RFR: 8367149: Add ergonomic construction for creating ad-hoc VMErrorCallback [v2]

[Axel Boldt-Christmas]

On Wed, 10 Sep 2025 14:17:42 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Add a class OnVMError which uses the VMErrorCallback mechanism which is an ergonomic construction for creating ad-hoc VMErrorCallback which automatically calls the provided invocable f if a VM crash occurs within its lifetime. Can be used to instrument a build for more detailed contextual information gathering. Especially useful when hunting down intermittent bugs, or issues only reproducible in environments where access to a debugger is not readily available. Example use:
>> ```C++
>>   {
>>     // Note the lambda is invoked after an error occurs within this thread,
>>     // and during on_error's lifetime. If state prior to the crash is required,
>>     // capture a copy of it first.
>>     auto important_value = get_the_value();
>> 
>>     OnVMError on_error([&](outputStream* st) {
>>       // Dump the important bits.
>>       st->print("Prior value: ");
>>       important_value.print_on(st);
>>       st->print("During crash: ")
>>       get_the_value().print_on(st);
>>       // Dump whole the whole state.
>>       this->print_on(st);
>>     });
>> 
>>     // Sometimes doing a thing will crash the VM.
>>     do_a_thing();
>>   }
>> 
>> 
>> C++17 class template argument deduction finally makes these sort of constructions ergonomic to use without the need for auto and helper construction methods.
>
> Axel Boldt-Christmas has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - Add a comment explaining the deduction rules
>  - Skip multiple inheritance and allow more than lambda like callables.
>  - Update doc example

I initially intended for this to just be allowed to take a lambda like temporary and just use that. But after looking at it some more and using it a bit I think it is fine to extend this to anything which behaves like a callable (via operator()(outputStream*)) and can be move/copy constructed. So you can now also use a lvalue lambda or function pointer.

Felt like it would be the least surprising outcome when trying to use this class. Even though using it as `OnVMError(<lambda type>&&)` is the expected use case (as presented in the example).

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PR Comment: https://git.openjdk.org/jdk/pull/27159#issuecomment-3277907074