--- a/spec/util_async_spec.lua	Mon Nov 29 14:11:24 2021 +0000
+++ b/spec/util_async_spec.lua	Mon Nov 29 14:14:30 2021 +0000
@@ -669,4 +669,50 @@
 			assert.spy(r.watchers.ready).was.called();
 		end);
 	end);
+
+	describe("#set_nexttick()", function ()
+		after_each(function ()
+			-- Restore to default
+			async.set_nexttick(nil);
+		end);
+		it("should work", function ()
+			local queue = {};
+			local nexttick = spy.new(function (f)
+				assert.is_function(f);
+				table.insert(queue, f);
+			end);
+			async.set_nexttick(nexttick);
+
+			local processed_item;
+			local wait, done;
+			local r = new(function (item)
+				wait, done = async.waiter();
+				wait();
+				processed_item = item;
+			end);
+			r:run("test");
+
+			-- Nothing happened, because the runner is waiting
+			assert.is_nil(processed_item);
+			assert.equal(r.state, "waiting");
+			assert.spy(nexttick).was_called(0);
+			assert.spy(r.watchers.waiting).was.called();
+			assert.spy(r.watchers.ready).was_not.called();
+
+			-- Mark the runner as ready, it should be scheduled for
+			-- the next tick
+			done();
+
+			assert.spy(nexttick).was_called(1);
+			assert.spy(nexttick).was_called_with(match.is_function());
+			assert.equal(1, #queue);
+
+			-- Pretend it's the next tick - call the pending function
+			queue[1]();
+
+			assert.equal(processed_item, "test");
+			assert.equal(r.state, "ready");
+			assert.spy(r.watchers.ready).was.called();
+		end);
+	end);
 end);

--- a/util/async.lua	Mon Nov 29 14:11:24 2021 +0000
+++ b/util/async.lua	Mon Nov 29 14:14:30 2021 +0000
@@ -13,6 +13,9 @@
 
 -- Configurable functions
 local schedule_task = nil; -- schedule_task(seconds, callback)
+local next_tick = function (f)
+	f();
+end
 
 local function runner_from_thread(thread)
 	local level = 0;
@@ -62,8 +65,10 @@
 		-- If state is 'ready', it is our responsibility to update runner.state from 'waiting'.
 		-- We also have to :run(), because the queue might have further items that will not be
 		-- processed otherwise. FIXME: It's probably best to do this in a nexttick (0 timer).
-		runner.state = "ready";
-		runner:run();
+		next_tick(function ()
+			runner.state = "ready";
+			runner:run();
+		end);
 	end
 	return true;
 end
@@ -286,5 +291,6 @@
 	wait_for = wait_for;
 	sleep = sleep;
 
+	set_nexttick = function(new_next_tick) next_tick = new_next_tick; end;
 	set_schedule_function = function (new_schedule_function) schedule_task = new_schedule_function; end;
 };