`_ITERATOR_DEBUG_LEVEL` 2: crash when iterators are destroyed while container is modified

[decden]

Iterator debug level 2 does not support concurrent deletion and invalidation of iterators from two different threads. When this happens, the application crashes with a nullptr access inside of the STL.

Command-line test case

C:\Temp>type repro.cpp
#include <iostream>
#include <unordered_set>
#include <future>
#include <thread>
#include <chrono>
#include <vector>

int main() {
	// Create a one element set
	std::unordered_set<int> set;
	set.insert(0);

	// Create a list of iterators to later destroy
	std::vector<std::unordered_set<int>::iterator> iters;
	for (int i = 0; i < 1000; ++i)
		iters.push_back(set.begin());

	// Concurrently destroy iterators and invalidate iterators
	{
		auto destroyIters = std::async(std::launch::async,
			[&](){
				iters.clear();
			});

		auto invalidateIters = std::async(std::launch::async,
			[&](){
				set.clear();
			});
	}

	std::cout << "Programm behaved as expected, and did not crash" << std::endl;
}

C:\Temp>cl /EHsc /W4 /WX /Zi /DEBUG /MTd /D_ITERATOR_DEBUG_LEVEL=2 .\repro.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.29.30038.1 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

repro.cpp
Microsoft (R) Incremental Linker Version 14.29.30038.1
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:repro.exe
/debug
repro.obj

C:\Temp>.\repro.exe
[application crashes]

Expected behavior
The application should not crash.

STL version
Microsoft Visual Studio Professional 2019 Version 16.10.2

[miscco]

So this is a race condition in the debug iterator machinery.

Note that due to constexpr containers and the restrictions during constant evaluation we need to have special paths for constant evaluation and plain runtime.

Now the bug is here:

    _CONSTEXPR20_CONTAINER void _Orphan_me_v2() noexcept {
        if (_Myproxy) { // adopted, remove self from list
#ifdef __cpp_lib_constexpr_dynamic_alloc
            if (_STD is_constant_evaluated()) {
                _Orphan_me_unlocked();
            } else
#endif // __cpp_lib_constexpr_dynamic_alloc
            {
                _Orphan_me_locked();
            }
        }
    }

We check _Myproxy before we take the lock, which introduces a race condition.

the proper solution is to only perform the if (_Myproxy) inside the _Orphan_me_unlocked call

[StephanTLavavej]

Totally unrelated, but passing /DEBUG to the compiler will define a macro named EBUG. Passing /MTd is what activates debug mode (and defines the macro _DEBUG).

(There's a linker option /DEBUG, which is distinct from "debug mode".)