Unknown variables from Variables file are now reported

[mwichmann]

Previously Update added everything from the file(s) to the local values dict by doing exec, and then unknown ones ended up silently dropped since they weren't declared as options. Now the result of the exec call is reprocessed, so unknowns can be collected.

No doc impacts - just making it work as documented (see linked issue).

Fixes #4645

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[bdbaddog]

This changes the previous behavior.
Previously unknown variables in variables files would still propagate in the Environment()..

Fairly certain I've seen such depended on in the wild.

[mwichmann]

They shouldn't - existing bit of Update:

        # put the variables in the environment:
        # (don't copy over variables that are not declared as options)
        for option in self.options:
            try:
                env[option.key] = values[option.key]
            except KeyError:
                pass

[mwichmann]

git blame (needed a --follow !!!) says that code snippet is old, indeed:

1523e6f372 src/engine/SCons/Options.py            (Steven Knight    2002-09-26 00:54:35 +0000 276)         # put the variables in the environment:
13985d5667 src/engine/SCons/Options/__init__.py   (Steven Knight    2007-07-15 13:42:20 +0000 277)         # (don't copy over variables that are not declared as options)
82bf497dd0 src/engine/SCons/Options.py            (Steven Knight    2002-10-13 12:52:05 +0000 278)         for option in self.options:
82bf497dd0 src/engine/SCons/Options.py            (Steven Knight    2002-10-13 12:52:05 +0000 279)             try:
82bf497dd0 src/engine/SCons/Options.py            (Steven Knight    2002-10-13 12:52:05 +0000 280)                 env[option.key] = values[option.key]
82bf497dd0 src/engine/SCons/Options.py            (Steven Knight    2002-10-13 12:52:05 +0000 281)             except KeyError:
82bf497dd0 src/engine/SCons/Options.py            (Steven Knight    2002-10-13 12:52:05 +0000 282)                 pass
1523e6f372 src/engine/SCons/Options.py            (Steven Knight    2002-09-26 00:54:35 +0000 283) 

So I think we can say that unknown options have never been copied to the environment.

However, the added check reading from the files doesn't have to exclude unknowns (since they'll be skipped by the mentioned chunk anyway) and just add to the unknown dict if that feels more comfortable...

[bdbaddog]

You are correct. I was mistaken.. sorry bout that..