title: Crashing on iOS 11 inside of CFNetwork
body: <p>I'm seeing dozens of crashes every few hours for an app, solely in iOS 11. The stack trace doesn't help and I've got no clue since the rest of the threads are doing something completely different in each of the crash logs. Thus I'm unable to figure out what exactly is triggering it. Anyone else seeing this?</p>
  
  <p>It always seems to be this line:</p>
  
  <p><code>TCPIOConnection::_handleTLSEvent(nw_tls_context*, tls_handshake_message_t, void () block_pointer, bool) + 640</code></p>
  
  <pre><code>Crashed: com.apple.CFNetwork.Connection
  0  libsystem_kernel.dylib         0x184f05338 __pthread_kill + 8
  1  libsystem_pthread.dylib        0x1850194d4 pthread_kill$VARIANT$mp + 360
  2  libsystem_c.dylib              0x184e74b0c abort + 140
  3  libsystem_c.dylib              0x184e485f0 basename_r + 314
  4  CFNetwork                      0x185aeca34 TCPIOConnection::_handleTLSEvent(nw_tls_context*, tls_handshake_message_t, void () block_pointer, bool) + 640
  5  libnetwork.dylib               0x1862a88c0 __tcp_connection_set_tls_message_handler_on_nw_connection_block_invoke + 168
  6  libboringssl.dylib             0x1861862c0 __boringssl_context_handshake_message_handler_block_invoke + 160
  7  libdispatch.dylib              0x184d69e04 _dispatch_call_block_and_release + 24
  8  libdispatch.dylib              0x184d69dc4 _dispatch_client_callout + 16
  9  libdispatch.dylib              0x184d73ef4 _dispatch_queue_serial_drain$VARIANT$mp + 532
  10 libdispatch.dylib              0x184d74944 _dispatch_queue_invoke$VARIANT$mp + 340
  11 libdispatch.dylib              0x184d73da8 _dispatch_queue_serial_drain$VARIANT$mp + 200
  12 libdispatch.dylib              0x184d74944 _dispatch_queue_invoke$VARIANT$mp + 340
  13 libdispatch.dylib              0x184d75510 _dispatch_root_queue_drain_deferred_wlh$VARIANT$mp + 428
  14 libdispatch.dylib              0x184d7e0a8 _dispatch_workloop_worker_thread$VARIANT$mp + 620
  15 libsystem_pthread.dylib        0x1850171e8 _pthread_wqthread + 924
  16 libsystem_pthread.dylib        0x185016e40 start_wqthread + 4
  </code></pre>
  
  <p>There does seem to be one more thread common between all these crashlogs and I suspect it's from Fabric itself. Anyone else seeing this as well?</p>
  
  <pre><code>trust
  0  libsystem_kernel.dylib         0x182550a24 mach_msg_trap + 8
  1  libsystem_kernel.dylib         0x18255089c mach_msg + 72
  2  libdispatch.dylib              0x1823ece00 _dispatch_mach_send_and_wait_for_reply + 656
  3  libdispatch.dylib              0x1823ed2d0 dispatch_mach_send_with_result_and_wait_for_reply$VARIANT$mp + 64
  4  libxpc.dylib                   0x1826bf9b4 xpc_connection_send_message_with_reply_sync + 196
  5  Security                       0x1836235bc securityd_message_with_reply_sync + 176
  6  Security                       0x1836238fc securityd_send_sync_and_do + 80
  7  Security                       0x1836a5a30 __SecTrustEvaluateIfNecessary_block_invoke_3 + 432
  8  Security                       0x183674494 SecOSStatusWith + 52
  9  Security                       0x1836a5864 __SecTrustEvaluateIfNecessary_block_invoke_2 + 96
  10 libsystem_trace.dylib          0x1826a0b68 _os_activity_initiate_impl + 60
  11 Security                       0x1836a57f4 __SecTrustEvaluateIfNecessary_block_invoke + 260
  12 libdispatch.dylib              0x1823d5dc4 _dispatch_client_callout + 16
  13 libdispatch.dylib              0x1823deb94 _dispatch_queue_barrier_sync_invoke_and_complete + 56
  14 Security                       0x1836a28e8 SecTrustEvaluateIfNecessary + 520
  15 Security                       0x1836a2c10 SecTrustEvaluate + 64
  16 MyApp                          0x10359c6e0 -[FABCertificatePinner evaluateTrust:result:] (FABCertificatePinner.m:52)
  17 MyApp                          0x10359c784 -[FABCertificatePinner handleChallenge:fromURLSession:completionHandler:] (FABCertificatePinner.m:76)
  18 MyApp                          0x10358dfc8 -[ANSNetworkRetryHandler handleChallenge:fromURLSession:completionHandler:] (ANSNetworkRetryHandler.m:55)
  19 MyApp                          0x103596ff4 -[ANSAnswersController uploader:session:didReceiveChallenge:completionHandler:] (ANSAnswersController.m:528)
  20 MyApp                          0x103591ac4 -[ANSUploader URLSession:didReceiveChallenge:completionHandler:] (ANSUploader.m:347)
  21 Foundation                     0x1834116f8 __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 16
  22 Foundation                     0x1833516f4 -[NSBlockOperation main] + 72
  23 Foundation                     0x1833412d4 -[__NSOperationInternal _start:] + 868
  24 libdispatch.dylib              0x1823d5dc4 _dispatch_client_callout + 16
  25 libdispatch.dylib              0x1823dd5e8 _dispatch_block_invoke_direct$VARIANT$mp + 260
  26 libdispatch.dylib              0x1823d5dc4 _dispatch_client_callout + 16
  27 libdispatch.dylib              0x1823dd5e8 _dispatch_block_invoke_direct$VARIANT$mp + 260
  28 libdispatch.dylib              0x1823dd4b0 dispatch_block_perform$VARIANT$mp + 104
  29 Foundation                     0x1834133d0 __NSOQSchedule_f + 376
  30 libdispatch.dylib              0x1823d5dc4 _dispatch_client_callout + 16
  31 libdispatch.dylib              0x1823de454 _dispatch_continuation_pop$VARIANT$mp + 456
  32 libdispatch.dylib              0x1823dcb94 _dispatch_async_redirect_invoke$VARIANT$mp + 648
  33 libdispatch.dylib              0x1823e36d0 _dispatch_root_queue_drain + 596
  34 libdispatch.dylib              0x1823e341c _dispatch_worker_thread4 + 56
  35 libsystem_pthread.dylib        0x182683338 _pthread_wqthread + 1260
  36 libsystem_pthread.dylib        0x182682e40 start_wqthread + 4
  </code></pre>