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[Objective-C] Fix thread-safety in template

Merged Administrator requested to merge github/fork/michalcichon/fix-objc-template into master

Created by: michalcichon

  • A static set needs a static lock or we get multiple locks if the class is inherited
  • If this fix is not included and SWGObject is accessed from a second thread then we get EXC_BAD_ACCESS error (crash)

Example crash:

Crashed: asyncRealm
0  libobjc.A.dylib                0x180f82088 objc_msgSend + 8
1  CoreFoundation                 0x1811e400c -[__NSSetM containsObject:] + 180
2  MyApp                          0x10094fc28 -[SWGObject initWithDictionary:error:] + 20 (SWGObject.m:20)
3  MyApp                          0x100e019e4 -[JSONModel __importDictionary:withKeyMapper:validation:error:] + 4310850020
4  MyApp                          0x100e00a28 -[JSONModel initWithDictionary:error:] + 4310845992
5  MyApp                          0x10094fcac -[SWGObject initWithDictionary:error:] + 29 (SWGObject.m:29)
6  MyApp                          0x100e019e4 -[JSONModel __importDictionary:withKeyMapper:validation:error:] + 4310850020
7  MyApp                          0x100e00a28 -[JSONModel initWithDictionary:error:] + 4310845992
8  MyApp                          0x10094fcac -[SWGObject initWithDictionary:error:] + 29 (SWGObject.m:29)
9  MyApp                          0x100e00700 -[JSONModel initWithData:error:] + 4310845184
10 MyApp                          0x100e0088c -[JSONModel initWithString:usingEncoding:error:] + 4310845580
11 MyApp                          0x100e007c4 -[JSONModel initWithString:error:] + 4310845380
12 MyApp                          0x100aa3138 specialized static PostPersistence.deleteOldPosts(realm:) + 4307317048 (PostPersistence.swift:4307317048)
13 MyApp                          0x100aa2238 closure #1 in static PostPersistence.persistPostsAsync(_:) + 23 (PostPersistence.swift:23)
14 MyApp                          0x1009e9840 thunk for @escaping @callee_guaranteed () -> () + 4306556992 (<compiler-generated>:4306556992)
15 libdispatch.dylib              0x180f2a610 _dispatch_call_block_and_release + 24
16 libdispatch.dylib              0x180f2b184 _dispatch_client_callout + 16
17 libdispatch.dylib              0x180f08710 _dispatch_lane_serial_drain$VARIANT$armv81 + 564
18 libdispatch.dylib              0x180f09128 _dispatch_lane_invoke$VARIANT$armv81 + 400
19 libdispatch.dylib              0x180f1243c _dispatch_workloop_worker_thread + 576
20 libsystem_pthread.dylib        0x180f7af88 _pthread_wqthread + 276
21 libsystem_pthread.dylib        0x180f7dad4 start_wqthread + 8

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