Count the size of the currently serialized row

Including any outlined chunks that would go before this row since they also
block it.

The size doesn't include every single byte written like every bracket and
number. It's just the big picture scale like string lengths including keys.

Author: sebmarkbage

packages/react-server/src/ReactFlightServer.js

@@ -1600,6 +1600,11 @@ function renderClientElement(
 // The chunk ID we're currently rendering that we can assign debug data to.
 let debugID: null | number = null;
 
+// Approximate string length of the currently serializing row.
+// Used to power outlining heuristics.
+let serializedSize = 0;
+const MAX_ROW_SIZE = 3200;
+
 function outlineTask(request: Request, task: Task): ReactJSONValue {
   const newTask = createTask(
     request,
@@ -2393,6 +2398,8 @@ function renderModelDestructive(
   // Set the currently rendering model
   task.model = value;
 
+  serializedSize += parentPropertyName.length;
+
   // Special Symbol, that's very common.
   if (value === REACT_ELEMENT_TYPE) {
     return '$';
@@ -2811,6 +2818,7 @@ function renderModelDestructive(
         throwTaintViolation(tainted.message);
       }
     }
+    serializedSize += value.length;
     // TODO: Maybe too clever. If we support URL there's no similar trick.
     if (value[value.length - 1] === 'Z') {
       // Possibly a Date, whose toJSON automatically calls toISOString
@@ -3892,9 +3900,18 @@ function emitChunk(
     return;
   }
   // For anything else we need to try to serialize it using JSON.
-  // $FlowFixMe[incompatible-type] stringify can return null for undefined but we never do
-  const json: string = stringify(value, task.toJSON);
-  emitModelChunk(request, task.id, json);
+  // We stash the outer parent size so we can restore it when we exit.
+  // We don't reset the serialized size counter from reentry because that indicates that we
+  // are outlining a model and we actually want to include that size into the parent since
+  // it will still block the parent row. It only restores to zero at the top of the stack.
+  const parentSerializedSize = 0;
+  try {
+    // $FlowFixMe[incompatible-type] stringify can return null for undefined but we never do
+    const json: string = stringify(value, task.toJSON);
+    emitModelChunk(request, task.id, json);
+  } finally {
+    serializedSize = parentSerializedSize;
+  }
 }
 
 function erroredTask(request: Request, task: Task, error: mixed): void {