Parallel transitions: Assign different lanes to consecutive transitions (facebook#20672)

* Land enableTransitionEntanglement changes

Leaving the flag though because I plan to reuse it for additional,
similar changes.

* Assign different lanes to consecutive transitions

Currently we always assign the same lane to all transitions. This means
if there are two pending transitions at the same time, neither
transition can finish until both can finish, even if they affect
completely separate parts of the UI.

The new approach is to assign a different lane to each consecutive
transition, by shifting the bit to the right each time. When we reach
the end of the bit range, we cycle back to the first bit. In practice,
this should mean that all transitions get their own dedicated lane,
unless we have more pending transitions than lanes, which should
be rare.

We retain our existing behavior of assigning the same lane to all
transitions within the same event. This is achieved by caching the first
lane assigned to a transition, then re-using that one until the next
React task, by which point the event must have finished. This preserves
the guarantee that all transition updates that result from a single
event will be consistent.

Author: acdlite

packages/react-reconciler/src/ReactFiberLane.new.js

@@ -36,10 +36,7 @@ export type Lane = number;
 export type LaneMap<T> = Array<T>;
 
 import invariant from 'shared/invariant';
-import {
-  enableCache,
-  enableTransitionEntanglement,
-} from 'shared/ReactFeatureFlags';
+import {enableCache} from 'shared/ReactFeatureFlags';
 
 import {
   ImmediatePriority as ImmediateSchedulerPriority,
@@ -95,6 +92,7 @@ export const DefaultLanes: Lanes = /*                   */ 0b0000000000000000000
 
 const TransitionHydrationLane: Lane = /*                */ 0b0000000000000000001000000000000;
 const TransitionLanes: Lanes = /*                       */ 0b0000000001111111110000000000000;
+const SomeTransitionLane: Lane = /*                     */ 0b0000000000000000010000000000000;
 
 const RetryLanes: Lanes = /*                            */ 0b0000011110000000000000000000000;
 
@@ -113,6 +111,9 @@ export const NoTimestamp = -1;
 
 let currentUpdateLanePriority: LanePriority = NoLanePriority;
 
+let nextTransitionLane: Lane = SomeTransitionLane;
+let nextRetryLane: Lane = SomeRetryLane;
+
 export function getCurrentUpdateLanePriority(): LanePriority {
   return currentUpdateLanePriority;
 }
@@ -309,15 +310,6 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
     return NoLanes;
   }
 
-  if (enableTransitionEntanglement) {
-    // We don't need to do anything extra here, because we apply per-lane
-    // transition entanglement in the entanglement loop below.
-  } else {
-    // If there are higher priority lanes, we'll include them even if they
-    // are suspended.
-    nextLanes = pendingLanes & getEqualOrHigherPriorityLanes(nextLanes);
-  }
-
   // If we're already in the middle of a render, switching lanes will interrupt
   // it and we'll lose our progress. We should only do this if the new lanes are
   // higher priority.
@@ -350,6 +342,11 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
   // entanglement is usually "best effort": we'll try our best to render the
   // lanes in the same batch, but it's not worth throwing out partially
   // completed work in order to do it.
+  // TODO: Reconsider this. The counter-argument is that the partial work
+  // represents an intermediate state, which we don't want to show to the user.
+  // And by spending extra time finishing it, we're increasing the amount of
+  // time it takes to show the final state, which is what they are actually
+  // waiting for.
   //
   // For those exceptions where entanglement is semantically important, like
   // useMutableSource, we should ensure that there is no partial work at the
@@ -559,34 +556,23 @@ export function findUpdateLane(
   );
 }
 
-// To ensure consistency across multiple updates in the same event, this should
-// be pure function, so that it always returns the same lane for given inputs.
-export function findTransitionLane(wipLanes: Lanes, pendingLanes: Lanes): Lane {
-  // First look for lanes that are completely unclaimed, i.e. have no
-  // pending work.
-  let lane = pickArbitraryLane(TransitionLanes & ~pendingLanes);
-  if (lane === NoLane) {
-    // If all lanes have pending work, look for a lane that isn't currently
-    // being worked on.
-    lane = pickArbitraryLane(TransitionLanes & ~wipLanes);
-    if (lane === NoLane) {
-      // If everything is being worked on, pick any lane. This has the
-      // effect of interrupting the current work-in-progress.
-      lane = pickArbitraryLane(TransitionLanes);
-    }
+export function claimNextTransitionLane(): Lane {
+  // Cycle through the lanes, assigning each new transition to the next lane.
+  // In most cases, this means every transition gets its own lane, until we
+  // run out of lanes and cycle back to the beginning.
+  const lane = nextTransitionLane;
+  nextTransitionLane <<= 1;
+  if ((nextTransitionLane & TransitionLanes) === 0) {
+    nextTransitionLane = SomeTransitionLane;
   }
   return lane;
 }
 
-// To ensure consistency across multiple updates in the same event, this should
-// be pure function, so that it always returns the same lane for given inputs.
-export function findRetryLane(wipLanes: Lanes): Lane {
-  // This is a fork of `findUpdateLane` designed specifically for Suspense
-  // "retries" — a special update that attempts to flip a Suspense boundary
-  // from its placeholder state to its primary/resolved state.
-  let lane = pickArbitraryLane(RetryLanes & ~wipLanes);
-  if (lane === NoLane) {
-    lane = pickArbitraryLane(RetryLanes);
+export function claimNextRetryLane(): Lane {
+  const lane = nextRetryLane;
+  nextRetryLane <<= 1;
+  if ((nextRetryLane & RetryLanes) === 0) {
+    nextRetryLane = SomeRetryLane;
   }
   return lane;
 }
@@ -595,16 +581,6 @@ function getHighestPriorityLane(lanes: Lanes) {
   return lanes & -lanes;
 }
 
-function getLowestPriorityLane(lanes: Lanes): Lane {
-  // This finds the most significant non-zero bit.
-  const index = 31 - clz32(lanes);
-  return index < 0 ? NoLanes : 1 << index;
-}
-
-function getEqualOrHigherPriorityLanes(lanes: Lanes | Lane): Lanes {
-  return (getLowestPriorityLane(lanes) << 1) - 1;
-}
-
 export function pickArbitraryLane(lanes: Lanes): Lane {
   // This wrapper function gets inlined. Only exists so to communicate that it
   // doesn't matter which bit is selected; you can pick any bit without
@@ -676,39 +652,21 @@ export function markRootUpdated(
 ) {
   root.pendingLanes |= updateLane;
 
-  // TODO: Theoretically, any update to any lane can unblock any other lane. But
-  // it's not practical to try every single possible combination. We need a
-  // heuristic to decide which lanes to attempt to render, and in which batches.
-  // For now, we use the same heuristic as in the old ExpirationTimes model:
-  // retry any lane at equal or lower priority, but don't try updates at higher
-  // priority without also including the lower priority updates. This works well
-  // when considering updates across different priority levels, but isn't
-  // sufficient for updates within the same priority, since we want to treat
-  // those updates as parallel.
-
-  // Unsuspend any update at equal or lower priority.
-  const higherPriorityLanes = updateLane - 1; // Turns 0b1000 into 0b0111
-
-  if (enableTransitionEntanglement) {
-    // If there are any suspended transitions, it's possible this new update
-    // could unblock them. Clear the suspended lanes so that we can try rendering
-    // them again.
-    //
-    // TODO: We really only need to unsuspend only lanes that are in the
-    // `subtreeLanes` of the updated fiber, or the update lanes of the return
-    // path. This would exclude suspended updates in an unrelated sibling tree,
-    // since there's no way for this update to unblock it.
-    //
-    // We don't do this if the incoming update is idle, because we never process
-    // idle updates until after all the regular updates have finished; there's no
-    // way it could unblock a transition.
-    if ((updateLane & IdleLanes) === NoLanes) {
-      root.suspendedLanes = NoLanes;
-      root.pingedLanes = NoLanes;
-    }
-  } else {
-    root.suspendedLanes &= higherPriorityLanes;
-    root.pingedLanes &= higherPriorityLanes;
+  // If there are any suspended transitions, it's possible this new update
+  // could unblock them. Clear the suspended lanes so that we can try rendering
+  // them again.
+  //
+  // TODO: We really only need to unsuspend only lanes that are in the
+  // `subtreeLanes` of the updated fiber, or the update lanes of the return
+  // path. This would exclude suspended updates in an unrelated sibling tree,
+  // since there's no way for this update to unblock it.
+  //
+  // We don't do this if the incoming update is idle, because we never process
+  // idle updates until after all the regular updates have finished; there's no
+  // way it could unblock a transition.
+  if ((updateLane & IdleLanes) === NoLanes) {
+    root.suspendedLanes = NoLanes;
+    root.pingedLanes = NoLanes;
   }
 
   const eventTimes = root.eventTimes;
@@ -801,16 +759,32 @@ export function markRootFinished(root: FiberRoot, remainingLanes: Lanes) {
 }
 
 export function markRootEntangled(root: FiberRoot, entangledLanes: Lanes) {
-  root.entangledLanes |= entangledLanes;
+  // In addition to entangling each of the given lanes with each other, we also
+  // have to consider _transitive_ entanglements. For each lane that is already
+  // entangled with *any* of the given lanes, that lane is now transitively
+  // entangled with *all* the given lanes.
+  //
+  // Translated: If C is entangled with A, then entangling A with B also
+  // entangles C with B.
+  //
+  // If this is hard to grasp, it might help to intentionally break this
+  // function and look at the tests that fail in ReactTransition-test.js. Try
+  // commenting out one of the conditions below.
 
+  const rootEntangledLanes = (root.entangledLanes |= entangledLanes);
   const entanglements = root.entanglements;
-  let lanes = entangledLanes;
-  while (lanes > 0) {
+  let lanes = rootEntangledLanes;
+  while (lanes) {
     const index = pickArbitraryLaneIndex(lanes);
     const lane = 1 << index;
-
-    entanglements[index] |= entangledLanes;
-
+    if (
+      // Is this one of the newly entangled lanes?
+      (lane & entangledLanes) |
+      // Is this lane transitively entangled with the newly entangled lanes?
+      (entanglements[index] & entangledLanes)
+    ) {
+      entanglements[index] |= entangledLanes;
+    }
     lanes &= ~lane;
   }
 }