Add duplicate update test

src/graph.luau

@@ -175,7 +175,7 @@ local function evaluate_node<T>(node: Node<T>, continue_on_error: boolean) -- if
             if not ok then
                 local msg = debug.traceback(`effect error: {new_value}`, 2)
                 if continue_on_error then
-                    task.spawn(error, msg, 0)
+                    print(msg)
                     return false
                 else
                     error(msg, 2)
@@ -199,7 +199,7 @@ local function evaluate_node<T>(node: Node<T>, continue_on_error: boolean) -- if
         if not ok then
             local msg = debug.traceback(`effect error: {new_value}`, 2)
             if continue_on_error then
-                task.spawn(error, msg, 0)
+                print(msg)
                 return false
             else
                 error(msg, 2)
@@ -307,7 +307,7 @@ local function flush_update_queue(from: number)
             update_queue_n = moveTo1
             if not changed then -- Can occur in recursive updates (where 'from' is > 0)
                 if from == 0 then
-                    warn("Some nodes failed to update", update_queue, node_deps_left, debug.traceback())
+                    print("Some nodes failed to update", update_queue, node_deps_left, debug.traceback())
                     reset_queue()
                 end
                 return

test/tests.luau

@@ -319,6 +319,102 @@ TEST("graph", function()
     end
 end)
 
+TEST("duplicate update", function()
+    local cases = {
+        {
+            name = "b",
+            err = "b",
+            e1 = {1, 2, 2, 2, 2},
+        },
+        {
+            name = "c",
+            err = "c",
+            e1 = {2, 1, 1, 2, 2},
+        },
+        {
+            name = "c2",
+            err = "c2",
+            e1 = {2, 2, 1, 2, 2},
+        },
+        {
+            name = "d",
+            err = "d",
+            e1 = {2, 2, 2, 1, 2},
+        },
+        {
+            name = "e",
+            err = "e",
+            e1 = {2, 2, 2, 2, 1},
+            e2 = {3, 3, 3, 3, 3}, -- e has a dependency on just 'a' after the first time, so it'll get updated 2x (only way to prevent this extra update is to yield, which isn't currently supported)
+        },
+        {
+            name = "no error",
+            err = "",
+            e1 = {2, 2, 2, 2, 2},
+        }
+    }
+    for _, case in cases do
+        CASE(case.name)
+        local destroy = root(function()
+            local e1, e2, err = case.e1, case.e2, case.err
+            if not e2 then
+                e2 = {}
+                for i, v in e1 do
+                    e2[i] = v + 1
+                end
+            end
+            local a = source(0)
+            local nb = 0
+            local b = derive(function()
+                if err == "b" and a() == 1 then error("user error") end
+                nb += 1
+                return a() + 1
+            end)
+            local nc = 0
+            local c = derive(function()
+                if err == "c" and a() == 1 then error("user error") end
+                nc += 1
+                return a() * 2
+            end)
+            local nc2 = 0
+            local c2 = derive(function()
+                if err == "c2" and c() == 2 then error("user error") end
+                nc2 += 1
+                return c()
+            end)
+            local nd = 0
+            local d = derive(function()
+                if err == "d" and b() == 2 then error("user error") end
+                nd += 1
+                return b() * 100 + c2()
+            end)
+            local ne = 0
+            effect(function()
+                if err == "e" and a() == 1 then error("user error") end
+                ne += 1
+                a(); b(); c(); c2(); d()
+            end)
+            local length = graph.get_update_queue_length()
+
+            a(1)
+            CHECK(nb == e1[1])
+            CHECK(nc == e1[2])
+            CHECK(nc2 == e1[3])
+            CHECK(nd == e1[4])
+            CHECK(ne == e1[5])
+            CHECK(graph.get_update_queue_length() == length)
+
+            a(2)
+            CHECK(nb == e2[1])
+            CHECK(nc == e2[2])
+            CHECK(nc2 == e2[3])
+            CHECK(nd == e2[4])
+            CHECK(ne == e2[5])
+        end)
+        destroy()
+    end
+end)
+
 TEST("mount()", function()
     local screen = create "ScreenGui" {}