Detect and report late assignment of Module API elements

[sbc100]

We had some bug reports recently when we switched to using async/await
in MODULUARIZE mode. One of the side effects was the --post-js code
then runs after module instantiation and startup, which means assignment
of properties like onRuntimeInitialized is too late if it happens in
--post-js.

This was always the case for sync instantiation mode.

When assertions are enabled, we now detect this too-late-assignment and abort
with a useful error.

Code size regression in this PR, as expected, only for debug builds.

See #23626 and #23420

[sbc100]

CC @sgbeal

[sgbeal]

CC @sgbeal

Thank you for the heads up! i'll keep an eye out for any fallout on our end.

When assertions are enabled, we now detect this too-late-assignment and abort with a useful error.

What happens when asserts are not enabled? Does that make such usage "legal but possibly ill-advised" or "undefined behavior"?

[sbc100]

CC @sgbeal

Thank you for the heads up! i'll keep an eye out for any fallout on our end.

When assertions are enabled, we now detect this too-late-assignment and abort with a useful error.

What happens when asserts are not enabled? Does that make such usage "legal but possibly ill-advised" or "undefined behavior"?

The same thing that happens today: nothing at all. The assignment will go undetected and un-used.

[sbc100]

CC @sgbeal

Thank you for the heads up! i'll keep an eye out for any fallout on our end.

When assertions are enabled, we now detect this too-late-assignment and abort with a useful error.

What happens when asserts are not enabled? Does that make such usage "legal but possibly ill-advised" or "undefined behavior"?

The same thing that happens today: nothing at all. The assignment will go undetected and un-used.

When we try to detect such API misused, or change the behaviour of emscripten like this, we normally focus on reported these errors only when assertions are enabled, to avoid increasing code size in release builds.

[kripken]

Good idea!