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Use double render to detect render phase mutation
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PR #20665 added a mechanism to detect when a `useMutableSource` source
is mutated during the render phase. It relies on the fact that we double
invoke components that error during development using
`invokeGuardedCallback`. If the version in the double render doesn't
match the first, that indicates there must have been a mutation during
render.

At first I thought it worked by detecting inside the *other* double
render, the one we do for Strict Mode. It turns out that while it does
warn then, the warning is suppressed, because we suppress all console
methods that occur during the Strict Mode double render. So it's really
the `invokeGuardedCallback` one that makes it work.

Anyway, let's set that aside that issue for a second. I realized during
review that errors that occur during the Strict Mode double render
reveal a useful property: A pure component will never throw during the
double render, because if it were pure, it would have also thrown during
the first render... in which case it wouldn't have double rendered! This
is true of all such errors, not just the one thrown by
`useMutableSource`.

Given this, we can simplify the `useMutableSource` mutation detection
mechanism. Instead of tracking and comparing the source's version, we
can instead check if we're inside a double render when the error is
thrown.

To get around the console suppression issue, I changed the warning to an
error. It errors regardless, in both dev and prod, so it doesn't have
semantic implications.

However, because of the paradox I described above, we arguably
_shouldn't_ throw an error in development, since we know that error
won't happen in production, because prod doesn't double render. (It's
still a tearing bug, but that doesn't mean the component will actually
throw.) I considered that, but that requires a larger conversation about
how to handle errors that we know are only possible in development. I
think we should probably be suppressing *all* errors (with a warning)
that occur during a double render.
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@acdlite
acdlite committed Jan 30, 2021
1 parent 28ab856 commit c16fe14
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Showing 4 changed files with 83 additions and 67 deletions.
47 changes: 31 additions & 16 deletions packages/react-reconciler/src/ReactFiberHooks.new.js
View file
Original file line number Diff line number Diff line change
Expand Up @@ -904,18 +904,6 @@ function readFromUnsubcribedMutableSource<Source, Snapshot>(
const getVersion = source._getVersion;
const version = getVersion(source._source);

let mutableSourceSideEffectDetected = false;
if (__DEV__) {
// Detect side effects that update a mutable source during render.
// See https://github.com/facebook/react/issues/19948
if (source._currentlyRenderingFiber !== currentlyRenderingFiber) {
source._currentlyRenderingFiber = currentlyRenderingFiber;
source._initialVersionAsOfFirstRender = version;
} else if (source._initialVersionAsOfFirstRender !== version) {
mutableSourceSideEffectDetected = true;
}
}

// Is it safe for this component to read from this source during the current render?
let isSafeToReadFromSource = false;

Expand Down Expand Up @@ -978,17 +966,44 @@ function readFromUnsubcribedMutableSource<Source, Snapshot>(
// but there's nothing we can do about that (short of throwing here and refusing to continue the render).
markSourceAsDirty(source);

// Intentioally throw an error to force React to retry synchronously. During
// the synchronous retry, it will block interleaved mutations, so we should
// get a consistent read. Therefore, the following error should never be
// visible to the user.
//
// If it were to become visible to the user, it suggests one of two things:
// a bug in React, or (more likely), a mutation during the render phase that
// caused the second re-render attempt to be different from the first.
//
// We know it's the second case if the logs are currently disabled. So in
// dev, we can present a more accurate error message.
if (__DEV__) {
if (mutableSourceSideEffectDetected) {
// eslint-disable-next-line react-internal/no-production-logging
if (console.log.__reactDisabledLog) {
// If the logs are disabled, this is the dev-only double render. This is
// only reachable if there was a mutation during render. Show a helpful
// error message.
//
// Something interesting to note: because we only double render in
// development, this error will never happen during production. This is
// actually true of all errors that occur during a double render,
// because if the first render had thrown, we would have exited the
// begin phase without double rendering. We should consider suppressing
// any error from a double render (with a warning) to more closely match
// the production behavior.
const componentName = getComponentName(currentlyRenderingFiber.type);
console.warn(
'A mutable source was mutated while the %s component was rendering. This is not supported. ' +
'Move any mutations into event handlers or effects.',
invariant(
false,
'A mutable source was mutated while the %s component was rendering. ' +
'This is not supported. Move any mutations into event handlers ' +
'or effects.',
componentName,
);
}
}

// We expect this error not to be thrown during the synchronous retry,
// because we blocked interleaved mutations.
invariant(
false,
'Cannot read from mutable source during the current render without tearing. This may be a bug in React. Please file an issue.',
Expand Down
47 changes: 31 additions & 16 deletions packages/react-reconciler/src/ReactFiberHooks.old.js
View file
Original file line number Diff line number Diff line change
Expand Up @@ -885,18 +885,6 @@ function readFromUnsubcribedMutableSource<Source, Snapshot>(
const getVersion = source._getVersion;
const version = getVersion(source._source);

let mutableSourceSideEffectDetected = false;
if (__DEV__) {
// Detect side effects that update a mutable source during render.
// See https://github.com/facebook/react/issues/19948
if (source._currentlyRenderingFiber !== currentlyRenderingFiber) {
source._currentlyRenderingFiber = currentlyRenderingFiber;
source._initialVersionAsOfFirstRender = version;
} else if (source._initialVersionAsOfFirstRender !== version) {
mutableSourceSideEffectDetected = true;
}
}

// Is it safe for this component to read from this source during the current render?
let isSafeToReadFromSource = false;

Expand Down Expand Up @@ -959,17 +947,44 @@ function readFromUnsubcribedMutableSource<Source, Snapshot>(
// but there's nothing we can do about that (short of throwing here and refusing to continue the render).
markSourceAsDirty(source);

// Intentioally throw an error to force React to retry synchronously. During
// the synchronous retry, it will block interleaved mutations, so we should
// get a consistent read. Therefore, the following error should never be
// visible to the user.
//
// If it were to become visible to the user, it suggests one of two things:
// a bug in React, or (more likely), a mutation during the render phase that
// caused the second re-render attempt to be different from the first.
//
// We know it's the second case if the logs are currently disabled. So in
// dev, we can present a more accurate error message.
if (__DEV__) {
if (mutableSourceSideEffectDetected) {
// eslint-disable-next-line react-internal/no-production-logging
if (console.log.__reactDisabledLog) {
// If the logs are disabled, this is the dev-only double render. This is
// only reachable if there was a mutation during render. Show a helpful
// error message.
//
// Something interesting to note: because we only double render in
// development, this error will never happen during production. This is
// actually true of all errors that occur during a double render,
// because if the first render had thrown, we would have exited the
// begin phase without double rendering. We should consider suppressing
// any error from a double render (with a warning) to more closely match
// the production behavior.
const componentName = getComponentName(currentlyRenderingFiber.type);
console.warn(
'A mutable source was mutated while the %s component was rendering. This is not supported. ' +
'Move any mutations into event handlers or effects.',
invariant(
false,
'A mutable source was mutated while the %s component was rendering. ' +
'This is not supported. Move any mutations into event handlers ' +
'or effects.',
componentName,
);
}
}

// We expect this error not to be thrown during the synchronous retry,
// because we blocked interleaved mutations.
invariant(
false,
'Cannot read from mutable source during the current render without tearing. This may be a bug in React. Please file an issue.',
Expand Down
52 changes: 19 additions & 33 deletions packages/react-reconciler/src/__tests__/useMutableSource-test.internal.js
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1744,24 +1744,14 @@ describe('useMutableSource', () => {
}

expect(() => {
expect(() => {
act(() => {
ReactNoop.render(<MutateDuringRead />);
});
}).toThrow(
'Cannot read from mutable source during the current render without tearing. This may be a bug in React. Please file an issue.',
);
}).toWarnDev([
// Warns twice because of the retry-on-error render pass. Should
// consider only warning during the first attempt, not during the
// retry. Or maybe vice versa.
'A mutable source was mutated while the MutateDuringRead component was rendering. This is not supported. ' +
'Move any mutations into event handlers or effects.\n' +
' in MutateDuringRead (at **)',
'A mutable source was mutated while the MutateDuringRead component was rendering. This is not supported. ' +
'Move any mutations into event handlers or effects.\n' +
' in MutateDuringRead (at **)',
]);
act(() => {
ReactNoop.render(<MutateDuringRead />);
});
}).toThrow(
'A mutable source was mutated while the MutateDuringRead component ' +
'was rendering. This is not supported. Move any mutations into ' +
'event handlers or effects.',
);

expect(Scheduler).toHaveYielded([
// First attempt
Expand Down Expand Up @@ -1795,21 +1785,17 @@ describe('useMutableSource', () => {
}

expect(() => {
expect(() => {
act(() => {
ReactNoop.renderLegacySyncRoot(
<React.StrictMode>
<MutateDuringRead />
</React.StrictMode>,
);
});
}).toThrow(
'Cannot read from mutable source during the current render without tearing. This may be a bug in React. Please file an issue.',
);
}).toWarnDev(
'A mutable source was mutated while the MutateDuringRead component was rendering. This is not supported. ' +
'Move any mutations into event handlers or effects.\n' +
' in MutateDuringRead (at **)',
act(() => {
ReactNoop.renderLegacySyncRoot(
<React.StrictMode>
<MutateDuringRead />
</React.StrictMode>,
);
});
}).toThrow(
'A mutable source was mutated while the MutateDuringRead component ' +
'was rendering. This is not supported. Move any mutations into ' +
'event handlers or effects.',
);

expect(Scheduler).toHaveYielded(['MutateDuringRead:initial']);
Expand Down
4 changes: 2 additions & 2 deletions scripts/error-codes/codes.json
View file
Original file line number Diff line number Diff line change
Expand Up @@ -339,7 +339,7 @@
"345": "Root did not complete. This is a bug in React.",
"348": "ensureListeningTo(): received a container that was not an element node. This is likely a bug in React.",
"349": "Expected a work-in-progress root. This is a bug in React. Please file an issue.",
"350": "Cannot read from mutable source during the current render without tearing. This is a bug in React. Please file an issue.",
"350": "Cannot read from mutable source during the current render without tearing. This may be a bug in React. Please file an issue.",
"351": "Unsupported server component type: %s",
"352": "React Lazy Components are not yet supported on the server.",
"353": "A server block should never encode any other slots. This is a bug in React.",
Expand Down Expand Up @@ -373,5 +373,5 @@
"382": "This query has received more parameters than the last time the same query was used. Always pass the exact number of parameters that the query needs.",
"383": "This query has received fewer parameters than the last time the same query was used. Always pass the exact number of parameters that the query needs.",
"384": "Refreshing the cache is not supported in Server Components.",
"385": "Cannot read from mutable source during the current render without tearing. This may be a bug in React. Please file an issue."
"385": "A mutable source was mutated while the %s component was rendering. This is not supported. Move any mutations into event handlers or effects."
}

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