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
 
+    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

@@ -186,11 +186,12 @@ end
 
 local _flushing = false
 local function flush_update_queue()
+    local n0 = update_queue.n
+
+    return function()
         assert(not _flushing, "recursive queue flush occured") -- todo
         _flushing = true
 
-    local n0 = 0
-
         local i = n0 + 1
         while i <= update_queue.n do
             local node = update_queue[i]
@@ -208,6 +209,7 @@ local function flush_update_queue()
 
         _flushing = false
     end
+end
 
 local function update_descendants<T>(root: SourceNode<T>)
     local n0 = update_queue.n

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()