util.promise: Add join() convenience method

Usage: promise.join(p1, p2, function (result1, result2)
  [...]
end)

spec/util_promise_spec.lua

@@ -459,6 +459,32 @@ describe("util.promise", function ()
 			assert.same({ foo = true }, result);
 		end);
 	end);
+	describe("join()", function ()
+		it("works", function ()
+			local r1, r2;
+			local res1, res2;
+			local p1, p2 = promise.new(function (resolve) r1 = resolve end), promise.new(function (resolve) r2 = resolve end);
+
+			local p = promise.join(p1, p2, function (_res1, _res2)
+				res1, res2 = _res1, _res2;
+				return promise.resolve("works");
+			end);
+
+			local result;
+			local cb = spy.new(function (v)
+				result = v;
+			end);
+			p:next(cb);
+			assert.spy(cb).was_called(0);
+			r2("yep");
+			assert.spy(cb).was_called(0);
+			r1("nope");
+			assert.spy(cb).was_called(1);
+			assert.same("works", result);
+			assert.equals("nope", res1);
+			assert.equals("yep", res2);
+		end);
+	end);
 	it("promises may be resolved by other promises", function ()
 		local r1, r2;
 		local p1, p2 = promise.new(function (resolve) r1 = resolve end), promise.new(function (resolve) r2 = resolve end);

util/promise.lua

@@ -2,6 +2,7 @@ local promise_methods = {};
 local promise_mt = { __name = "promise", __index = promise_methods };
 
 local xpcall = require "util.xpcall".xpcall;
+local unpack = table.unpack or unpack; --luacheck: ignore 113
 
 function promise_mt:__tostring()
 	return  "promise (" .. (self._state or "invalid") .. ")";
@@ -137,6 +138,15 @@ local function all_settled(promises)
 	end);
 end
 
+local function join(...)
+	local promises, n = { ... }, select("#", ...);
+	local handler = promises[n];
+	promises[n] = nil;
+	return all(promises):next(function (results)
+		return handler(unpack(results, 1, n - 1));
+	end);
+end
+
 local function race(promises)
 	return new(function (resolve, reject)
 		for i = 1, #promises do
@@ -180,6 +190,7 @@ end
 return {
 	new = new;
 	resolve = resolve;
+	join = join;
 	reject = reject;
 	all = all;
 	all_settled = all_settled;