[iOS] convert NSNull to nil before checking type in readAsDataURL

[haileyok]

Summary:

This issue original arose out of bluesky-social/social-app#5100. Copying the description (with my general understanding of the problem) from the patch PR to here as well.

There's a crash that comes up in the following, pretty specific scenario:

• Have a response that has an empty body
• Do not include a content-type header in the response
• Set the x-content-type-options header to nosniff

RN handles the response for a request in this block of code:

react-native/packages/react-native/Libraries/Blob/RCTBlobManager.mm

Lines 314 to 326 in 303e0ed

	- (id)handleNetworkingResponse:(NSURLResponse *)response data:(NSData *)data
	{
	// An empty body will have nil for data, in this case we need to return
	// an empty blob as per the XMLHttpRequest spec.
	data = data ?: [NSData new];
	return @{
	@"blobId" : [self store:data],
	@"offset" : @0,
	@"size" : @(data.length),
	@"name" : RCTNullIfNil([response suggestedFilename]),
	@"type" : RCTNullIfNil([response MIMEType]),
	};
	}

Here, we see that values of nil - which [response MIMEType] will return when no content-type is provided in the response and the actual type cannot be determined (https://developer.apple.com/documentation/foundation/nsurlresponse/1411613-mimetype) - gets converted to NSNull by RCTNullIfNil.

When we get back over to readAsDataURL, we see that we grab the type from the dictionary and check if its nil before calling length on the string.

react-native/packages/react-native/Libraries/Blob/RCTFileReaderModule.mm

Lines 74 to 77 in 303e0ed

	NSString *type = [RCTConvert NSString:blob[@"type"]];
	NSString *text = [NSString stringWithFormat:@"data:%@;base64,%@",
	type != nil && [type length] > 0 ? type : @"application/octet-stream",
	[data base64EncodedStringWithOptions:0]];

However, this check is dubious, because the value will never actually be nil. It will always either be NSString or NSNull because of the RCTNullIfNil call made above and [RCTConvert NSString] seems to just return the input if it is NSNull.

Changelog:

[IOS] [FIXED] - Convert NSNull to nil before checking type in readAsDataURL

Test Plan:

This is a little awkward to test, but essentially this comes up in the following scenario that is described (and "tested" as being fixed by tweaking) in bluesky-social/social-app#5100. I have personally tested by using Cloudflare rules to add/remove that particular header from an empty body response. You could also test this with a little local web server if you want.

Before

Screen.Recording.2024-09-24.at.2.20.07.PM.mov

After

Screen.Recording.2024-09-24.at.2.23.09.PM.mov

[facebook-github-bot]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@cipolleschi merged this pull request in 99ab845.

[react-native-bot]

This pull request was successfully merged by @haileyok in 99ab845

^{When will my fix make it into a release? | How to file a pick request?}