Avoid race conditions when setting RCTNetworkTask status

[pxpeterxu]

Summary

While using a library that made a call to React Native's "loadImageWithURLRequest", I noticed that once in every 5 calls, we'd get the following error message:

Task orphaned for request <NSMutableURLRequest: [[SOME_HEX_CODE]]> { URL: [[IMG_URL]] }

This was despite everything working just fine: the image load would call the success callback without any problems.

For more context: loadImageWithURLRequest was called from a thread that had a lower priority than default.

After investigating this by stepping through with XCode, what was happening was that:

1. Thread 1: loadImageWithURLRequest would call [task start] on a NetworkTask to send the request for the image
2. Thread 1: [task start] would send the request using the corresponding handler
3. In my case, I was using a ph:// photo/asset library URL, which gets handled by RCTAssetsLibraryRequestHandler. In this case, even though the request was sent from a background thread, the result seems to always be returned on the main thread
4. Thread 2: After completing the request:
   • RCTAssetsLibraryRequestHandler will call NetworkTask's (void)URLRequest:(id)requestToken didCompleteWithError:(NSError *)error
   • Which sets the status to RCTNetworkTaskFinished
   • AND then calls NetworkTask's invalidate
   • Which sets the _requestToken to nil
5. Thread 1: Some time later, the thread resumes, and sets the status to RCTNetworkTaskInProgress, even though it's actually already finished!
6. Thread 1: Later on, when the ImageLoader iterates through the network tasks again, it finds the odd case where there's no requestToken yet it's marked as in progress.

This may address some, but not all causes of #12152

Changelog

[iOS] [Fixed] - Fix race conditions in NetworkTask that caused ImageLoader to report Task Orphaned

Test Plan

Tested using RNTester; also, verified that the warning messages for "Task orphaned" no longer appear in test app

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[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	6,764,741	0
android	hermes	armeabi-v7a	6,432,395	0
android	hermes	x86	7,152,590	0
android	hermes	x86_64	7,042,493	0
android	jsc	arm64-v8a	8,936,131	0
android	jsc	armeabi-v7a	8,596,166	0
android	jsc	x86	8,767,068	0
android	jsc	x86_64	9,342,599	0

Base commit: 8d613c6

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: 8d613c6

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