[gentoo-dev] 'Local' USE_EXPAND flags -- aka plugins are not that special

[Michał Górny]

Hello,

I've did a quick statistic of how often USE_EXPAND flags are used.
In order to obtain the results, I have put all known USE_EXPANDs into
${flags[@]} and then did:

  for a in "${flags[@]}"; do
    echo -n "$a: "
    grep -l IUSE=.*${a}_ */* | xargs qatom | cut -d' ' -f1-2 | sort -u
  | wc -l done | tee /tmp/use-expand.txt

The result was a table of USE_EXPAND flags with counts of how many
*different* packages declare them in IUSE.

The results are:

   package count | n flags
  ---------------+---------
               0 |       3
               1 |      24
               2 |       4
             3-8 |       3
             >10 |      11

As you can see, we have *a lot* of USE_EXPAND variables which are only
used by a *single* package. This feels a lot like people are overusing
USE_EXPANDs in place of local USE flags, just to have them prefixed.
And if you take a look at the detailed list (attached for
completeness), you can see that they really do.

And if anyone wondered why it's wrong -- it's wrong because it's much
like declaring global USE flags for a very specific features which are
not only specific to a single package, but are even defined in a way
that they will *never* be suitable to anything else.

Although most of the cases need to be handled separately
and specifically to a particular flag, a common mistake is to introduce
USE_EXPAND every time plugins or modules are involved.

People, plugins are *not* special. There are regular features, just
packaged in a bit different way.

Let's take an example of a mail client. We have two mail clients, one
tightly integrated and the other plugin-based. Both have optional HTML
message support, the latter in the form of a plugin.

Now, let's assume we have global USE=html (why the heck we don't have
this and instead a dozen of local USE=html, then USE=webkit?!). It is
described as something vague like:

  enable HTML support

If I take this USE=html and see it in a mail client, I add 2+2 and get
that USE=html enables HTML message support. Now, why the heck I am
supposed to find out that the latter mail client instead of this
USE=html uses FOO_PLUGINS=htmlviewer?

This looks like a double crime to me. First of all, it's introducing
a specific, local flag for something that we have a global flag for.
Secondly, it's introducing that local flag in a global manner via
USE_EXPAND.

While we actually avoided having that USE_EXPAND for that specific mail
client, there are other cases which are very similar to this.

We have three HTTP servers which define USE_EXPANDs for their modules.
Each of them uses a completely separate namespace and different names
for the *same* features. We have global USE=cgi yet for apache we have
to use apache2_modules_cgi. People are reinventing USE=ssl like this
as well.

And CURL_SSL is a complete disaster. Does supporting 6 different SSL
backends (what for?!) justify inventing a global USE_EXPAND? Most of
those backends have their local (or even global) flags already. Why do
I have to repeat my preference twice?

People, get some reason! The major point of having global USE flags is
to let user express his preferences *once*. Extensively using
USE_EXPAND as a cheap namespace for local flags forces him to re-state
those preferences for every single affected package (USE_EXPAND set).

-- 
Best regards,
Michał Górny