Fixed an issue with nested contexts unwinding when server rendering. …

…Issue #12984 (#12985)

* Fixed an issue with nested contexts unwinding when server rendering. GitHub issue #12984

* Fixed an issue with search direction and stricter false checking

* Use decrement infix operator

* Streamlined existence checks

* Streamlined assignment. Removed redundant comment. Use null for array values

* Made prettier

* Relaxed type checking and improved comment

* Improve test coverage

packages/react-dom/src/__tests__/ReactDOMServerIntegrationNewContext-test.js

@@ -191,5 +191,63 @@ describe('ReactDOMServerIntegration', () => {
       expect(e.querySelector('#theme').textContent).toBe('light');
       expect(e.querySelector('#language').textContent).toBe('english');
     });
+
+    itRenders('nested context unwinding', async render => {
+      const Theme = React.createContext('dark');
+      const Language = React.createContext('french');
+
+      const App = () => (
+        <div>
+          <Theme.Provider value="light">
+            <Language.Provider value="english">
+              <Theme.Provider value="dark">
+                <Theme.Consumer>
+                  {theme => <div id="theme1">{theme}</div>}
+                </Theme.Consumer>
+              </Theme.Provider>
+              <Theme.Consumer>
+                {theme => <div id="theme2">{theme}</div>}
+              </Theme.Consumer>
+              <Language.Provider value="sanskrit">
+                <Theme.Provider value="blue">
+                  <Theme.Provider value="red">
+                    <Language.Consumer>
+                      {() => (
+                        <Language.Provider value="chinese">
+                          <Language.Provider value="hungarian" />
+                          <Language.Consumer>
+                            {language => <div id="language1">{language}</div>}
+                          </Language.Consumer>
+                        </Language.Provider>
+                      )}
+                    </Language.Consumer>
+                  </Theme.Provider>
+                  <Language.Consumer>
+                    {language => (
+                      <React.Fragment>
+                        <Theme.Consumer>
+                          {theme => <div id="theme3">{theme}</div>}
+                        </Theme.Consumer>
+                        <div id="language2">{language}</div>
+                      </React.Fragment>
+                    )}
+                  </Language.Consumer>
+                </Theme.Provider>
+              </Language.Provider>
+            </Language.Provider>
+          </Theme.Provider>
+          <Language.Consumer>
+            {language => <div id="language3">{language}</div>}
+          </Language.Consumer>
+        </div>
+      );
+      let e = await render(<App />);
+      expect(e.querySelector('#theme1').textContent).toBe('dark');
+      expect(e.querySelector('#theme2').textContent).toBe('light');
+      expect(e.querySelector('#theme3').textContent).toBe('blue');
+      expect(e.querySelector('#language1').textContent).toBe('chinese');
+      expect(e.querySelector('#language2').textContent).toBe('sanskrit');
+      expect(e.querySelector('#language3').textContent).toBe('french');
+    });
   });
 });

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

@@ -689,14 +689,23 @@ class ReactDOMServerRenderer {
     this.providerStack[this.providerIndex] = null;
     this.providerIndex -= 1;
     const context: ReactContext<any> = provider.type._context;
-    if (this.providerIndex < 0) {
-      context._currentValue = context._defaultValue;
+
+    // Find the closest parent provider of the same type and use its value.
+    // TODO: it would be nice to avoid this being O(N).
+    let contextPriorProvider = null;
+    for (let i = this.providerIndex; i >= 0; i--) {
+      // We assume this Flow type is correct because of the index check above
+      // and because pushProvider() enforces the correct type.
+      const priorProvider: ReactProvider<any> = (this.providerStack[i]: any);
+      if (priorProvider.type === provider.type) {
+        contextPriorProvider = priorProvider;
+        break;
+      }
+    }
+    if (contextPriorProvider !== null) {
+      context._currentValue = contextPriorProvider.props.value;
     } 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;
+      context._currentValue = context._defaultValue;
     }
   }