fix recursive queue flush

src/batch.luau

@@ -6,17 +6,18 @@ local graph = require(script.Parent.graph)
 
 local function batch(setter: () -> ())
     local already_batching = flags.batch
+    local flush
 
-    flags.batch = true
+    if not already_batching then
+        flags.batch = true
+        flush = graph.flush_update_queue()
+    end
 
     local ok, err: string? = pcall(setter)
 
     if not already_batching then
         flags.batch = false
-        
-        if not already_batching then
-            graph.flush_update_queue()
-        end
+        flush()
     end
 
     if not ok then throw(`error occured while batching updates: {err}`) end

src/graph.luau

@@ -186,27 +186,29 @@ end
 
 local _flushing = false
 local function flush_update_queue()
-    assert(not _flushing, "recursive queue flush occured") -- todo
-    _flushing = true
+    local n0 = update_queue.n
 
-    local n0 = 0
+    return function()
+        assert(not _flushing, "recursive queue flush occured") -- todo
+        _flushing = true
 
-    local i = n0 + 1
-    while i <= update_queue.n do
-        local node = update_queue[i]
-        --assert(node.effect)
+        local i = n0 + 1
+        while i <= update_queue.n do
+            local node = update_queue[i]
+            --assert(node.effect)
 
-        if node.owner and evaluate_node(node) then
-            queue_children_for_update(node)
+            if node.owner and evaluate_node(node) then
+                queue_children_for_update(node)
+            end
+
+            update_queue[i] = false :: any
+            i += 1
         end
 
-        update_queue[i] = false :: any
-        i += 1
+        update_queue.n = n0
+
+        _flushing = false
     end
-
-    update_queue.n = n0
-
-    _flushing = false
 end
 
 local function update_descendants<T>(root: SourceNode<T>)

test/tests.luau

@@ -1920,6 +1920,34 @@ TEST("batch()", wrap_root(function()
         CHECK(b2() == 3)
         CHECK(b3() == 4)
     end
+
+    do CASE "recursive queue flush"
+
+        local a0 = source(0)
+        local a1 = source(1)
+
+        derive(function()
+            a0(a1() + 1)
+        end)
+
+        derive(function()
+            print("update")
+
+            a0()
+            
+            batch(function()
+            
+            end)
+
+            return 1
+        end)
+
+        a1(2)
+
+        -- if it didnt error, all is fine!
+        CHECK(true)
+
+    end
 end))
 
 TEST("read()", wrap_root(function()