Memory leak when exception is thrown in adl_serializer::to_json

[mjerabek]

Description

Throwing an exception in serializer function after the output json object is already initialized results in a memory leak (detected with both ASan and valgrind):

void adl_serializer<Foo>::to_json(json& j, Foo const& f) {
    j["a"] = 42;
    throw std::runtime_error("error");
}

Reproduction steps

Reproducer here: https://godbolt.org/z/dvorMExWh

Expected vs. actual results

Expected: no memory leak

Minimal code example

No response

Error messages

No response

Compiler and operating system

linux, clang 15, gcc 12.2

Library version

3.11.2, godbolt trunk, 7f72eed

Validation

• The bug also occurs if the latest version from the develop branch is used.
• I can successfully compile and run the unit tests.

[gregmarr]

This is throwing inside the constructor. When the constructor doesn't complete without exception, then the destructor is not run, meaning that m_value.destroy(m_type); is not called. Cleaning up from this is going to be tricky.
https://isocpp.org/wiki/faq/exceptions#selfcleaning-members

Off the top of my head, the constructor code that calls out to the adl serializer could detect an exception and call m_value.destroy(m_type); manually before rethrowing.

[barcode]

This can be solved by either using a base class or a member var to do the cleanup.
I would prefer one of those solutions, since they solve the problem of leaking memory permanently.

The stl uses base classes for containers managing memory (https://github.com/gcc-mirror/gcc/blob/7c755fd9018821b79ddc32ee507897860510986c/libstdc%2B%2B-v3/include/bits/stl_vector.h#L423).
If we use a base class, we do not need to change the variable names. It would be enough to move some code into this base class.

I will implement a solution using a base class in #3901.

/edit
I went with the solution using a member var, since it requires less changes to the existing code and the resulting code is simpler to understand.