# HG changeset patch
# User Matthew Wild <mwild1@gmail.com>
# Date 1638195270 0
# Node ID 9a70a543c727a9a6b73fc3441e0a2fb724a8c5fe
# Parent  542a9a503073e414e86f8b7778dad8459183df58
util.async: Add next-tick configuration

Running woken runners in the next iteration of the event loop prevents
unexpected recursion, unexpected tracebacks, and is generally more
predictable.

The pattern is borrowed from util.promise, where we're now doing the same.

diff -r 542a9a503073 -r 9a70a543c727 spec/util_async_spec.lua
--- 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);
diff -r 542a9a503073 -r 9a70a543c727 util/async.lua
--- 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;
 };