Re: Question about 'provides' and emacs packaging

[Ken Brown <kbrown@cornell.edu>]

On 10/5/2021 2:24 PM, Achim Gratz wrote:
> Ken Brown via Cygwin-apps writes:
>> There are currently five emacs packages: emacs-common, emacs,
>> emacs-X11, emacs-w32, and emacs-lucid.  The first includes things that
>> are needed by each of the other four, and those four each include an
>> emacs binary.  The binary in the emacs package is
>> /usr/bin/emacs-nox.exe.  The other packages contain
>> /usr/bin/emacs-X11.exe, and so on.
>>
>> This way of naming the packages doesn't really reflect the contents of
>> the emacs package.  It also means that anyone who installs emacs gets
>> emacs-nox.exe, even if they plan to use one of the other three
>> binaries.
>>
>> I would rather rename the current emacs-common package to emacs and
>> the current emacs package to emacs-nox.  But then the new emacs would
>> have to have a way of requiring the installation of at least one of
>> emacs-nox, emacs-X11, emacs-w32, or emacs-lucid.  Is there any way to
>> do this with our current setup machinery?
> 
> I don't think we have the transaction capability that would be necessary
> to specify that the meaning of an already existing package name (two,
> actually) changes in this manner.  You might have to use new package
> names and place the appropriate obsoletions w.r.t. old names instead.
> 
>> My idea three years ago was to have the new emacs package require a
>> "feature" called, for instance, emacs-bin, and then have each of
>> emacs-nox, emacs-X11, emacs-w32, emacs-lucid "provide" that feature.
> 
> I have no idea if multiple packages can provide the same feature. It's
> worth trying if it does what you want (you must pick at least one of
> those).

This seems to work, with one caveat.  Suppose package P requires feature f, and 
packages Q, R, S,... provide f.  If the user selects P and one or more of Q, R, 
S,..., setup is happy.  But if the user simply selects P, then setup/libsolv 
will choose among Q, R, S,... the one whose name is alphabetically first.  In 
the emacs case, this would be emacs-lucid, which is a stupid default.  The 
default ought to be emacs-nox.  So I can make it work if I call that package 
emacs-basic instead of emacs-nox.

I've only tested setup so far, not calm.  Jon, if you're reading this, does calm 
allow 'requires' and 'provides' to contain arbitrary names that are not package 
names?

Ken