Facebook-ios-sdk: Crash on FBSDKAppLinkNavigation

[Kry256]

Checklist

• [x] I've updated to the latest released version of the SDK
• [x] I've searched for existing GitHub issues
• [x] I've looked for existing answers on Stack Overflow, the Facebook Developer Community Forum and the Facebook Developers Group
• [x] I've read the Code of Conduct
• [x] This issue is not security related and can safely be disclosed publicly on GitHub

Environment

Xcode Version: 12.2
Swift Version: 5.3
Installation Platform & Verison: Swift Package
The crash happens on all Facebook iOS 8.x.x SDK versions.

Steps to Reproduce

I'm facing into a crash when invoking
+ (void)resolveAppLink:(NSURL *)destination handler:(FBSDKAppLinkBlock)handler
of FBSDKAppLinkNavigation class

I use it inside
- (BOOL)application:(UIApplication *)application continueUserActivity:(NSUserActivity *)userActivity restorationHandler:(void(^)(NSArray<id<UIUserActivityRestoring>> * __nullable restorableObjects))restorationHandler
of my app delegate class in this way
[FBSDKAppLinkNavigation resolveAppLink:userActivity.webpageURL
handler:^(FBSDKAppLink * _Nullable appLink, NSError * _Nullable error) {

Every time the method is invoked, it makes the app crash, below the log, hoping it could be useful

I installed FacebookCore SDK via swift package (actually 8.2.0 version), so I was able to navigate into the code, and I saw that this crash happens in the line number 167 of FBSDKWebViewAppLinkResolver class, specifically when try to initialize an MKWebView inside the method
- (void)appLinkFromURL:(NSURL *)url handler:(FBSDKAppLinkBlock)handler
..
..
WKWebView *webView = [[WKWebView alloc] init]; (line 167)

At the very beginning of the method I can see those lines
dispatch_async(dispatch_get_main_queue(), ^{
[self followRedirects:url handler:^(NSDictionary<NSString *,id> *result, NSError * _Nullable error) {

IMO I think that followRedirects:url are not responding on main thread, so maybe putting
dispatch_async(dispatch_get_main_queue(), ^{
inside the followRedirects:url handler, could solve it.

Anyone else with the same issue?

[joesus]

I'm trying to work out what UI is being called from a background thread since the appLinkFromURL:handler: callback is already dispatched on the main thread and that's what would display UI.

I think it might have to do with how you're using

- (BOOL)application:(UIApplication *)application continueUserActivity:(NSUserActivity *)userActivity restorationHandler:(void(^)(NSArray<id<UIUserActivityRestoring>> * __nullable restorableObjects))restorationHandler

According to the Apple Docs:

restorationHandler
A block to execute if your app creates objects to perform the task. Calling this block is optional and you can copy this block > and call it at a later time. When calling a saved copy of the block, you must call it from the app’s main thread.

Can you confirm that you are calling the restoration handler on the main thread?

[Kry256]

Yes I can confirm that.

As I mentioned, my suspicious is that the issue is inside the completion handler of
[self followRedirects:url handler:^(NSDictionary<NSString *,id> *result, NSError * _Nullable error) {
It is not sufficient to call the entire method on the main thread, if the completion handler responds on another thread.

Taking a look at the code, that handler is called as a completion handler of an NSURLSessionDataTask that responds in a thread different than the main

[Kry256]

I updated to master branch, now I can facing another crash invoking the same method, but now the class involved isFBSDKWebViewAppLinkResolverWebViewDelegate

In particular there is a missed check inside this method

- (void)webView:(WKWebView *)webView decidePolicyForNavigationAction:(WKNavigationAction *)navigationAction decisionHandler:(void (^)(WKNavigationActionPolicy))decisionHandler
{
if (self.hasLoaded) {
self.didFinishLoad(webView);
decisionHandler(WKNavigationActionPolicyCancel);
}
self.hasLoaded = YES;
decisionHandler(WKNavigationActionPolicyAllow);
}`

that makes the app crash because of

Thread 1: "Completion handler passed to -[FBSDKWebViewAppLinkResolverWebViewDelegate webView:decidePolicyForNavigationAction:decisionHandler:] was called more than once"

[joesus]

This fix looks reasonable enough. Can you share a little more about the use case here? I'm trying to sort out how to test the fixes.

Is it accurate that you're handling a universal link, extracting app link data out of the metadata from that universal link, and that the metadata includes a al:ios:url meta tag that has a url that redirects?

[Kry256]

Yes it is correct, it is exactely what I'm trying to do.

Unfortunately I can't share with you the link(s) that I'm using, btw this issues started for me when moved from the old Bolt framework to this one integrated in CoreKit

[joesus]

This fix should make it into the next patch release. Thanks for the help!