensure
 					@gardener = nil
-					self.close
+
+					# During cancellation, busy resources will never be released,
+					# so force-retire everything instead of draining gracefully.
+					while (resource, _usage = @resources.first)
+						retire(resource)
+					end
+					@available.clear
 				end
 			end
 

test/async/pool/controller.rb

@@ -301,6 +301,26 @@
 
 			expect(events).to be == [:acquire, :close, :release, :closed]
 		end
+
+		it "does not deadlock when Sync scope exits with unreleased resources" do
+			thread = Thread.new do
+				Sync do
+					policy = proc{|pool| pool.prune(0)}
+					pool = subject.new(Async::Pool::Resource, policy: policy)
+
+					# Acquire a resource (starts the gardener) but don't release it
+					pool.acquire
+
+					# Let gardener start and enter its wait loop
+					sleep 0.05
+				end
+			end
+
+			result = thread.join(5)
+			thread.kill if result.nil?
+
+			expect(result).not.to be_nil
+		end
 	end
 
 	with "#to_s" do