[Flight] Track Awaits on I/O as Debug Info

packages/react-server/src/ReactFlightAsyncSequence.js

@@ -0,0 +1,41 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+export const IO_NODE = 0;
+export const PROMISE_NODE = 1;
+export const AWAIT_NODE = 2;
+
+export type IONode = {
+  tag: 0,
+  stack: Error, // callsite that spawned the I/O
+  start: number, // start time when the first part of the I/O sequence started
+  end: number, // we typically don't use this. only when there's no promise intermediate.
+  awaited: null, // I/O is only blocked on external.
+  previous: null | AwaitNode, // the preceeding await that spawned this new work
+};
+
+export type PromiseNode = {
+  tag: 1,
+  stack: Error, // callsite that created the Promise
+  start: number, // start time when the Promise was created
+  end: number, // end time when the Promise was resolved.
+  awaited: null | AsyncSequence, // the thing that ended up resolving this promise
+  previous: null, // where we created the promise is not interesting since creating it doesn't mean waiting.
+};
+
+export type AwaitNode = {
+  tag: 2,
+  stack: Error, // callsite that awaited (using await, .then(), Promise.all(), ...)
+  start: -1.1, // not used. We use the timing of the awaited promise.
+  end: -1.1, // not used.
+  awaited: null | AsyncSequence, // the promise we were waiting on
+  previous: null | AsyncSequence, // the sequence that was blocking us from awaiting in the first place
+};
+
+export type AsyncSequence = IONode | PromiseNode | AwaitNode;