[Rpm-maint] [rpm-software-management/rpm] Discussion: How to handle Python extras (#1061)

[Miro Hrončok]

Here's my idea:

```lua
%python_extras_subpkg(n:i:f:) %{expand:%{lua:
    local value_n = rpm.expand('%{-n*}')
    local value_i = rpm.expand('%{-i*}')
    local value_f = rpm.expand('%{-f*}')
    local args = rpm.expand('%{*}')
    if value_n == '' then
        rpm.expand('%{error:%%%0: missing option -n (name of the base package)}')
    end
    if value_i == '' and value_f == '' then
        rpm.expand('%{error:%%%0: missing option -i (buildroot path to metadata) or -f (builddir path to a filelist)}')
    end
    if value_i ~= '' and value_f ~= '' then
        rpm.expand('%{error:%%%0: simultaneous -i (buildroot path to metadata) and -f (builddir path to a filelist) is not supported}')
    end
    if args == '' then
        rpm.expand('%{error:%%%0 requires at least one argument with "extras" name}')
    end
    local requires = 'Requires: ' .. value_n .. ' = %{?epoch:%{epoch}:}%{version}-%{release}'
    for extras in args:gmatch('%w+') do
        local rpmname = value_n .. '+' .. extras
        local pyname = value_n .. '[' .. extras .. ']'
        local pkgdef = '%package -n ' .. rpmname
        local summary = 'Summary: Metapackage requiring requires for ' .. pyname
        local description = '%description -n ' .. rpmname .. '\\\n' ..
                            'This is a metapackage bringing in requires for ' .. pyname .. '.\\\n' ..
                            'It contains no code, just makes sure the dependencies are installed.\\\n'
        local files
        if value_i ~= '' then
            files = '%files -n ' .. rpmname .. '\\\n' .. '%ghost ' .. value_i
        else
            files = '%files -n ' .. rpmname .. ' -f ' .. value_f
        end
        for i, line in ipairs({pkgdef, summary, requires, description, files, ''}) do
            print(line .. '\\\n')
        end
    end
}}
```

You'd put this to your specfile near other subpackage declarations:

```
%python_extras_subpkg -n python3-setuptools_scm -i %{python3_sitelib}/*.egg-info toml
```

It expands to:

```
%package -n python3-setuptools_scm+toml
Summary: Metapackage requiring requires for python3-setuptools_scm[toml]
Requires: python3-setuptools_scm = %{?epoch:%{epoch}:}%{version}-%{release}
%description -n python3-setuptools_scm+toml
This is a metapackage bringing in requires for python3-setuptools_scm[toml].
It contains no code, just makes sure the dependencies are installed.

%files -n python3-setuptools_scm+toml
%ghost /usr/lib/python3.8/site-packages/*.egg-info
```

The rest would be handled by the Python dist dependency generator (taking package name as argument, parsing out the `+toml` part of it, and finding extras named `toml` in the package, eventually providing `python3dist(setuptools_scm[toml]) = 1.2.3` and requiring whatever that requires. If extras `toml` is not found, the dependency generator will error ([somehow](https://github.com/rpm-software-management/rpm/issues/1183)).

Side note: The `-f` option allows to do:

```
%python_extras_subpkg -n python3-setuptools_scm -f %{python_extras_filelist} toml
```

When other automation is at play.

Side note 2: The subpackage should really require `python3-setuptools_scm%{?_isa}` if arched, but I am unsure how to detect if `python3-setuptools_scm` is arched or noarch, so I will just hope for the best :crossed_fingers: 

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