Avoid sharing mutable attributed string across threads in RCTBaseTextInputShadowView

Summary:
The `NSMutableAttributedString` was initialized with the same backing store as the cached attributed string on the shadow queue (background thread), but then passed to the main thread and ultimately the JS thread.

This explicitly copies the mutable attributed string into an immutable one on the shadow thread before passed off to other threads, which hopefully will address the `convertIdToFollyDynamic` crash that always includes `RCTBaseTextInputShadowView` touching an attributed string on the shadow queue in the trace.

Changelog:
[iOS][Fixed] - Possible fix for convertIdToFollyDynamic crash in RCTBaseTextInputView and RCTEventDispatcher

Reviewed By: sammy-SC

Differential Revision: D38133150

fbshipit-source-id: f371da5d17a32c3341287cd3e9730b31a98495f9

Author: christophpurrer

Libraries/Text/TextInput/RCTBaseTextInputShadowView.m

@@ -144,8 +144,7 @@ - (void)uiManagerWillPerformMounting
 
   RCTTextAttributes *textAttributes = [self.textAttributes copy];
 
-  NSMutableAttributedString *attributedText =
-    [[NSMutableAttributedString alloc] initWithAttributedString:[self attributedTextWithBaseTextAttributes:nil]];
+  NSMutableAttributedString *attributedText = [[self attributedTextWithBaseTextAttributes:nil] mutableCopy];
 
   // Removing all references to Shadow Views and tags to avoid unnecessary retaining
   // and problems with comparing the strings.
@@ -162,13 +161,13 @@ - (void)uiManagerWillPerformMounting
     [attributedText insertAttributedString:propertyAttributedText atIndex:0];
   }
 
-  BOOL isAttributedTextChanged = NO;
+  NSAttributedString *newAttributedText;
   if (![_previousAttributedText isEqualToAttributedString:attributedText]) {
     // We have to follow `set prop` pattern:
     // If the value has not changed, we must not notify the view about the change,
     // otherwise we may break local (temporary) state of the text input.
-    isAttributedTextChanged = YES;
-    _previousAttributedText = [attributedText copy];
+    newAttributedText = [attributedText copy];
+    _previousAttributedText = newAttributedText;
   }
 
   NSNumber *tag = self.reactTag;
@@ -183,10 +182,10 @@ - (void)uiManagerWillPerformMounting
     baseTextInputView.reactBorderInsets = borderInsets;
     baseTextInputView.reactPaddingInsets = paddingInsets;
 
-    if (isAttributedTextChanged) {
+    if (newAttributedText) {
       // Don't set `attributedText` if length equal to zero, otherwise it would shrink when attributes contain like `lineHeight`.
-      if (attributedText.length != 0) {
-        baseTextInputView.attributedText = attributedText;
+      if (newAttributedText.length != 0) {
+        baseTextInputView.attributedText = newAttributedText;
       } else {
         baseTextInputView.attributedText = nil;
       }