Allow the re-initialization of contracts

[Amxx]

Fixes #2901

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

This is a sensitive piece of code. It needs comments explaining in depth what behavior it implements and why.

We also need to develop and document the alternative to constructor() initializer {}.

[Amxx]

We also need to develop and document the alternative to constructor() initializer {}.

Should we add a lockImplementation modifier that could be used for constructors?

[frangio]

lockImplementation sounds too specific for upgrades. It also doesn't need to be a modifier, does it? What about something like this:

constructor() {
  _preventInitialize();
}

[Amxx]

How should this function behave? Should it check that the contract is not yet initialized? Should it be allowed to run within the scope of an initializer?

Here are possible implementation

    function _preventInitialize() internal virtual {
        require(!_initializing && _initialized == 0, "Initializable: contract initializing of already initialized");
        _initialized = type(uint8).max;
        _initializing = false;
    }

    function _preventInitialize() internal virtual {
         if (_setInitializeStep(type(uint8).max)) {
            _initializing = false;
        }
    }

[frangio]

I think we want the second option:

    function _preventInitialize() internal virtual {
        if (_setInitializeStep(type(uint8).max)) {
            _initializing = false;
        }
    }

The behavior that results is that after this function is called no more initializer or reinitializer functions will be callable. As a result, onlyInitializing functions will not be callable either.

[Amxx]

not, it will be incorrect to write

constructor() initializer {
    _preventInitialize();
}

meant to comment

[frangio]

Looks good to me. Thank you!