util.async: Make parameters to async.runner() optional

spec/util_async_spec.lua

@@ -94,6 +94,26 @@ describe("util.async", function()
 			assert.equal(last_item, values[#values]);
 		end);
 
+		it("should work with no parameters", function ()
+			local item = "fail";
+			local r = async.runner();
+			local f = spy.new(function () item = "success"; end);
+			r:run(f);
+			assert.spy(f).was.called();
+			assert.equal(item, "success");
+		end);
+
+		it("supports a default error handler", function ()
+			local item = "fail";
+			local r = async.runner();
+			local f = spy.new(function () error("test error"); end);
+			assert.error_matches(function ()
+				r:run(f);
+			end, "test error");
+			assert.spy(f).was.called();
+			assert.equal(item, "fail");
+		end);
+
 		describe("#errors", function ()
 			describe("should notify", function ()
 				local last_processed_item, last_error;

util/async.lua

@@ -130,10 +130,14 @@ local function runner_create_thread(func, self)
 	return thread;
 end
 
-local empty_watchers = {};
+local function default_error_watcher(runner, err)
+	runner:log("error", "Encountered error: %s", err);
+	error(err);
+end
+local function default_func(f) f(); end
 local function runner(func, watchers, data)
-	return setmetatable({ func = func, thread = false, state = "ready", notified_state = "ready",
-		queue = {}, watchers = watchers or empty_watchers, data = data, id = new_id() }
+	return setmetatable({ func = func or default_func, thread = false, state = "ready", notified_state = "ready",
+		queue = {}, watchers = watchers or { error = default_error_watcher }, data = data, id = new_id() }
 	, runner_mt);
 end