[0.73-stable] Make socketConnections NSDictionary thread-safe (#1986)

#### Please select one of the following
- [ ] I am removing an existing difference between facebook/react-native
and microsoft/react-native-macos 👍
- [ ] I am cherry-picking a change from Facebook's react-native into
microsoft/react-native-macos 👍
- [x] I am making a fix / change for the macOS implementation of
react-native
- [ ] I am making a change required for Microsoft usage of react-native

## Summary:

Cherry-pick of commits from #1980.

## Changelog:

[IOS] [FIXED] - Improve `RCTInspectorDevServerHelper` thread safety

## Test Plan:

Tested this change in an internal app that consumes React Native and
confirmed that it doesn't crash. (Without this change, crashes happened
intermittently but frequently enough that it was noticeable.)

Author: amgleitman

packages/react-native/React/DevSupport/RCTInspectorDevServerHelper.mm

@@ -61,12 +61,15 @@ @implementation RCTInspectorDevServerHelper
 RCT_NOT_IMPLEMENTED(-(instancetype)init)
 
 static NSMutableDictionary<NSString *, RCTInspectorPackagerConnection *> *socketConnections = nil;
+static NSLock *connectionsLock = [NSLock new];
 
 static void sendEventToAllConnections(NSString *event)
 {
+  [connectionsLock lock]; // [macOS]
   for (NSString *socketId in socketConnections) {
     [socketConnections[socketId] sendEventToAllConnections:event];
   }
+  [connectionsLock unlock]; // [macOS]
 }
 
 + (void)openDebugger:(NSURL *)bundleURL withErrorMessage:(NSString *)errorMessage
@@ -101,9 +104,11 @@ + (RCTInspectorPackagerConnection *)connectWithBundleURL:(NSURL *)bundleURL
   // Note, using a static dictionary isn't really the greatest design, but
   // the packager connection does the same thing, so it's at least consistent.
   // This is a static map that holds different inspector clients per the inspectorURL
+  [connectionsLock lock]; // [macOS]
   if (socketConnections == nil) {
     socketConnections = [NSMutableDictionary new];
   }
+  [connectionsLock unlock]; // [macOS]
 
   NSString *key = [inspectorURL absoluteString];
   // [macOS safety check to avoid a crash
@@ -112,18 +117,23 @@ + (RCTInspectorPackagerConnection *)connectWithBundleURL:(NSURL *)bundleURL
     return nil;
   }
   // macOS]
-  RCTInspectorPackagerConnection *connection = socketConnections[key];
+
+  RCTInspectorPackagerConnection *connection;
+
+  [connectionsLock lock]; // [macOS]
+  connection = socketConnections[key];
   if (!connection || !connection.isConnected) {
     connection = [[RCTInspectorPackagerConnection alloc] initWithURL:inspectorURL];
     // [macOS safety check to avoid a crash
-    if (connection == nil) {
+    if (connection != nil) {
+      socketConnections[key] = connection;
+      [connection connect];
+    } else {
       RCTLogError(@"failed to initialize RCTInspectorPackagerConnection");
-      return nil;
     }
     // macOS]
-    socketConnections[key] = connection;
-    [connection connect];
   }
+  [connectionsLock unlock]; // [macOS]
 
   return connection;
 }