EH termination does not call __cxa_begin_catch

[aheejin]

When an exception thrown ends up calling std::terminate, for example, because an exception is thrown within a noexcept function or an exception is thrown from __cxa_end_catch during handling the previous exception, the libc++abi spec says we are supposed to call __cxa_begin_catch before std::terminate: https://libcxxabi.llvm.org/spec.html

When the personality routine encounters a termination condition, it will call __cxa_begin_catch() to mark the exception as handled and then call terminate(), which shall not return to its caller.

But currently Emscripten does NOT call __cxa_begin_catch before terminate. For example:

#include <iostream>
#include <exception>

int main() noexcept {
  std::set_terminate([] {
    auto ptr = std::current_exception();
    if (ptr)
      std::cerr << "exception_ptr is NOT null" << std::endl;
    else
      std::cerr << "exception_ptr is null" << std::endl;
    std::abort();
  });
  throw 3;
}

When you call std::current_exception() within the custom terminate handler, it should print

exception_ptr is NOT null

But Wasm EH prints

exception_ptr is null

The reason is that we replaced the call to __clang_call_terminate, a generated function that calls __cxa_begin_catch and std::terminate, with just a call to std::terminate in llvm/llvm-project@561abd8 because this caused some tricky transformation problems for Wasm EH.

(Emscripten EH (accidentally) had the same problem after llvm/llvm-project@561abd8 but it should be fixed by llvm/llvm-project#129020.)

This should be fixable with sufficient efforts but has not been a priority. This issues documents the problem so it is not forgotten and can possibly be fixed later.

This problem was first reported in #23720.

[aheejin]

cc @stevenwdv, in case this gets fixed in future