[Mono-dev] New MonoError API for 2.8

[Rodrigo Kumpera]

Hi,

Today landed the initial version (r144686) of the MonoError API that will
eventually unify all error handling code in the runtime.
Here is an overview of how it's meant to be used across the runtime.

The central structure is MonoError, which carry success/error result and all
details about what went wrong.

The usage model is that a MonoError object is defined at entry point
functions or error contexts. Entry point functions
are things like icalls or trampoline'd functions; an error context is, for
example, type loading, which has to trap the error
to itself.

All function that can set an error must call mono_error_init or
mono_error_init_flags at its very beginning. This sets the result
as success so nothing else needs to be done unless an error happens. Error
setting functions are modeled around the managed
exceptions that might be thrown in the end.

After all call sites that pass a MonoError there must be a call to
mono_error_ok to check the result. If an error mono_error_cleanup
must be called as well.

The idea is to hook up all entrypoints and embedding API calls with a
MonoError parameter. For public API functions we'll add
a _checked version of them and mark the former as deprecated. For internal
functions we'll just change the signature.

Besides that, with MonoError in place, the plan is to remove all possible
asserts from the runtime to make things less crashy.

There are a few concerns about how this currently are that deserves to be
discussed:

-Should we backport this to 2.6? To me it makes sense to do so for ML2, but
without exposing new API entrypoints.

-Right now the whole API is made of functions, should we move the functions
used in the fast path to macros? This is basically
mono_error_init and mono_error_ok.

Please review the change and give some feedback about the design. r144688
changes mono_class_inflate_generic_type as an example.

Cheers,
Rodrigo
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