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Force unwind work loop during selective hydration #25695

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Nov 17, 2022
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Force unwind work loop during selective hydration #25695

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8 changes: 3 additions & 5 deletions packages/react-dom/src/__tests__/ReactDOMServerSelectiveHydration-test.internal.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1496,12 +1496,10 @@ describe('ReactDOMServerSelectiveHydration', () => {
// Start rendering. This will force the first boundary to hydrate
// by scheduling it at one higher pri than Idle.
expect(Scheduler).toFlushAndYieldThrough([
// An update was scheduled to force hydrate the boundary, but React will
// continue rendering at Idle until the next time React yields. This is
// fine though because it will switch to the hydration level when it
// re-enters the work loop.
'App',
'AA',

// Start hydrating A
'A',
]);

// Hover over A which (could) schedule at one higher pri than Idle.
Expand Down
32 changes: 26 additions & 6 deletions packages/react-reconciler/src/ReactFiberBeginWork.new.js
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Original file line number Diff line number Diff line change
Expand Up @@ -280,6 +280,14 @@ import {

const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;

// A special exception that's used to unwind the stack when an update flows
// into a dehydrated boundary.
export const SelectiveHydrationException: mixed = new Error(
"This is not a real error. It's an implementation detail of React's " +
"selective hydration feature. If this leaks into userspace, it's a bug in " +
'React. Please file an issue.',
);

let didReceiveUpdate: boolean = false;

let didWarnAboutBadClass;
Expand Down Expand Up @@ -2810,6 +2818,16 @@ function updateDehydratedSuspenseComponent(
attemptHydrationAtLane,
eventTime,
);

// Throw a special object that signals to the work loop that it should
// interrupt the current render.
//
// Because we're inside a React-only execution stack, we don't
// strictly need to throw here — we could instead modify some internal
// work loop state. But using an exception means we don't need to
// check for this case on every iteration of the work loop. So doing
// it this way moves the check out of the fast path.
throw SelectiveHydrationException;
} else {
// We have already tried to ping at a higher priority than we're rendering with
// so if we got here, we must have failed to hydrate at those levels. We must
Expand All @@ -2820,15 +2838,17 @@ function updateDehydratedSuspenseComponent(
}
}

// If we have scheduled higher pri work above, this will just abort the render
// since we now have higher priority work. We'll try to infinitely suspend until
// we yield. TODO: We could probably just force yielding earlier instead.
renderDidSuspendDelayIfPossible();
// If we rendered synchronously, we won't yield so have to render something.
// This will cause us to delete any existing content.
// If we did not selectively hydrate, we'll continue rendering without
// hydrating. Mark this tree as suspended to prevent it from committing
// outside a transition.
//
// This path should only happen if the hydration lane already suspended.
// Currently, it also happens during sync updates because there is no
// hydration lane for sync updates.
// TODO: We should ideally have a sync hydration lane that we can apply to do
// a pass where we hydrate this subtree in place using the previous Context and then
// reapply the update afterwards.
renderDidSuspendDelayIfPossible();
return retrySuspenseComponentWithoutHydrating(
current,
workInProgress,
Expand Down
32 changes: 26 additions & 6 deletions packages/react-reconciler/src/ReactFiberBeginWork.old.js
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Original file line number Diff line number Diff line change
Expand Up @@ -280,6 +280,14 @@ import {

const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;

// A special exception that's used to unwind the stack when an update flows
// into a dehydrated boundary.
export const SelectiveHydrationException: mixed = new Error(
"This is not a real error. It's an implementation detail of React's " +
"selective hydration feature. If this leaks into userspace, it's a bug in " +
'React. Please file an issue.',
);

let didReceiveUpdate: boolean = false;

let didWarnAboutBadClass;
Expand Down Expand Up @@ -2810,6 +2818,16 @@ function updateDehydratedSuspenseComponent(
attemptHydrationAtLane,
eventTime,
);

// Throw a special object that signals to the work loop that it should
// interrupt the current render.
//
// Because we're inside a React-only execution stack, we don't
// strictly need to throw here — we could instead modify some internal
// work loop state. But using an exception means we don't need to
// check for this case on every iteration of the work loop. So doing
// it this way moves the check out of the fast path.
throw SelectiveHydrationException;
} else {
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Is this branch just a bug in React? Not sure how it happens exactly.

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Same answer as here: cd34ec3#r1024411227

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This link is broken.

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It happens when the hydration lane is suspended. That indicates we already attempted to hydrate but we couldn't because the code/data isn't ready. In that case we switch back to the original update lane and render without hydrating. Without this branch it would keep restarting in a loop until the data resolved.

// We have already tried to ping at a higher priority than we're rendering with
// so if we got here, we must have failed to hydrate at those levels. We must
Expand All @@ -2820,15 +2838,17 @@ function updateDehydratedSuspenseComponent(
}
}

// If we have scheduled higher pri work above, this will just abort the render
// since we now have higher priority work. We'll try to infinitely suspend until
// we yield. TODO: We could probably just force yielding earlier instead.
renderDidSuspendDelayIfPossible();
// If we rendered synchronously, we won't yield so have to render something.
// This will cause us to delete any existing content.
// If we did not selectively hydrate, we'll continue rendering without
// hydrating. Mark this tree as suspended to prevent it from committing
// outside a transition.
//
// This path should only happen if the hydration lane already suspended.
// Currently, it also happens during sync updates because there is no
// hydration lane for sync updates.
// TODO: We should ideally have a sync hydration lane that we can apply to do
// a pass where we hydrate this subtree in place using the previous Context and then
// reapply the update afterwards.
renderDidSuspendDelayIfPossible();
return retrySuspenseComponentWithoutHydrating(
current,
workInProgress,
Expand Down
55 changes: 47 additions & 8 deletions packages/react-reconciler/src/ReactFiberWorkLoop.new.js
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Original file line number Diff line number Diff line change
Expand Up @@ -178,7 +178,10 @@ import {
lanesToEventPriority,
} from './ReactEventPriorities.new';
import {requestCurrentTransition, NoTransition} from './ReactFiberTransition';
import {beginWork as originalBeginWork} from './ReactFiberBeginWork.new';
import {
SelectiveHydrationException,
beginWork as originalBeginWork,
} from './ReactFiberBeginWork.new';
import {completeWork} from './ReactFiberCompleteWork.new';
import {unwindWork, unwindInterruptedWork} from './ReactFiberUnwindWork.new';
import {
Expand Down Expand Up @@ -316,12 +319,13 @@ let workInProgress: Fiber | null = null;
// The lanes we're rendering
let workInProgressRootRenderLanes: Lanes = NoLanes;

opaque type SuspendedReason = 0 | 1 | 2 | 3 | 4;
opaque type SuspendedReason = 0 | 1 | 2 | 3 | 4 | 5;
const NotSuspended: SuspendedReason = 0;
const SuspendedOnError: SuspendedReason = 1;
const SuspendedOnData: SuspendedReason = 2;
const SuspendedOnImmediate: SuspendedReason = 3;
const SuspendedAndReadyToUnwind: SuspendedReason = 4;
const SuspendedOnHydration: SuspendedReason = 5;

// When this is true, the work-in-progress fiber just suspended (or errored) and
// we've yet to unwind the stack. In some cases, we may yield to the main thread
Expand Down Expand Up @@ -1775,6 +1779,18 @@ function handleThrow(root, thrownValue): void {
workInProgressSuspendedReason = shouldAttemptToSuspendUntilDataResolves()
? SuspendedOnData
: SuspendedOnImmediate;
} else if (thrownValue === SelectiveHydrationException) {
// An update flowed into a dehydrated boundary. Before we can apply the
// update, we need to finish hydrating. Interrupt the work-in-progress
// render so we can restart at the hydration lane.
//
// The ideal implementation would be able to switch contexts without
// unwinding the current stack.
//
// We could name this something more general but as of now it's the only
// case where we think this should happen.
workInProgressSuspendedThenableState = null;
workInProgressSuspendedReason = SuspendedOnHydration;
} else {
// This is a regular error. If something earlier in the component already
// suspended, we must clear the thenable state to unblock the work loop.
Expand Down Expand Up @@ -1965,6 +1981,9 @@ export function renderHasNotSuspendedYet(): boolean {
return workInProgressRootExitStatus === RootInProgress;
}

// TODO: Over time, this function and renderRootConcurrent have become more
// and more similar. Not sure it makes sense to maintain forked paths. Consider
// unifying them again.
function renderRootSync(root: FiberRoot, lanes: Lanes) {
const prevExecutionContext = executionContext;
executionContext |= RenderContext;
Expand Down Expand Up @@ -2004,7 +2023,7 @@ function renderRootSync(root: FiberRoot, lanes: Lanes) {
markRenderStarted(lanes);
}

do {
outer: do {
try {
if (
workInProgressSuspendedReason !== NotSuspended &&
Expand All @@ -2020,11 +2039,23 @@ function renderRootSync(root: FiberRoot, lanes: Lanes) {
// function and fork the behavior some other way.
const unitOfWork = workInProgress;
const thrownValue = workInProgressThrownValue;
workInProgressSuspendedReason = NotSuspended;
workInProgressThrownValue = null;
unwindSuspendedUnitOfWork(unitOfWork, thrownValue);

// Continue with the normal work loop.
switch (workInProgressSuspendedReason) {
case SuspendedOnHydration: {
// Selective hydration. An update flowed into a dehydrated tree.
// Interrupt the current render so the work loop can switch to the
// hydration lane.
workInProgress = null;
workInProgressRootExitStatus = RootDidNotComplete;
break outer;
}
default: {
// Continue with the normal work loop.
workInProgressSuspendedReason = NotSuspended;
workInProgressThrownValue = null;
unwindSuspendedUnitOfWork(unitOfWork, thrownValue);
break;
}
}
}
workLoopSync();
break;
Expand Down Expand Up @@ -2160,6 +2191,14 @@ function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
workInProgressSuspendedReason = SuspendedAndReadyToUnwind;
break outer;
}
case SuspendedOnHydration: {
// Selective hydration. An update flowed into a dehydrated tree.
// Interrupt the current render so the work loop can switch to the
// hydration lane.
workInProgress = null;
workInProgressRootExitStatus = RootDidNotComplete;
break outer;
}
default: {
workInProgressSuspendedReason = NotSuspended;
workInProgressThrownValue = null;
Expand Down
55 changes: 47 additions & 8 deletions packages/react-reconciler/src/ReactFiberWorkLoop.old.js
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Original file line number Diff line number Diff line change
Expand Up @@ -178,7 +178,10 @@ import {
lanesToEventPriority,
} from './ReactEventPriorities.old';
import {requestCurrentTransition, NoTransition} from './ReactFiberTransition';
import {beginWork as originalBeginWork} from './ReactFiberBeginWork.old';
import {
SelectiveHydrationException,
beginWork as originalBeginWork,
} from './ReactFiberBeginWork.old';
import {completeWork} from './ReactFiberCompleteWork.old';
import {unwindWork, unwindInterruptedWork} from './ReactFiberUnwindWork.old';
import {
Expand Down Expand Up @@ -316,12 +319,13 @@ let workInProgress: Fiber | null = null;
// The lanes we're rendering
let workInProgressRootRenderLanes: Lanes = NoLanes;

opaque type SuspendedReason = 0 | 1 | 2 | 3 | 4;
opaque type SuspendedReason = 0 | 1 | 2 | 3 | 4 | 5;
const NotSuspended: SuspendedReason = 0;
const SuspendedOnError: SuspendedReason = 1;
const SuspendedOnData: SuspendedReason = 2;
const SuspendedOnImmediate: SuspendedReason = 3;
const SuspendedAndReadyToUnwind: SuspendedReason = 4;
const SuspendedOnHydration: SuspendedReason = 5;

// When this is true, the work-in-progress fiber just suspended (or errored) and
// we've yet to unwind the stack. In some cases, we may yield to the main thread
Expand Down Expand Up @@ -1775,6 +1779,18 @@ function handleThrow(root, thrownValue): void {
workInProgressSuspendedReason = shouldAttemptToSuspendUntilDataResolves()
? SuspendedOnData
: SuspendedOnImmediate;
} else if (thrownValue === SelectiveHydrationException) {
// An update flowed into a dehydrated boundary. Before we can apply the
// update, we need to finish hydrating. Interrupt the work-in-progress
// render so we can restart at the hydration lane.
//
// The ideal implementation would be able to switch contexts without
// unwinding the current stack.
//
// We could name this something more general but as of now it's the only
// case where we think this should happen.
workInProgressSuspendedThenableState = null;
workInProgressSuspendedReason = SuspendedOnHydration;
} else {
// This is a regular error. If something earlier in the component already
// suspended, we must clear the thenable state to unblock the work loop.
Expand Down Expand Up @@ -1965,6 +1981,9 @@ export function renderHasNotSuspendedYet(): boolean {
return workInProgressRootExitStatus === RootInProgress;
}

// TODO: Over time, this function and renderRootConcurrent have become more
// and more similar. Not sure it makes sense to maintain forked paths. Consider
// unifying them again.
function renderRootSync(root: FiberRoot, lanes: Lanes) {
const prevExecutionContext = executionContext;
executionContext |= RenderContext;
Expand Down Expand Up @@ -2004,7 +2023,7 @@ function renderRootSync(root: FiberRoot, lanes: Lanes) {
markRenderStarted(lanes);
}

do {
outer: do {
try {
if (
workInProgressSuspendedReason !== NotSuspended &&
Expand All @@ -2020,11 +2039,23 @@ function renderRootSync(root: FiberRoot, lanes: Lanes) {
// function and fork the behavior some other way.
const unitOfWork = workInProgress;
const thrownValue = workInProgressThrownValue;
workInProgressSuspendedReason = NotSuspended;
workInProgressThrownValue = null;
unwindSuspendedUnitOfWork(unitOfWork, thrownValue);

// Continue with the normal work loop.
switch (workInProgressSuspendedReason) {
case SuspendedOnHydration: {
// Selective hydration. An update flowed into a dehydrated tree.
// Interrupt the current render so the work loop can switch to the
// hydration lane.
workInProgress = null;
workInProgressRootExitStatus = RootDidNotComplete;
break outer;
}
default: {
// Continue with the normal work loop.
workInProgressSuspendedReason = NotSuspended;
workInProgressThrownValue = null;
unwindSuspendedUnitOfWork(unitOfWork, thrownValue);
break;
}
}
}
workLoopSync();
break;
Expand Down Expand Up @@ -2160,6 +2191,14 @@ function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
workInProgressSuspendedReason = SuspendedAndReadyToUnwind;
break outer;
}
case SuspendedOnHydration: {
// Selective hydration. An update flowed into a dehydrated tree.
// Interrupt the current render so the work loop can switch to the
// hydration lane.
workInProgress = null;
workInProgressRootExitStatus = RootDidNotComplete;
break outer;
}
default: {
workInProgressSuspendedReason = NotSuspended;
workInProgressThrownValue = null;
Expand Down
3 changes: 2 additions & 1 deletion scripts/error-codes/codes.json
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Original file line number Diff line number Diff line change
Expand Up @@ -445,5 +445,6 @@
"457": "acquireHeadResource encountered a resource type it did not expect: \"%s\". This is a bug in React.",
"458": "Currently React only supports one RSC renderer at a time.",
"459": "Expected a suspended thenable. This is a bug in React. Please file an issue.",
"460": "Suspense Exception: This is not a real error! It's an implementation detail of `use` to interrupt the current render. You must either rethrow it immediately, or move the `use` call outside of the `try/catch` block. Capturing without rethrowing will lead to unexpected behavior.\n\nTo handle async errors, wrap your component in an error boundary, or call the promise's `.catch` method and pass the result to `use`"
"460": "Suspense Exception: This is not a real error! It's an implementation detail of `use` to interrupt the current render. You must either rethrow it immediately, or move the `use` call outside of the `try/catch` block. Capturing without rethrowing will lead to unexpected behavior.\n\nTo handle async errors, wrap your component in an error boundary, or call the promise's `.catch` method and pass the result to `use`",
"461": "This is not a real error. It's an implementation detail of React's selective hydration feature. If this leaks into userspace, it's a bug in React. Please file an issue."
}