Fixed race condition in RCTUIManager's mounting process

Summary:
See the comment in code.
This picture also illustrates the problem: https://cl.ly/250z0D1B0c17.
https://pxl.cl/ckBq

Reviewed By: fkgozali

Differential Revision: D7343566

fbshipit-source-id: e45a691add5e43434dea8708be4b76ff18e52131

React/Modules/RCTUIManager.m

@@ -69,6 +69,9 @@ NSString *const RCTUIManagerWillUpdateViewsDueToContentSizeMultiplierChangeNotif
 
   // Keyed by viewName
   NSDictionary *_componentDataByName;
+
+  BOOL _isMountingInProgress;
+  BOOL _isRemountingNeeded;
 }
 
 @synthesize bridge = _bridge;
@@ -1046,6 +1049,19 @@ RCT_EXPORT_METHOD(dispatchViewManagerCommand:(nonnull NSNumber *)reactTag
  */
 - (void)_layoutAndMount
 {
+  // Mounting is (usually, not always) an asynchronous process.
+  // So, we have to ensure that there is no any case of self-interleaving
+  // (when another mourning process is initiated before the previous
+  // one finishes) here.
+  if (_isMountingInProgress) {
+    // In a case when another mounting process hasn't completed yet,
+    // we have to run another pass right after it finishes.
+    _isRemountingNeeded = YES;
+    return;
+  }
+
+  _isMountingInProgress = YES;
+
   [self _dispatchPropsDidChangeEvents];
   [self _dispatchChildrenDidChangeEvents];
 
@@ -1063,6 +1079,13 @@ RCT_EXPORT_METHOD(dispatchViewManagerCommand:(nonnull NSNumber *)reactTag
 
   [self flushUIBlocksWithCompletion:^{
     [self->_observerCoordinator uiManagerDidPerformMounting:self];
+
+    self->_isMountingInProgress = NO;
+
+    if (self->_isRemountingNeeded) {
+      self->_isRemountingNeeded = NO;
+      [self _layoutAndMount];
+    }
   }];
 }