[gentoo-dev] USE flag how are they supposed to work?

[gentoo-dev] USE flag how are they supposed to work?

[Steen Eugen Poulsen]

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I'm trying to write a Replicator for /etc/portage and that leads me to 
work with USE flags, trying to design the replication of them among 
similar systems, but I can't find the golden set of rules for how best 
to apply USE flags.

There seem to be a global/local USE flag system, but many so called 
global flags has duplicated description marking them as local flags, or 
they enable unneeded optional support.

Lets take one of the big ones python. This use flag enables optional 
python support in many packages, I don't see my system ever using the 
python support in most of these. Seems like it really should be a local 
pr. packet flag for most, a pythonapi flag perhaps.

There is also a few cases where package X requires package Y to be 
compiled with an USE flag, but no testing is being done in the ebuild 
for it and the compilation fails. Example gnome-system-tools requires 
libxml2 to be compiled with python support.

Then there is flags like alsa, it's marked as a global flag and then 
virtualbox also has it marked as a local flag. I personally think 
virtualbox is somewhat right in saying it's a local flag, but perhaps it 
would be better if the flag didn't have a double meaning.

As I see it, Gentoo's USE flag system is one of it's greatest strength, 
but at the moment seems like there is missing some overall design for 
how to implement USE flags, making it a lot harder to use USE flags, as 
there is no clear definition of global or local flags.


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Re: [gentoo-dev] USE flag how are they supposed to work?

[Alistair Bush]

Firstly...

>From http://devmanual.gentoo.org/general-concepts/use-flags/index.html

"Local and Global USE Flags

USE flags are categorised as either local or global. A global USE flag
must satisfy several criteria:

    * It is used by many different packages.
    * It has a general non-specific purpose.

The second point is important. If the effect of the thing USE flag upon
pkg-one is substantially different from the effect it has upon pkg-two,
then thing is not a suitable candidate for being made a global flag. In
particular, note that if client and server USE flags are ever
introduced, they can not be global USE flags for this reason.

Before introducing a new global USE flag, it must be discussed on the
gentoo-dev mailing list."


Steen Eugen Poulsen wrote:
> I'm trying to write a Replicator for /etc/portage and that leads me to
> work with USE flags, trying to design the replication of them among
> similar systems, but I can't find the golden set of rules for how best
> to apply USE flags.
> 
> There seem to be a global/local USE flag system, but many so called
> global flags has duplicated description marking them as local flags, or
> they enable unneeded optional support.

Unneeded by whom? As for the duplicate local USE flags,  They are most
probably redundant.

> 
> Lets take one of the big ones python. This use flag enables optional
> python support in many packages, I don't see my system ever using the
> python support in most of these. Seems like it really should be a local
> pr. packet flag for most, a pythonapi flag perhaps.

See above.  Also note that you are not enabling support for pythonapi,
you are enabling support for python.  Would you enable support for
linuxheaders when you are without a linux kernel.

> 
> There is also a few cases where package X requires package Y to be
> compiled with an USE flag, but no testing is being done in the ebuild
> for it and the compilation fails. Example gnome-system-tools requires
> libxml2 to be compiled with python support.

If that is the case, then I would suggest you search bugzilla and then
file a bug.

> 
> Then there is flags like alsa, it's marked as a global flag and then
> virtualbox also has it marked as a local flag. I personally think
> virtualbox is somewhat right in saying it's a local flag, but perhaps it
> would be better if the flag didn't have a double meaning.

See above.  I can only assume that virtualbox doesn't need to have a
local alsa use flag.

> 
> As I see it, Gentoo's USE flag system is one of it's greatest strength,
> but at the moment seems like there is missing some overall design for
> how to implement USE flags, making it a lot harder to use USE flags, as
> there is no clear definition of global or local flags.
> 
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Re: [gentoo-dev] USE flag how are they supposed to work?

[Steen Eugen Poulsen]

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Alistair Bush skrev:
>     * It is used by many different packages.

yes, this is the rubber rule. It pretty much allows any use flag to be 
promoted to global when it has XX packages with it, the confusion comes 
because the number of package using a flag is no indication whatever you 
should set the flag globally or pr. package.

Seem to me that the word global is used in the portage tree to mean one 
thing and then when we edit make.conf and /etc/portage we get another 
global/local meaning.

>     * It has a general non-specific purpose.

Shouldn't that be "It has a general specific purpose".

> The second point is important. If the effect of the thing USE flag upon
> pkg-one is substantially different from the effect it has upon pkg-two,
> then thing is not a suitable candidate for being made a global flag. In
> particular, note that if client and server USE flags are ever
> introduced, they can not be global USE flags for this reason.

Again you seem to talk about specific purpose, but the quoted text say 
non-specific, excuse me for being confused.

>> I'm trying to write a Replicator for /etc/portage and that leads me to
>> work with USE flags, trying to design the replication of them among
>> similar systems, but I can't find the golden set of rules for how best
>> to apply USE flags.
>>
>> There seem to be a global/local USE flag system, but many so called
>> global flags has duplicated description marking them as local flags, or
>> they enable unneeded optional support.
> 
> Unneeded by whom?

The package in order for it to work. You don't need Java, Python, Perl, 
Lua, whatever scripting support in most packages. For most of the ones 
I've seen, I have to go write a Java/Python/Perl/Lua program, before I 
actually need it.

 > As for the duplicate local USE flags,  They are most
> probably redundant.



> See above.  Also note that you are not enabling support for pythonapi,
> you are enabling support for python.  Would you enable support for
> linuxheaders when you are without a linux kernel.

I don't understand what support for python is.

> If that is the case, then I would suggest you search bugzilla and then
> file a bug.

Off course, but thats hardly the point of mentioning here, it was for 
bringing general attention to a ebuild mistake, that happens again and 
again.

> See above.  I can only assume that virtualbox doesn't need to have a
> local alsa use flag.

alsa: Adds support for media-labs/alsa-lib (Advanced Linux Sound 
Architecture)

Local Flag: Enable support for ALSA instead of OSS 
(app-emulation/virtualbox)

In box's case it's an "alsa not oss" flag, the global flag tend to be 
"also alsa", though I wont put my head on the block claiming it's always 
that meaning it has.

>> As I see it, Gentoo's USE flag system is one of it's greatest strength,
>> but at the moment seems like there is missing some overall design for
>> how to implement USE flags, making it a lot harder to use USE flags, as
>> there is no clear definition of global or local flags.

The words is given different meaning depending on whatever I'm looking 
at the portage tree or working on configuring emerge. The portage trees 
global flag, is no indication whatever I should put the flag in USE="" 
in make.conf, in many cases a portage tree global flag is more an 
indication that I should use it locally pr. package.


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Re: [gentoo-dev] USE flag how are they supposed to work?

[Alistair Bush]

Ok, so I think I understand where you are coming from...

Firstly,  USE flags are not meant to have a specific purpose. Enabling
python support is non-specific as it doesn't describe how it is enabled
or what python support actually is.

Lets compare 2 packages to demonstrate...

subversion has a python use flag,  this flag will install a subversion
api for python.  so within python you an interact with subversion.

as another example.

vim also has a python use flag.  this flag enables the ability to
execute python commands from within vim.

_But_ it is important to note that nowhere does the python use flag
attempt to distinguish between the 2 types of support. This is where I
believe you are getting stuck.

The reason for having non-specific use flags is otherwise we would have
hundreds of duplicate local use flags whose descriptions differ only
slightly. We are not attempting to document all the functionality of
each package.  It is up to you to discover what packages you want to
have python support.

> 
> The words is given different meaning depending on whatever I'm looking
> at the portage tree or working on configuring emerge. The portage trees
> global flag, is no indication whatever I should put the flag in USE=""
> in make.conf, in many cases a portage tree global flag is more an
> indication that I should use it locally pr. package.
> 

well I would only agree with that if you correlate the global status of
a USE flag to the number of packages that use it and you are wanting to
only enable the bare minimum use flags for the pacakges that you require.

But anyway, in summary

If you want to find out what a packages USE flag does,  it is better to
read the upstream documentation.
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Re: [gentoo-dev] USE flag how are they supposed to work?

[Vlastimil Babka]

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Steen Eugen Poulsen wrote:
> Alistair Bush skrev:
>>     * It is used by many different packages.
> 
> yes, this is the rubber rule. It pretty much allows any use flag to be
> promoted to global when it has XX packages with it, the confusion comes
> because the number of package using a flag is no indication whatever you
> should set the flag globally or pr. package.

Right, the only indication is if you want the functionality of the flag
globally or per package :)

> Seem to me that the word global is used in the portage tree to mean one
> thing and then when we edit make.conf and /etc/portage we get another
> global/local meaning.

That's right. Global/local use flag descriptions have no relation to
global/local setting via make.conf/package.use. As a non-dev you might
just ignore the first classification.

>>> I'm trying to write a Replicator for /etc/portage and that leads me to
>>> work with USE flags, trying to design the replication of them among
>>> similar systems, but I can't find the golden set of rules for how best
>>> to apply USE flags.
>>>
>>> There seem to be a global/local USE flag system, but many so called
>>> global flags has duplicated description marking them as local flags, or
>>> they enable unneeded optional support.
>>
>> Unneeded by whom?
> 
> The package in order for it to work. You don't need Java, Python, Perl,
> Lua, whatever scripting support in most packages. For most of the ones
> I've seen, I have to go write a Java/Python/Perl/Lua program, before I
> actually need it.

Well then don't enable those flags globally (that they are "global"
doesn't mean you have to), enable them only for packages which have
depending packages requiring them. Which you currently find out only by
errors (graceful die messages, except bugs like the one you mentioned :)
until there are finally use deps.

> The words is given different meaning depending on whatever I'm looking
> at the portage tree or working on configuring emerge. The portage trees
> global flag, is no indication whatever I should put the flag in USE=""
> in make.conf, in many cases a portage tree global flag is more an
> indication that I should use it locally pr. package.

Right, as mentioned above, there is no relation, although the
"global/local" notion may suggest it. One might even ask why we have
separate use.desc and use.local.desc then. Good question :) IMHO it's
mostly administrative thing so people don't add many new global flags
without consultation, but still can quickly add local flags just for
their package.

- --
Vlastimil Babka (Caster)
Gentoo/Java
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