bug: `moc` allows declaration of actor method `_motoko_async_helper`

[crusso]

... which conflicts with the compiler generated one.

Possible fixes:

1. reject shared functions called _motoko_async_helper (or more generally, beginning with double underscore, reserving that for compiler generated methods).

2. choose an inaccessible internal name.

The replica actually use the presence of this caniser method to detect motoko canisters and other tooling might too (icrocks?). Perhaps 1. is a better solution.

c.f. '__get_candid_interface_tmp_hack' and other secret entrypoints.

Repro in #3118

[nomeata]

The replica actually use the presence of this caniser method to detect motoko canisters and other tooling might too (icrocks?). Perhaps 1. is a better solution.

Sounds like a great reason to use 2 :-D

[chenyan-dfinity]

There are external apps calling the _get_candid_interface_tmp_hack method, so better to keep that name unchanged.

[nomeata]

Right! with the async helper, I wouldn't hesitate to change the name. For the candid one, you are absolutely right.

[Web3NL]

Shared function ending with one underscore foo_ will generate an corresponding Candid name foo.

Shared function ending with two underscores foo__ will generate an corresponding Candid name foo_ (with one underscore)

three underscores foo___ gives foo__ (with two underscores)

Is this intended (and if so what is its use)?

@crusso @chenyan-dfinity

[chenyan-dfinity]

Yes, it's used to escape reserved keywords in Candid. See https://github.com/dfinity/motoko/blob/master/design/IDL-Motoko.md#type-name-mangling