Nested event loops

[parasyte]

This application nests the winit event loop inside the tokio event loop:

example-blog-client/src/main.rs

Lines 17 to 26 in 7b42b9a

	#[tokio::main]
	async fn main() {
	let native_options = eframe::NativeOptions::default();
	eframe::run_native(
	"Blog-Demo for async / tokio",
	native_options,
	Box::new(|cc| Box::new(BlogClient::new(cc))),
	)
	.unwrap();
	}

Doing this is undefined behavior. It causes serious problems on platforms like macOS, and blocks one of the tokio worker threads. This flavor of UB is visible on macOS where the app will crash with an uncaught exception when closing the window:

Trace/BPT trap: 5

Process:               example_blog_client [88174]
Path:                  /Users/USER/*/example_blog_client
Identifier:            example_blog_client
Version:               ???
Code Type:             ARM-64 (Native)
Parent Process:        bash [21143]
Responsible:           Terminal [7005]
User ID:               501

Date/Time:             2024-10-23 07:32:37.4558 -0700
OS Version:            macOS 14.6.1 (23G93)
Report Version:        12
Anonymous UUID:        CFCF464F-AF6C-A205-19C6-73C7EBE19B5C

Sleep/Wake UUID:       F6360395-3827-4373-A55D-758348322FE5

Time Awake Since Boot: 1000000 seconds
Time Since Wake:       329 seconds

System Integrity Protection: enabled

Crashed Thread:        0  main  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_BREAKPOINT (SIGTRAP)
Exception Codes:       0x0000000000000001, 0x000000018e10f758

Termination Reason:    Namespace SIGNAL, Code 5 Trace/BPT trap: 5
Terminating Process:   exc handler [88174]

Application Specific Backtrace 0:
0   CoreFoundation                      0x000000018a64a2ec __exceptionPreprocess + 176
1   libobjc.A.dylib                     0x000000018a12e158 objc_exception_throw + 60
2   Foundation                          0x000000018b6fc36c -[NSObject(NSKeyValueObserverRegistration) _removeObserver:forProperty:] + 628
3   Foundation                          0x000000018b6fc0a4 -[NSObject(NSKeyValueObserverRegistration) removeObserver:forKeyPath:] + 136
4   Foundation                          0x000000018b6fbd70 -[NSObject(NSKeyValueObserverRegistration) removeObserver:forKeyPath:context:] + 196
5   AppKit                              0x000000018e5393e4 -[_NSTouchBarFinderObservation invalidate] + 264
6   AppKit                              0x000000018e53a738 ___NSTouchBarFinderSetNeedsUpdateOnMain_block_invoke_2 + 628
7   AppKit                              0x000000018ded7cec NSDisplayCycleObserverInvoke + 168
8   AppKit                              0x000000018ded7948 NSDisplayCycleFlush + 644
9   QuartzCore                          0x00000001927ca98c _ZN2CA11Transaction19run_commit_handlersE18CATransactionPhase + 120
10  QuartzCore                          0x00000001927c972c _ZN2CA11Transaction6commitEv + 320
11  AppKit                              0x000000018df5bac8 __62+[CATransaction(NSCATransaction) NS_setFlushesWithDisplayLink]_block_invoke + 272
12  AppKit                              0x000000018e91c4e4 ___NSRunLoopObserverCreateWithHandler_block_invoke + 64
13  CoreFoundation                      0x000000018a5d487c __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 36
14  CoreFoundation                      0x000000018a5d4768 __CFRunLoopDoObservers + 536
15  CoreFoundation                      0x000000018a5d3d94 __CFRunLoopRun + 776
16  CoreFoundation                      0x000000018a5d3434 CFRunLoopRunSpecific + 608
17  HIToolbox                           0x0000000194d7d19c RunCurrentEventLoopInMode + 292
18  HIToolbox                           0x0000000194d7ce2c ReceiveNextEventCommon + 220
19  HIToolbox                           0x0000000194d7cd30 _BlockUntilNextEventMatchingListInModeWithFilter + 76
20  AppKit                              0x000000018de32cc8 _DPSNextEvent + 660
21  AppKit                              0x000000018e6294d0 -[NSApplication(NSEventRouting) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 700
22  AppKit                              0x000000018de25ffc -[NSApplication run] + 476
23  example_blog_client                 0x000000010216cc60 _ZN5winit13platform_impl8platform10event_loop18EventLoop$LT$T$GT$10run_return17hd204e57310b01024E + 380
24  example_blog_client                 0x000000010217ae00 _ZN6eframe6native3run14run_and_return17h98b04053861a1c38E + 572
25  example_blog_client                 0x000000010216b268 _ZN6eframe6native3run16glow_integration8run_glow17h73e6eff3e463615aE + 596
26  example_blog_client                 0x000000010217b29c _ZN6eframe10run_native17hf2ac08c50c3f68a8E + 428
27  example_blog_client                 0x0000000102075a14 _ZN5tokio7runtime4park16CachedParkThread8block_on17hfbfd26e6bada6b29E + 224
28  example_blog_client                 0x00000001020796e4 _ZN5tokio7runtime7runtime7Runtime8block_on17h6ea9cf69a2a8ee76E + 88
29  example_blog_client                 0x0000000102052dbc _ZN19example_blog_client4main17h0eddff21d4633986E + 112
30  example_blog_client                 0x0000000102077ba4 _ZN3std3sys9backtrace28__rust_begin_short_backtrace17h1f48f483b07c3691E + 12
31  example_blog_client                 0x0000000102074398 _ZN3std2rt10lang_start28_$u7b$$u7b$closure$u7d$$u7d$17h9a6c470f04bee322E.llvm.18315569416295985373 + 16
32  example_blog_client                 0x00000001023bbdc0 _ZN3std2rt19lang_start_internal17h738180e634528c71E + 812
33  example_blog_client                 0x0000000102052e7c main + 52
34  dyld                                0x000000018a16b154 start + 2476


Thread 0 Crashed:: main Dispatch queue: com.apple.main-thread
0   AppKit                        	       0x18e10f758 -[NSApplication _crashOnException:] + 240
1   AppKit                        	       0x18df5bc3c __62+[CATransaction(NSCATransaction) NS_setFlushesWithDisplayLink]_block_invoke + 644
2   AppKit                        	       0x18e91c4e4 ___NSRunLoopObserverCreateWithHandler_block_invoke + 64
3   CoreFoundation                	       0x18a5d487c __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 36
4   CoreFoundation                	       0x18a5d4768 __CFRunLoopDoObservers + 536
5   CoreFoundation                	       0x18a5d3d94 __CFRunLoopRun + 776
6   CoreFoundation                	       0x18a5d3434 CFRunLoopRunSpecific + 608
7   HIToolbox                     	       0x194d7d19c RunCurrentEventLoopInMode + 292
8   HIToolbox                     	       0x194d7ce2c ReceiveNextEventCommon + 220
9   HIToolbox                     	       0x194d7cd30 _BlockUntilNextEventMatchingListInModeWithFilter + 76
10  AppKit                        	       0x18de32cc8 _DPSNextEvent + 660
11  AppKit                        	       0x18e6294d0 -[NSApplication(NSEventRouting) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 700
12  AppKit                        	       0x18de25ffc -[NSApplication run] + 476
13  example_blog_client           	       0x10216cc60 winit::platform_impl::platform::event_loop::EventLoop$LT$T$GT$::run_return::hd204e57310b01024 + 380
14  example_blog_client           	       0x10217ae00 eframe::native::run::run_and_return::h98b04053861a1c38 + 572
15  example_blog_client           	       0x10216b268 eframe::native::run::glow_integration::run_glow::h73e6eff3e463615a + 596
16  example_blog_client           	       0x10217b29c eframe::run_native::hf2ac08c50c3f68a8 + 428
17  example_blog_client           	       0x102075a14 tokio::runtime::park::CachedParkThread::block_on::hfbfd26e6bada6b29 + 224
18  example_blog_client           	       0x1020796e4 tokio::runtime::runtime::Runtime::block_on::h6ea9cf69a2a8ee76 + 88
19  example_blog_client           	       0x102052dbc example_blog_client::main::h0eddff21d4633986 + 112
20  example_blog_client           	       0x102077ba4 std::sys::backtrace::__rust_begin_short_backtrace::h1f48f483b07c3691 + 12
21  example_blog_client           	       0x102074398 std::rt::lang_start::_$u7b$$u7b$closure$u7d$$u7d$::h9a6c470f04bee322 (.llvm.18315569416295985373) + 16
22  example_blog_client           	       0x1023bbdc0 std::rt::lang_start_internal::h738180e634528c71 + 812
23  example_blog_client           	       0x102052e7c main + 52
24  dyld                          	       0x18a16b154 start + 2476

Thread 1:: tokio-runtime-worker
0   libsystem_kernel.dylib        	       0x18a4baf40 kevent + 8
1   example_blog_client           	       0x1023a2b1c mio::poll::Poll::poll::h2e4310d4ea0fbade + 116
2   example_blog_client           	       0x10238fe98 tokio::runtime::io::Driver::turn::hf0008c1ab73a6f72 (.llvm.16663742601449969147) + 88
3   example_blog_client           	       0x102390660 tokio::runtime::time::Driver::park_internal::hd799af5bbfe1604b (.llvm.16663742601449969147) + 688
4   example_blog_client           	       0x10238aa80 tokio::runtime::scheduler::multi_thread::park::Parker::park::hd1c8149a5a640831 + 440
5   example_blog_client           	       0x1023853d4 tokio::runtime::scheduler::multi_thread::worker::Context::park_timeout::hd9d677c8cbb051cc + 152
6   example_blog_client           	       0x102384c58 tokio::runtime::scheduler::multi_thread::worker::Context::run::h67dd108fa651a15a + 1260
7   example_blog_client           	       0x1023962b8 tokio::macros::scoped_tls::ScopedKey$LT$T$GT$::set::hc5fbf11fa4d71847 + 84
8   example_blog_client           	       0x102384668 tokio::runtime::scheduler::multi_thread::worker::run::h17700621f5c59ec3 + 132
9   example_blog_client           	       0x10238595c _$LT$tokio..runtime..blocking..task..BlockingTask$LT$T$GT$$u20$as$u20$core..future..future..Future$GT$::poll::h7ef90a31b5cc97ca + 80
10  example_blog_client           	       0x10239a6c4 tokio::runtime::task::core::Core$LT$T$C$S$GT$::poll::h6b1ea7dfeb277b85 + 148
11  example_blog_client           	       0x1023913c4 tokio::runtime::task::harness::Harness$LT$T$C$S$GT$::poll::h36aa34a38b3efdb2 + 176
12  example_blog_client           	       0x10238bf70 tokio::runtime::blocking::pool::Inner::run::h2cb318c4b884e453 + 408
13  example_blog_client           	       0x10238108c std::sys::backtrace::__rust_begin_short_backtrace::hd9606544b2d39132 + 80
14  example_blog_client           	       0x102399284 core::ops::function::FnOnce::call_once$u7b$$u7b$vtable.shim$u7d$$u7d$::h886c10b4a08115c0 + 188
15  example_blog_client           	       0x1023c5214 std::sys::pal::unix::thread::Thread::new::thread_start::h7f24d7d921e6556e + 52
16  libsystem_pthread.dylib       	       0x18a4f5f94 _pthread_start + 136
17  libsystem_pthread.dylib       	       0x18a4f0d34 thread_start + 8

Thread 2:
0   libsystem_pthread.dylib       	       0x18a4f0d20 start_wqthread + 0

Thread 3:
0   libsystem_pthread.dylib       	       0x18a4f0d20 start_wqthread + 0

Thread 4:
0   libsystem_pthread.dylib       	       0x18a4f0d20 start_wqthread + 0

Thread 5:
0   libsystem_pthread.dylib       	       0x18a4f0d20 start_wqthread + 0

Thread 6:: tokio-runtime-worker
0   libsystem_kernel.dylib        	       0x18a4b85ec __psynch_cvwait + 8
1   libsystem_pthread.dylib       	       0x18a4f655c _pthread_cond_wait + 1228
2   example_blog_client           	       0x1023c5c04 std::sys::sync::condvar::pthread::Condvar::wait_timeout::h9c14fc6b89193d63 + 256
3   example_blog_client           	       0x10238bea8 tokio::runtime::blocking::pool::Inner::run::h2cb318c4b884e453 + 208
4   example_blog_client           	       0x10238108c std::sys::backtrace::__rust_begin_short_backtrace::hd9606544b2d39132 + 80
5   example_blog_client           	       0x102399284 core::ops::function::FnOnce::call_once$u7b$$u7b$vtable.shim$u7d$$u7d$::h886c10b4a08115c0 + 188
6   example_blog_client           	       0x1023c5214 std::sys::pal::unix::thread::Thread::new::thread_start::h7f24d7d921e6556e + 52
7   libsystem_pthread.dylib       	       0x18a4f5f94 _pthread_start + 136
8   libsystem_pthread.dylib       	       0x18a4f0d34 thread_start + 8

Thread 7:: tokio-runtime-worker
0   libsystem_kernel.dylib        	       0x18a4b85ec __psynch_cvwait + 8
1   libsystem_pthread.dylib       	       0x18a4f655c _pthread_cond_wait + 1228
2   example_blog_client           	       0x1023c5c04 std::sys::sync::condvar::pthread::Condvar::wait_timeout::h9c14fc6b89193d63 + 256
3   example_blog_client           	       0x10238bea8 tokio::runtime::blocking::pool::Inner::run::h2cb318c4b884e453 + 208
4   example_blog_client           	       0x10238108c std::sys::backtrace::__rust_begin_short_backtrace::hd9606544b2d39132 + 80
5   example_blog_client           	       0x102399284 core::ops::function::FnOnce::call_once$u7b$$u7b$vtable.shim$u7d$$u7d$::h886c10b4a08115c0 + 188
6   example_blog_client           	       0x1023c5214 std::sys::pal::unix::thread::Thread::new::thread_start::h7f24d7d921e6556e + 52
7   libsystem_pthread.dylib       	       0x18a4f5f94 _pthread_start + 136
8   libsystem_pthread.dylib       	       0x18a4f0d34 thread_start + 8

Thread 8:: com.apple.NSEventThread
0   libsystem_kernel.dylib        	       0x18a4b4df4 mach_msg2_trap + 8
1   libsystem_kernel.dylib        	       0x18a4c75e4 mach_msg2_internal + 80
2   libsystem_kernel.dylib        	       0x18a4bd9c4 mach_msg_overwrite + 476
3   libsystem_kernel.dylib        	       0x18a4b5178 mach_msg + 24
4   CoreFoundation                	       0x18a5d5680 __CFRunLoopServiceMachPort + 160
5   CoreFoundation                	       0x18a5d3f44 __CFRunLoopRun + 1208
6   CoreFoundation                	       0x18a5d3434 CFRunLoopRunSpecific + 608
7   AppKit                        	       0x18df5d280 _NSEventThread + 144
8   libsystem_pthread.dylib       	       0x18a4f5f94 _pthread_start + 136
9   libsystem_pthread.dylib       	       0x18a4f0d34 thread_start + 8


Thread 0 crashed with ARM Thread State (64-bit):
    x0: 0x000000013f945400   x1: 0x000000013f94ad39   x2: 0xfffffffffffffff8   x3: 0x000000013f946318
    x4: 0x0000000000004a01   x5: 0x0000000000000020   x6: 0x000000013f945400   x7: 0x0000000000000f58
    x8: 0x00000001f28a5000   x9: 0x8ba4586afe9700d6  x10: 0x8ba4586afe9700d6  x11: 0x000000000000007f
   x12: 0x0000000000000017  x13: 0x000000013f926960  x14: 0x01000001f235c589  x15: 0x00000001f235c588
   x16: 0x000000018a5261a0  x17: 0x00000001fca42aa8  x18: 0x0000000000000000  x19: 0x00006000031452f0
   x20: 0x0000600002a09540  x21: 0x0000168d57ab2854  x22: 0x000000013f009050  x23: 0x000000016ddbfec0
   x24: 0x0000000000000000  x25: 0x0000000000000000  x26: 0x0000600000e1d360  x27: 0x0000600003144f00
   x28: 0x000000018a5d4e04   fp: 0x000000016ddbfd30   lr: 0x102e80018e10f750
    sp: 0x000000016ddbfd00   pc: 0x000000018e10f758 cpsr: 0x60001000
   far: 0x0000000000000000  esr: 0xf2000001 (Breakpoint) brk 1

Binary Images:
       0x10cfd4000 -        0x10cfdffff libobjc-trampolines.dylib (*) <7d6ca1cb-c5e6-38b6-9618-2c580486fc6b> /usr/lib/libobjc-trampolines.dylib
       0x102da0000 -        0x102e0bfff com.apple.AppleMetalOpenGLRenderer (1.0) <ddc96be4-9f21-325d-b266-1bbe5fe29812> /System/Library/Extensions/AppleMetalOpenGLRenderer.bundle/Contents/MacOS/AppleMetalOpenGLRenderer
       0x10203c000 -        0x10277bfff example_blog_client (*) <4c4c442c-5555-3144-a146-ed08b13d75d6> /Users/USER/*/example_blog_client
       0x18ddf9000 -        0x18f137fff com.apple.AppKit (6.9) <7acbeef9-a453-3e4d-98ac-f9441d86e183> /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit
       0x18a557000 -        0x18aa2ffff com.apple.CoreFoundation (6.9) <55720ee0-d8c3-33a1-9854-8e61a6f44895> /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation
       0x194d4a000 -        0x19500dfff com.apple.HIToolbox (2.1.1) <4166f6cb-b599-306c-8568-823bd29a74c3> /System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/HIToolbox
       0x18a165000 -        0x18a1ee507 dyld (*) <f635824e-318b-3f0c-842c-c369737f2b68> /usr/lib/dyld
               0x0 - 0xffffffffffffffff ??? (*) <00000000-0000-0000-0000-000000000000> ???
       0x18a522000 -        0x18a529fe7 libsystem_platform.dylib (*) <b4bf9f89-31d7-3742-8ce7-ab3554f9f525> /usr/lib/system/libsystem_platform.dylib
       0x18a4b4000 -        0x18a4eeffb libsystem_kernel.dylib (*) <71ff45b8-f14e-3666-9e96-6cf58315b91d> /usr/lib/system/libsystem_kernel.dylib
       0x18a4ef000 -        0x18a4fbfff libsystem_pthread.dylib (*) <e03e8478-6f5c-3d21-a79a-58408f514000> /usr/lib/system/libsystem_pthread.dylib

External Modification Summary:
  Calls made by other processes targeting this process:
    task_for_pid: 0
    thread_create: 0
    thread_set_state: 0
  Calls made by this process:
    task_for_pid: 0
    thread_create: 0
    thread_set_state: 0
  Calls made by all processes on this machine:
    task_for_pid: 0
    thread_create: 0
    thread_set_state: 0

VM Region Summary:
ReadOnly portion of Libraries: Total=837.7M resident=0K(0%) swapped_out_or_unallocated=837.7M(100%)
Writable regions: Total=2.3G written=0K(0%) resident=0K(0%) swapped_out=0K(0%) unallocated=2.3G(100%)

                                VIRTUAL   REGION 
REGION TYPE                        SIZE    COUNT (non-coalesced) 
===========                     =======  ======= 
Accelerate framework               128K        1 
Activity Tracing                   256K        1 
CG image                           160K        7 
ColorSync                          624K       29 
CoreAnimation                      336K       16 
CoreGraphics                        32K        2 
CoreUI image data                 1248K        9 
Foundation                          16K        1 
Kernel Alloc Once                   32K        1 
MALLOC                             2.3G       72 
MALLOC guard page                  192K       12 
OpenGL GLSL                        256K        3 
STACK GUARD                        128K        8 
Stack                             16.7M        9 
Stack Guard                       56.0M        1 
VM_ALLOCATE                        704K       30 
VM_ALLOCATE (reserved)             512K        4         reserved VM address space (unallocated)
__AUTH                             940K      207 
__AUTH_CONST                      15.0M      368 
__CTF                               824        1 
__DATA                            3596K      356 
__DATA_CONST                      18.9M      372 
__DATA_DIRTY                       924K      116 
__FONT_DATA                        2352        1 
__GLSLBUILTINS                    5174K        1 
__LINKEDIT                       527.7M        4 
__OBJC_RO                         71.9M        1 
__OBJC_RW                         2200K        1 
__TEXT                           310.0M      386 
dyld private memory                272K        2 
mapped file                      179.5M       25 
shared memory                     1392K       15 
===========                     =======  ======= 
TOTAL                              3.5G     2062 
TOTAL, minus reserved VM space     3.5G     2062 

Further reading:

• Integration with async ecosystem rust-windowing/winit#1199
• https://tokio.rs/tokio/topics/bridging
• Tokio + Winit? tokio-rs/tokio#2953

[ChrisRega]

Thanks for the report!