util.promise: Remove some redundant checks, add tests confirming redundancy

This lines don't appear to do anything useful, and all tests pass when they
are removed. Discovered via mutation testing.

I added extra tests to exercise this code, because I wasn't certain that there
were no side-effects caused by removal. Everything appears to be fine, thanks
to the "pending" check at the start of promise_settle().

2 files changed, 21 insertions, 4 deletions

diff --git a/spec/util_promise_spec.lua b/spec/util_promise_spec.lua
index 597b56f8..75e9294e 100644
--- a/spec/util_promise_spec.lua
+++ b/spec/util_promise_spec.lua
@@ -30,6 +30,27 @@ describe("util.promise", function ()
 		r("foo");
 		assert.spy(cb).was_called(1);
 	end);
+	it("ignores resolve/reject of settled promises", function ()
+		local res, rej;
+		local p = promise.new(function (resolve, reject)
+			res, rej = resolve, reject;
+		end);
+		local cb = spy.new(function (v)
+			assert.equal("foo", v);
+		end);
+		p:next(cb, cb);
+		assert.spy(cb).was_called(0);
+		res("foo");
+		assert.spy(cb).was_called(1);
+		rej("bar");
+		assert.spy(cb).was_called(1);
+		rej(promise.resolve("bar"));
+		assert.spy(cb).was_called(1);
+		res(promise.reject("bar"));
+		assert.spy(cb).was_called(1);
+		res(promise.resolve("bar"));
+		assert.spy(cb).was_called(1);
+	end);
 	it("allows chaining :next() calls", function ()
 		local r;
 		local result;
diff --git a/util/promise.lua b/util/promise.lua
index 1762d501..d8f0b3d6 100644
--- a/util/promise.lua
+++ b/util/promise.lua
@@ -57,9 +57,7 @@ local function promise_settle(promise, new_state, new_next, cbs, value)
 end
 
 local function new_resolve_functions(p)
-	local resolved = false;
 	local function _resolve(v)
-		if resolved then return; end
 		resolved = true;
 		if is_promise(v) then
 			v:next(new_resolve_functions(p));
@@ -69,8 +67,6 @@ local function new_resolve_functions(p)
 
 	end
 	local function _reject(e)
-		if resolved then return; end
-		resolved = true;
 		if promise_settle(p, "rejected", next_rejected, p._pending_on_rejected, e) then
 			p.reason = e;
 		end