From ce126fbb231f9110151b4c93cff765fd882a9f64 Mon Sep 17 00:00:00 2001
From: Andrew Clark <git@andrewclark.io>
Date: Tue, 23 Apr 2019 16:41:05 -0700
Subject: [PATCH] Fix priority inference of next level of work (#15478)

Bugfix for `inferPriorityFromExpirationTime` function. It happened to
work in our existing tests because we use virtual time.

Flow would have caught this if expiration times were an opaque type. We
should consider that in the future. (The downside of opaque types is
that all operations would have to go through helper functions, which may
or may not get inlined by Closure.)
---
 .../src/ReactFiberExpirationTime.js              |  6 +++---
 .../ReactSchedulerIntegration-test.internal.js   | 16 ++++++++++++++++
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/packages/react-reconciler/src/ReactFiberExpirationTime.js b/packages/react-reconciler/src/ReactFiberExpirationTime.js
index e28e888545a29..a8594a57a3fe6 100644
--- a/packages/react-reconciler/src/ReactFiberExpirationTime.js
+++ b/packages/react-reconciler/src/ReactFiberExpirationTime.js
@@ -111,14 +111,14 @@ export function inferPriorityFromExpirationTime(
     return IdlePriority;
   }
   const msUntil =
-    msToExpirationTime(expirationTime) - msToExpirationTime(currentTime);
+    expirationTimeToMs(expirationTime) - expirationTimeToMs(currentTime);
   if (msUntil <= 0) {
     return ImmediatePriority;
   }
-  if (msUntil <= HIGH_PRIORITY_EXPIRATION) {
+  if (msUntil <= HIGH_PRIORITY_EXPIRATION + HIGH_PRIORITY_BATCH_SIZE) {
     return UserBlockingPriority;
   }
-  if (msUntil <= LOW_PRIORITY_EXPIRATION) {
+  if (msUntil <= LOW_PRIORITY_EXPIRATION + LOW_PRIORITY_BATCH_SIZE) {
     return NormalPriority;
   }
 
diff --git a/packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.internal.js b/packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.internal.js
index d558a578a20b5..dec4bfbfc57b1 100644
--- a/packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.internal.js
@@ -150,6 +150,22 @@ describe('ReactSchedulerIntegration', () => {
     ]);
   });
 
+  it('after completing a level of work, infers priority of the next batch based on its expiration time', () => {
+    function App({label}) {
+      Scheduler.yieldValue(`${label} [${getCurrentPriorityAsString()}]`);
+      return label;
+    }
+
+    // Schedule two separate updates at different priorities
+    runWithPriority(UserBlockingPriority, () => {
+      ReactNoop.render(<App label="A" />);
+    });
+    ReactNoop.render(<App label="B" />);
+
+    // The second update should run at normal priority
+    expect(Scheduler).toFlushAndYield(['A [UserBlocking]', 'B [Normal]']);
+  });
+
   // TODO
   it.skip('passive effects have render priority even if they are flushed early', () => {});
 });