Add back concurrent renderer warning

Only warn for two concurrent primary or two concurrent secondary renderers.

packages/react-reconciler/src/ReactFiberNewContext.js

@@ -18,13 +18,21 @@ export type NewContext = {
   getContextChangedBits(context: ReactContext<any>): number,
 };
 
+import warning from 'fbjs/lib/warning';
+
 export default function(stack: Stack, isPrimaryRenderer: boolean) {
   const {createCursor, push, pop} = stack;
 
   const providerCursor: StackCursor<Fiber | null> = createCursor(null);
   const valueCursor: StackCursor<mixed> = createCursor(null);
   const changedBitsCursor: StackCursor<number> = createCursor(0);
 
+  let rendererSigil;
+  if (__DEV__) {
+    // Use this to detect multiple renderers using the same context
+    rendererSigil = {};
+  }
+
   function pushProvider(providerFiber: Fiber): void {
     const context: ReactContext<any> = providerFiber.type._context;
 
@@ -35,13 +43,31 @@ export default function(stack: Stack, isPrimaryRenderer: boolean) {
 
       context._currentValue = providerFiber.pendingProps.value;
       context._changedBits = providerFiber.stateNode;
+      if (__DEV__) {
+        warning(
+          context._currentRenderer === null ||
+            context._currentRenderer === rendererSigil,
+          'Detected multiple renderers concurrently rendering the ' +
+            'same context provider. This is currently unsupported.',
+        );
+        context._currentRenderer = rendererSigil;
+      }
     } else {
       push(changedBitsCursor, context._changedBits_secondary, providerFiber);
       push(valueCursor, context._currentValue_secondary, providerFiber);
       push(providerCursor, providerFiber, providerFiber);
 
       context._currentValue_secondary = providerFiber.pendingProps.value;
       context._changedBits_secondary = providerFiber.stateNode;
+      if (__DEV__) {
+        warning(
+          context._currentRenderer_secondary === null ||
+            context._currentRenderer_secondary === rendererSigil,
+          'Detected multiple renderers concurrently rendering the ' +
+            'same context provider. This is currently unsupported.',
+        );
+        context._currentRenderer_secondary = rendererSigil;
+      }
     }
   }
 

packages/react-reconciler/src/__tests__/ReactNewContext-test.internal.js

@@ -724,6 +724,46 @@ describe('ReactNewContext', () => {
     }
   });
 
+  it('warns if multiple renderers concurrently render the same context', () => {
+    spyOnDev(console, 'error');
+    const Context = React.createContext(0);
+
+    function Foo(props) {
+      ReactNoop.yield('Foo');
+      return null;
+    }
+
+    function App(props) {
+      return (
+        <Context.Provider value={props.value}>
+          <Foo />
+          <Foo />
+        </Context.Provider>
+      );
+    }
+
+    ReactNoop.render(<App value={1} />);
+    // Render past the Provider, but don't commit yet
+    ReactNoop.flushThrough(['Foo']);
+
+    // Get a new copy of ReactNoop
+    jest.resetModules();
+    ReactFeatureFlags = require('shared/ReactFeatureFlags');
+    React = require('react');
+    ReactNoop = require('react-noop-renderer');
+
+    // Render the provider again using a different renderer
+    ReactNoop.render(<App value={1} />);
+    ReactNoop.flush();
+
+    if (__DEV__) {
+      expect(console.error.calls.argsFor(0)[0]).toContain(
+        'Detected multiple renderers concurrently rendering the same ' +
+          'context provider. This is currently unsupported',
+      );
+    }
+  });
+
   it('warns if consumer child is not a function', () => {
     spyOnDev(console, 'error');
     const Context = React.createContext(0);

packages/react/src/ReactContext.js

@@ -57,6 +57,7 @@ export function createContext<T>(
 
   if (__DEV__) {
     context._currentRenderer = null;
+    context._currentRenderer_secondary = null;
   }
 
   return context;

packages/shared/ReactTypes.js

@@ -91,6 +91,7 @@ export type ReactContext<T> = {
 
   // DEV only
   _currentRenderer?: Object | null,
+  _currentRenderer_secondary?: Object | null,
 };
 
 export type ReactPortal = {