Failing test for Context bug in ReactPartialRenderer

[aweary]

A failing test for the bug reported in #12968

A summary of the issue is that rendering this combination of consumers and providers for multiple contexts causes a bug where the value from one context is being used in the other in subsequent renders.

This is possible because context is stored on the Context element itself, so it's mutated as the components are rendered. In this case, the wrong value is being set on a provider when its being popped:

react/packages/react-dom/src/server/ReactPartialRenderer.js

Lines 692 to 700 in 36546b5

	if (this.providerIndex < 0) {
	context._currentValue = context._defaultValue;
	} else {
	// We assume this type is correct because of the index check above.
	const previousProvider: ReactProvider<any> = (this.providerStack[
	this.providerIndex
	]: any);
	context._currentValue = previousProvider.props.value;
	}

previousProvider ends up being the provider for the other context, so the next time it renders it has the wrong value. The assumption that the context type is correct because of the index check isn't accurate. It's also strange that it's always setting the context value to previousProvider.props.value, even if the provider doesn't have a value.

Not sure what the right fix is, but I'd like to figure it out 😄

cc @acdlite @gaearon

[gaearon]

It's also strange that it's always setting the context value to previousProvider.props.value, even if the provider doesn't have a value.

What do you mean by "doesn't have a value"?

[aweary]

What do you mean by "doesn't have a value"?

@gaearon I mean when a Provider doesn't have a value prop and is relying on the default value passed in to React.createContext

[gaearon]

If Provider doesn't have a value prop then you'll get undefined. The defaultValue is only used when there's no matching Provider.

[aweary]

@gaearon sure, but it sets _currentValue not _defaultValue

react/packages/react-dom/src/server/ReactPartialRenderer.js

Line 699 in 36546b5

context._currentValue = previousProvider.props.value;

The only place I see _defaultValue being used is when the provider stack is empty

react/packages/react-dom/src/server/ReactPartialRenderer.js

Lines 692 to 694 in 36546b5

	if (this.providerIndex < 0) {
	context._currentValue = context._defaultValue;
	} else {

So I'm not sure how setting it to undefined is doing anything

[aweary]

Closing in favor of #12985