util.promise: Support delayed promise execution

2 files changed, 26 insertions, 5 deletions

diff --git a/spec/util_promise_spec.lua b/spec/util_promise_spec.lua
index 67f33416..2fd89d03 100644
--- a/spec/util_promise_spec.lua
+++ b/spec/util_promise_spec.lua
@@ -668,4 +668,18 @@ describe("util.promise", function ()
 			assert.spy(on_rejected).was_called_with(test_error);
 		end);
 	end);
+	describe("set_nexttick()", function ()
+		it("works", function ()
+			local next_tick = spy.new(function (f)
+				f();
+			end)
+			local cb = spy.new();
+			promise.set_nexttick(next_tick);
+			promise.new(function (y, _)
+				y("okay");
+			end):next(cb);
+			assert.spy(next_tick).was.called();
+			assert.spy(cb).was.called_with("okay");
+		end);
+	end)
 end);
diff --git a/util/promise.lua b/util/promise.lua
index ea30898c..c4e166ed 100644
--- a/util/promise.lua
+++ b/util/promise.lua
@@ -78,14 +78,20 @@ local function new_resolve_functions(p)
 	return _resolve, _reject;
 end
 
+local next_tick = function (f)
+	f();
+end
+
 local function new(f)
 	local p = setmetatable({ _state = "pending", _next = next_pending, _pending_on_fulfilled = {}, _pending_on_rejected = {} }, promise_mt);
 	if f then
-		local resolve, reject = new_resolve_functions(p);
-		local ok, ret = xpcall(f, debug.traceback, resolve, reject);
-		if not ok and p._state == "pending" then
-			reject(ret);
-		end
+		next_tick(function()
+			local resolve, reject = new_resolve_functions(p);
+			local ok, ret = xpcall(f, debug.traceback, resolve, reject);
+			if not ok and p._state == "pending" then
+				reject(ret);
+			end
+		end);
 	end
 	return p;
 end
@@ -203,4 +209,5 @@ return {
 	race = race;
 	try = try;
 	is_promise = is_promise;
+	set_nexttick = function(new_next_tick) next_tick = new_next_tick; end;
 }