Warn about setState inside getChildContext

Stack uses a module called ReactInvalidSetStateWarningHook, but all it
does is warn about setState inside getChildContext. Doesn't seem worth
keeping around, so I didn't use it for Fiber, but if I'm mistaken I'll
change it.

scripts/fiber/tests-passing-except-dev.txt

@@ -106,7 +106,6 @@ src/renderers/__tests__/ReactComponentTreeHook-test.native.js
 * does not report top-level wrapper as a root
 
 src/renderers/__tests__/ReactCompositeComponent-test.js
-* should warn about `setState` in getChildContext
 * should disallow nested render calls
 
 src/renderers/__tests__/ReactHostOperationHistoryHook-test.js

scripts/fiber/tests-passing.txt

@@ -578,6 +578,7 @@ src/renderers/__tests__/ReactCompositeComponent-test.js
 * should warn about `setState` on unmounted components
 * should silently allow `setState`, not call cb on unmounting components
 * should warn about `setState` in render
+* should warn about `setState` in getChildContext
 * should cleanup even if render() fatals
 * should call componentWillUnmount before unmounting
 * should warn when shouldComponentUpdate() returns undefined

src/renderers/shared/fiber/ReactFiberContext.js

@@ -47,6 +47,21 @@ let didPerformWorkStackCursor : StackCursor<boolean> = createCursor(false);
 // pushed the next context provider, and now need to merge their contexts.
 let previousContext : Object = emptyObject;
 
+if (__DEV__) {
+  var _isProcessingChildContext = false;
+  var onBeginProcessingChildContext = function() {
+    _isProcessingChildContext = true;
+  };
+  exports.onBeginProcessingChildContext = onBeginProcessingChildContext;
+  var onEndProcessingChildContext = function() {
+    _isProcessingChildContext = false;
+  };
+  exports.onEndProcessingChildContext = onEndProcessingChildContext;
+  exports.isProcessingChildContext = function() : boolean {
+    return _isProcessingChildContext;
+  };
+}
+
 function getUnmaskedContext(workInProgress : Fiber) : Object {
   const hasOwnContext = isContextProvider(workInProgress);
   if (hasOwnContext) {
@@ -144,6 +159,10 @@ exports.pushTopLevelContextObject = function(fiber : Fiber, context : Object, di
 };
 
 function processChildContext(fiber : Fiber, parentContext : Object, isReconciling : boolean): Object {
+  if (__DEV__) {
+    onBeginProcessingChildContext();
+  }
+
   const instance = fiber.stateNode;
   const childContextTypes = fiber.type.childContextTypes;
 
@@ -189,6 +208,11 @@ function processChildContext(fiber : Fiber, parentContext : Object, isReconcilin
     checkReactTypeSpec(childContextTypes, childContext, 'child context', name);
     ReactDebugCurrentFrame.current = null;
   }
+
+  if (__DEV__) {
+    onEndProcessingChildContext();
+  }
+
   return {...parentContext, ...childContext};
 }
 exports.processChildContext = processChildContext;

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -90,6 +90,10 @@ if (__DEV__) {
   var warning = require('warning');
   var ReactFiberInstrumentation = require('ReactFiberInstrumentation');
   var ReactDebugCurrentFiber = require('ReactDebugCurrentFiber');
+  var {
+    isProcessingChildContext,
+    onEndProcessingChildContext,
+  } = require('ReactFiberContext');
 
   var warnAboutUpdateOnUnmounted = function(instance : ReactClass<any>) {
     const ctor = instance.constructor;
@@ -103,14 +107,21 @@ if (__DEV__) {
     );
   };
 
-  var warnAboutUpdateInRender = function(instance : ReactClass<any>) {
-    warning(
-      ReactCurrentOwner.current == null,
-      'Cannot update during an existing state transition (such as within ' +
-      '`render` or another component\'s constructor). Render methods should ' +
-      'be a pure function of props and state; constructor side-effects are ' +
-      'an anti-pattern, but can be moved to `componentWillMount`.'
-    );
+  var warnAboutInvalidUpdates = function(instance : ReactClass<any>) {
+    if (isProcessingChildContext()) {
+      warning(
+        false,
+        'setState(...): Cannot call setState() inside getChildContext()',
+      );
+    } else if (ReactCurrentOwner.current != null) {
+      warning(
+        false,
+        'Cannot update during an existing state transition (such as within ' +
+        '`render` or another component\'s constructor). Render methods should ' +
+        'be a pure function of props and state; constructor side-effects are ' +
+        'an anti-pattern, but can be moved to `componentWillMount`.'
+      );
+    }
   };
 }
 
@@ -869,6 +880,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     ReactCurrentOwner.current = null;
     if (__DEV__) {
       ReactDebugCurrentFiber.current = null;
+      onEndProcessingChildContext();
     }
     // It is no longer valid because this unit of work failed.
     nextUnitOfWork = null;
@@ -1115,7 +1127,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(config : HostConfig<T, P,
     if (__DEV__) {
       if (fiber.tag === ClassComponent) {
         const instance = fiber.stateNode;
-        warnAboutUpdateInRender(instance);
+        warnAboutInvalidUpdates(instance);
       }
     }