timing fix for RCTCxxBridge.executeApplicationScript

[ahimberg]

Summary

In one of our test apps (actually on Mac not iOS, but same code) we very consistently crash in RCTCxxBridge.executeApplicationScript when js debugging, due to a timing issue where another thread has reset _reactInstance in between the null check on self->_reactInstance and usage of it on these lines:

    } else if (self->_reactInstance) {
      self->_reactInstance->loadScriptFromString(std::make_unique<NSDataBigString>(script),

The thread doing the reset is doing so switching the executorClass to WebSocketExecutor.
In the scenario we crash, the packager has a bundle ready and quickly returns it, though its a 34MB string being passed to NSDataBigString which must be taking long enough for the other thread to get a chance to reset.

Changelog

[iOS] [Fixed] - Fix crash in RCTCxxBridge.executeApplicationScript

Test Plan

Ran apple code path in normal from bundle file and js debugging scenarios.

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[ahimberg]

alternatively an additional null check could be added after creating the NSDataBigString, this seemed a little cleaner

[zhongwuzw]

@ahimberg Hi, thanks for the PR! Would you mind show the crash stack backtrace? I think we already ensure read and write it on JS thread.

[ahimberg]

Sure, this is the callstack where I see the crash (the JS thread is where the reset happened):

Thread 14 Queue : com.apple.root.user-interactive-qos (concurrent)
#0	0x000000000000f000 in 0x0000f000 ()
#1	0x0000000100323fd4 in ::__51-[RCTCxxBridge executeApplicationScript:url:async:]_block_invoke() at src/react-native/React/CxxBridge/RCTCxxBridge.mm:1317
#2	0x000000010032abfe in decltype(std::__1::forward<void () block_pointer __strong&>(fp)()) std::__1::__invoke<void () block_pointer __strong&>(&&, decltype(std::__1::forward<void () block_pointer __strong&>(fp)())&&...) at /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/type_traits:4339
#3	0x000000010032abad in void std::__1::__invoke_void_return_wrapper<void>::__call<void () block_pointer __strong&>(void () block_pointer __strong&) at /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/__functional_base:349
#4	0x0000000100329901 in std::__1::__function::__func<void () block_pointer __strong, std::__1::allocator<std::__1::allocator>, void ()>::operator()() at /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/functional:1562
#5	0x000000010037b525 in std::__1::function<void ()>::operator()() const at /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/functional:1913
#6	0x000000010037b248 in facebook::react::tryAndReturnError(std::__1::function<void ()> const&) at src/react-native/React/CxxModule/RCTCxxUtils.mm:72
#7	0x0000000100314590 in ::-[RCTCxxBridge _tryAndHandleError:](dispatch_block_t) at src/react-native/React/CxxBridge/RCTCxxBridge.mm:264
#8	0x00000001003237a8 in ::-[RCTCxxBridge executeApplicationScript:url:async:](NSData *, NSURL *, BOOL) at rc/react-native/React/CxxBridge/RCTCxxBridge.mm:1300
#9	0x000000010032314c in ::-[RCTCxxBridge enqueueApplicationScript:url:onComplete:](NSData *, NSURL *, dispatch_block_t) at /src/react-native/React/CxxBridge/RCTCxxBridge.mm:1280
#10	0x000000010031dddc in ::-[RCTCxxBridge executeSourceCode:sync:](NSData *, BOOL) at /src/react-native/React/CxxBridge/RCTCxxBridge.mm:904
#11	0x000000010031658f in ::__21-[RCTCxxBridge start]_block_invoke.191() at /src/react-native/React/CxxBridge/RCTCxxBridge.mm:380
#12	0x0000000102fb4e7c in _dispatch_call_block_and_release ()
#13	0x0000000102fb5f1b in _dispatch_client_callout ()
#14	0x0000000102fc8a06 in _dispatch_root_queue_drain ()
#15	0x0000000102fc92da in _dispatch_worker_thread2 ()
#16	0x000000010302f0b7 in _pthread_wqthread ()
#17	0x000000010302ee01 in start_wqthread ()
Enqueued from com.apple.main-thread (Thread 1) Queue : com.apple.main-thread (serial)
#0	0x0000000102fb71d9 in _dispatch_group_wake ()
#1	0x0000000102fc45af in _dispatch_main_queue_callback_4CF ()
#2	0x00007fff2f1c0cd7 in __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ ()
#3	0x00007fff2f1c0401 in __CFRunLoopRun ()
#4	0x00007fff2f1bf8be in CFRunLoopRunSpecific ()
#5	0x00007fff2e4ab96b in RunCurrentEventLoopInMode ()
#6	0x00007fff2e4ab6a5 in ReceiveNextEventCommon ()
#7	0x00007fff2e4ab436 in _BlockUntilNextEventMatchingListInModeWithFilter ()
#8	0x00007fff2c845987 in _DPSNextEvent ()
#9	0x00007fff2c84471f in -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] ()
#10	0x00007fff2c83e83c in -[NSApplication run] ()
#11	0x00007fff2c82dd7c in NSApplicationMain ()
#12	0x0000000100103c3d in main at /Users/######
#13	0x00007fff5b0ee3d5 in start ()

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[basisworldwide]

Will this change be merged in anytime soon? It resolves the timing crash for me on app start.

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

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This pull request was successfully merged by @ahimberg in 0c2db32.

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