Bug 301998 – Function=(const Function&&) is a footgun in waiting

• Component: Web Template Framework
• Status: NEW

[Jean-Yves Avenard [:jya]]

Today I encountered an issue that is rather problematic.

the code was
```
    ensureOnDispatcherWithConnection([callback = WTFMove(callback)](auto& renderer, auto& connection) {
        assertIsCurrent(queueSingleton());
        renderer.m_hasAvailableVideoFrameCallback = WTFMove(callback);
        connection.send(Messages::RemoteAudioVideoRendererProxyManager::NotifyWhenHasAvailableVideoFrame(renderer.m_identifier, !!renderer.m_hasAvailableVideoFrameCallback), 0);
    });
```

the code crashed with an infinite recursion. At first I was surprised it even compiled as I had forgotten the `mutable` keyword and didn't expect `renderer.m_hasAvailableVideoFrameCallback = WTFMove(callback);` to even compile.

the stack trace was
```
thread #2, queue = 'AudioVideoRendererRemote', stop reason = EXC_BAD_ACCESS (code=2, address=0x16b1e7fd0)
  * frame #0: 0x0000000131360128 JavaScriptCore`WTF::assertMallocRestrictionForCurrentThreadScope() at MallocCommon.cpp:58
    frame #1: 0x0000000131305994 JavaScriptCore`WTF::fastMalloc(size=16) at FastMalloc.cpp:537:5 [opt]
    frame #2: 0x000000011b5587c8 WebKit`WTF::Detail::CallableWrapperAllocatorBase::operator new(size=16) at Function.h:38:5
    frame #3: 0x000000011f79665c WebKit`std::__1::unique_ptr<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, std::__1::default_delete<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>>> std::__1::make_unique[abi:sn200100]<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const, 0>(__args=0x00000001190cc1a8) at unique_ptr.h:767:26
    frame #4: 0x000000011f7965a4 WebKit`decltype(auto) WTF::makeUnique<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const>(args=0x00000001190cc1a8) at StdLibExtras.h:871:12
    frame #5: 0x000000011f79685c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>::Function<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001193cf538, callable=0x00000001190cc1a8) at Function.h:80:29
    frame #6: 0x000000011f79677c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>::Function<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001193cf538, callable=0x00000001190cc1a8) at Function.h:80:130
    frame #7: 0x000000011f796744 WebKit`WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>::CallableWrapper(this=0x00000001193cf530, callable=0x00000001190cc1a8) at Function.h:56:11
    frame #8: 0x000000011f7966a8 WebKit`WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>::CallableWrapper(this=0x00000001193cf530, callable=0x00000001190cc1a8) at Function.h:56:41
    frame #9: 0x000000011f796668 WebKit`std::__1::unique_ptr<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, std::__1::default_delete<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>>> std::__1::make_unique[abi:sn200100]<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const, 0>(__args=0x00000001190cc1a8) at unique_ptr.h:767:30
    frame #10: 0x000000011f7965a4 WebKit`decltype(auto) WTF::makeUnique<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const>(args=0x00000001190cc1a8) at StdLibExtras.h:871:12
    frame #11: 0x000000011f79685c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>::Function<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001193cf528, callable=0x00000001190cc1a8) at Function.h:80:29
    frame #12: 0x000000011f79677c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>::Function<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001193cf528, callable=0x00000001190cc1a8) at Function.h:80:130
    frame #13: 0x000000011f796744 WebKit`WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>::CallableWrapper(this=0x00000001193cf520, callable=0x00000001190cc1a8) at Function.h:56:11
    frame #14: 0x000000011f7966a8 WebKit`WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>::CallableWrapper(this=0x00000001193cf520, callable=0x00000001190cc1a8) at Function.h:56:41
    frame #15: 0x000000011f796668 WebKit`std::__1::unique_ptr<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, std::__1::default_delete<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>>> std::__1::make_unique[abi:sn200100]<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const, 0>(__args=0x00000001190cc1a8) at unique_ptr.h:767:30
    frame #16: 0x000000011f7965a4 WebKit`decltype(auto) WTF::makeUnique<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const>(args=0x00000001190cc1a8) at StdLibExtras.h:871:12
    frame #17: 0x000000011f79685c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>::Function<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001193cf518, callable=0x00000001190cc1a8) at Function.h:80:29
    frame #18: 0x000000011f79677c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>::Function<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001193cf518, callable=0x00000001190cc1a8) at Function.h:80:130
    frame #19: 0x000000011f796744 WebKit`WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>::CallableWrapper(this=0x00000001193cf510, callable=0x00000001190cc1a8) at Function.h:56:11
    frame #20: 0x000000011f7966a8 WebKit`WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>::CallableWrapper(this=0x00000001193cf510, callable=0x00000001190cc1a8) at Function.h:56:41
    frame #21: 0x000000011f796668 WebKit`std::__1::unique_ptr<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, std::__1::default_delete<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void,
[....] repeat 4000 times
WTF::MediaTime const&, double>>> std::__1::make_unique[abi:sn200100]<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const, 0>(__args=0x00000001190cc1a8) at unique_ptr.h:767:30
    frame #10480: 0x000000011f7965a4 WebKit`decltype(auto) WTF::makeUnique<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const>(args=0x00000001190cc1a8) at StdLibExtras.h:871:12
    frame #10481: 0x000000011f79685c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>::Function<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001193c8768, callable=0x00000001190cc1a8) at Function.h:80:29
    frame #10482: 0x000000011f79677c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>::Function<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001193c8768, callable=0x00000001190cc1a8) at Function.h:80:130
    frame #10483: 0x000000011f796744 WebKit`WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>::CallableWrapper(this=0x00000001193c8760, callable=0x00000001190cc1a8) at Function.h:56:11
    frame #10484: 0x000000011f7966a8 WebKit`WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>::CallableWrapper(this=0x00000001193c8760, callable=0x00000001190cc1a8) at Function.h:56:41
    frame #10485: 0x000000011f796668 WebKit`std::__1::unique_ptr<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, std::__1::default_delete<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>>> std::__1::make_unique[abi:sn200100]<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const, 0>(__args=0x00000001190cc1a8) at unique_ptr.h:767:30
    frame #11128: 0x000000011f7965a4 WebKit`decltype(auto) WTF::makeUnique<WTF::Detail::CallableWrapper<WTF::Function<void (WTF::MediaTime const&, double)> const, void, WTF::MediaTime const&, double>, WTF::Function<void (WTF::MediaTime const&, double)> const>(args=0x00000001190cc1a8) at StdLibExtras.h:871:12
    frame #11129: 0x000000011f79645c WebKit`WTF::Function<void (WTF::MediaTime const&, double)>& WTF::Function<void (WTF::MediaTime const&, double)>::operator=<WTF::Function<void (WTF::MediaTime const&, double)> const>(this=0x00000001192602e0, callable=0x00000001190cc1a8) at Function.h:112:29
    frame #11130: 0x000000011f7963b0 WebKit`_ZZN6WebKit24AudioVideoRendererRemote32notifyWhenHasAvailableVideoFrameEON3WTF8FunctionIFvRKNS1_9MediaTimeEdEEEENK3$_0clIS0_N3IPC10ConnectionEEEDaRT_RT0_(this=0x00000001190cc1a8, renderer=0x0000000119260210, connection=0x0000000119262100) at AudioVideoRendererRemote.cpp:207:51
    frame #11131: 0x000000011f7962d0 WebKit`WTF::Detail::CallableWrapper<WebKit::AudioVideoRendererRemote::notifyWhenHasAvailableVideoFrame(WTF::Function<void (WTF::MediaTime const&, double)>&&)::$_0, void, WebKit::AudioVideoRendererRemote&, IPC::Connection&>::call(this=0x00000001190cc1a0, in=0x0000000119260210, in=0x0000000119262100) at Function.h:59:39
    frame #11132: 0x000000011f7afd90 WebKit`WTF::Function<void (WebKit::AudioVideoRendererRemote&, IPC::Connection&)>::operator()(this=0x00000001190d81d8, in=0x0000000119260210, in=0x0000000119262100) const at Function.h:103:35
    frame #11133: 0x000000011f7afca0 WebKit`WebKit::AudioVideoRendererRemote::ensureOnDispatcherWithConnection(WTF::Function<void (WebKit::AudioVideoRendererRemote&, IPC::Connection&)>&&)::$_0::operator()(this=0x00000001190d81c8) at AudioVideoRendererRemote.cpp:647:9
    frame #11134: 0x000000011f7afb2c WebKit`WTF::Detail::CallableWrapper<WebKit::AudioVideoRendererRemote::ensureOnDispatcherWithConnection(WTF::Function<void (WebKit::AudioVideoRendererRemote&, IPC::Connection&)>&&)::$_0, void>::call(this=0x00000001190d81c0) at Function.h:59:39
    frame #11135: 0x00000001312d7614 JavaScriptCore`WTF::Function<void ()>::operator()(this=0x00000001190cc1b0) const at Function.h:103:35
    frame #11136: 0x00000001314d0c2c JavaScriptCore`WTF::(anonymous namespace)::DispatchWorkItem::operator()(this=0x00000001190cc1b0) at WorkQueueCocoa.cpp:40:25
    frame #11137: 0x00000001314cf85c JavaScriptCore`void WTF::dispatchWorkItem<WTF::(anonymous namespace)::DispatchWorkItem>(dispatchContext=0x00000001190cc1b0) at WorkQueueCocoa.cpp:48:5
    frame #11138: 0x0000000104de14dc libdispatch.dylib`_dispatch_client_callout + 16
    frame #11139: 0x0000000104dcd7bc libdispatch.dylib`_dispatch_lane_serial_drain + 820
    frame #11140: 0x0000000104dce65c libdispatch.dylib`_dispatch_lane_invoke + 440
    frame #11141: 0x0000000104ddc220 libdispatch.dylib`_dispatch_root_queue_drain_deferred_wlh + 664
    frame #11142: 0x0000000104ddb700 libdispatch.dylib`_dispatch_workloop_worker_thread + 752
    frame #11143: 0x0000000104e677e4 libsystem_pthread.dylib`_pthread_wqthread + 292
```

The code for the operator=
Function=(const Function&&) starts with doing
```
    Function& operator=(CallableType&& callable)
    {
        m_callableWrapper = makeUnique<Detail::CallableWrapper<CallableType, Out, In...>>(std::forward<CallableType>(callable));
        return *this;
    }
```
and from there CallableWrapper tries to create a new function and so forth.

It may be best to check that neither CallableType nor FunctionType are const.

[Radar WebKit Bug Importer]

<rdar://problem/164544374>