[No QA] [APP-7B2] Fix iOS deadlock in ComponentDescriptorRegistry::add() during HybridApp transition

[JakubKorytko]

Explanation of Change

Patches ComponentDescriptorRegistry::add() in React Native 0.83.1 to move descriptor construction outside the unique_lock(mutex_). The lock is now only held for the two map insertions (_registryByHandle, _registryByName), breaking the deadlock where the background thread holds unique_lock while waiting for the main queue (via RCTUnsafeExecuteOnMainQueueSync in legacy interop descriptor construction), while the main thread is blocked on shared_lock in findComponentDescriptorByHandle_DO_NOT_USE_THIS_IS_BROKEN during an animation tick.

A separate PR with a simulated reproduction exists to deterministically validate this $ #91607

Fixed Issues

$ #91292
PROPOSAL: N/A

Tests

This is a [No QA] fix - the deadlock requires a precise race condition between a native animation tick and lazy legacy interop component registration during OldDot→NewDot transition on iOS HybridApp. It cannot be reliably reproduced manually.

Probable reproduction steps (non-deterministic, narrow race window):

1. Use an iOS HybridApp build on a real device (iPhone 13 Pro or similar - device class 3)
2. Ensure the user is a migrated user (so MigratedUserWelcomeModalGuard triggers navigation)
3. Cold launch the app (first launch scenario with hasUrl: false, withBootsplash: true)
4. The app navigates Home → MigratedUserWelcomeModal_Root
5. During this initial render, if a native Animated.timing animation is actively ticking (SidePanelContextProvider runs on mount) and JS simultaneously creates a node for a not-yet-registered legacy interop component, the deadlock occurs
6. Without fix: App freezes - iOS watchdog kills it after ~2s
7. With fix: Transition completes normally, no hang

A deterministic reproduction is difficult because it requires precise timing, but it can be made more likely by adding artificial delay to LegacyViewManagerInteropComponentDescriptor construction or adding more legacy interop components. A separate PR with a simulated local reproduction exists to validate the fix deterministically: #91607

Offline tests

N/A - this is a native C++ mutex ordering fix, no network/Onyx involvement.

QA Steps

N/A, cannot be reliably reproduced without the simulated reproduction patch. The race window requires precise timing between a CADisplayLink animation tick and lazy component descriptor registration during HybridApp transition.

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
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Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[github-actions]

⚠️ This PR is possibly changing native code and/or updating libraries, it may cause problems with HybridApp. Please check if any patch updates are required in the HybridApp repo and run an AdHoc build to verify that HybridApp will not break. Ask Contributor Plus for help if you are not sure how to handle this. ⚠️

[melvin-bot]

@jayeshmangwani Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

[Copilot]

Pull request overview

This PR adds a React Native 0.83.1 patch to prevent a fatal iOS HybridApp hang by avoiding a mutex deadlock in Fabric’s ComponentDescriptorRegistry::add() during legacy interop descriptor registration.

Changes:

• Patch ComponentDescriptorRegistry::add() to construct component descriptors outside the registry mutex, only locking for final map insertions.
• Document the new patch in patches/react-native/details.md with rationale and links.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
patches/react-native/react-native+0.83.1+037+fix-deadlock-APP-7B2.patch	Moves descriptor construction outside the unique_lock to break a main-thread/background-thread deadlock scenario.
patches/react-native/details.md	Adds an entry describing the new patch, its motivation, and relevant upstream/E/App links.

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[jayeshmangwani]

Reviewer Checklist

• I have verified the author checklist is complete (all boxes are checked off).
• I verified the correct issue is linked in the ### Fixed Issues section above
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  • I verified the steps for local testing are in the Tests section
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  • I verified the steps cover any possible failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
• I checked that screenshots or videos are included for tests on all platforms
• I included screenshots or videos for tests on all platforms
• I verified that the composer does not automatically focus or open the keyboard on mobile unless explicitly intended. This includes checking that returning the app from the background does not unexpectedly open the keyboard.
• I verified tests pass on all platforms & I tested again on:
  • Android: HybridApp
  • Android: mWeb Chrome
  • iOS: HybridApp
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
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  • I verified any copy / text shown in the product is localized by adding it to src/languages/* files and using the translation method
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  • I verified any copy / text that was added to the app is grammatically correct in English. It adheres to proper capitalization guidelines (note: only the first word of header/labels should be capitalized), and is either coming verbatim from figma or has been approved by marketing (in order to get marketing approval, ask the Bug Zero team member to add the Waiting for copy label to the issue)
  • I verified proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
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Screenshots/Videos

Android: HybridApp

Android.mov

Android: mWeb Chrome

iOS: HybridApp

ios.mov

iOS: mWeb Safari

MacOS: Chrome / Safari

[jayeshmangwani]

@mountiny tested the iOS app on the simulator. I performed general testing by leaving it idle for 5 minutes, soft-killing it, and then reopening it from the background. Everything worked well, and I did not encounter any app initialization issues.

[jayeshmangwani]

LGTM 🚀

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