Work around missing function support

[dominicletz]

This fixes two issues:

1. crypto.hash_equals can be present but unsupported (in which case it throws)

2. When compiling beam files on an older otp revision than deploying , the compile time detection reports false positives. Instead we're using runtime detection now always.

[josevalim]

When compiling beam files on a newer otp revision than deploying , the compile time detection reports false positives. Instead we're using runtime detection now always.

We can expand on this a bit? I don't believe Elixir nor the Erlang VM actually supports compiling on a newer OTP version. You should always compile on the lowest version that you plan to support.

[bcardarella]

@dominicletz is there feedback you can provide to Jose, as we're also in need of this PR for LVN

[dominicletz]

Yes, sorry it's the other way around. E.g. compiling on an older version then the newer functions become not accessible even when running on a new OTP. I've fixed that comment. And yes OTP supports running older beam code on newer OTP releases. I think two major release steps are supported.

The one exception here is hash_equals as mentioned above, the function can be 'present' but non-functional e.g. always throwing. In that case too we want to use the legacy work-around.

[josevalim]

Sorry, it is still unclear what is going on here. There are two possibilities:

1. Some functions are not available in some OSes. That part I understand and we can better support it

2. You moved the detection to runtime but:
   a) using the compile-time fallback should not be an issue in such places anyway
   b) if it happens for Erlang/OTP 24, then it is no longer an officially supported version anyway

I'd like to break this apart, perhaps as two separate PRs? The ones for number 1 above we can definitely merge. Thanks.

[dominicletz]

I've created a new PR just with the hash_equals fallback https://github.com/elixir-plug/plug_crypto/pull/45/files

[josevalim]

Thank you, it has been merged. Please let me know if this is enough to squash iOS issues.

[bcardarella]

We'll try this today on our end. Thank you

[carson-katri]

main is working for me on iOS

[josevalim]

New package is out.

[josevalim]

Thanks @dominicletz! If the other patch is ready, let me know, but given that it is about OTP 24, we should be fine. :)