en/glep/glep-0033.txt

GLEP: 33
Title: Eclass Restructure/Redesign
Version: $Revision: 1.0 $
Last-Modified: $Date: 2005/02/15 00:00:00 $
Author: John Mylchreest <johnm@gentoo.org>, Brian Harring <ferringb@gentoo.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 29-Jan-2005
Post-History: 29-Jan-2005


Abstract
========

For any design, the transition from theoretical to applied exposes inadequacies 
in the original design.  This document is intended to document, and propose a 
revision of the current eclass setup to address current eclass inadequacies.

This document proposes several thing- the creation of ebuild libraries, 'elibs', 
a narrowing of the focus of eclasses, a move of eclasses w/in the tree, the 
addition of changelogs, and a way to allow for simple eclass gpg signing.
In general, a large scale restructuring of what eclasses are and how they're
implemented.  Essentially version two of the eclass setup.


Terminology
===========

From this point on, the proposed eclass setup will be called 'new eclasses', the
existing crop (as of this writing) will be referenced as 'old eclasses'.  The
destinction is elaborated on within this document.


Motivation and Rationale
========================

Eclasses within the tree currently are a bit of a mess- they're forced to
maintain backwards compatability w/ all previous functionality.  In effect,
their api is constant, and can only be added to- never changing the existing
functionality. This obviously is quite limiting, and leads to cruft accrueing in
eclasses as a eclasses design is refined.  This needs to be dealt with prior to
eclass code  reaching a critical mass where they become unmanagable/fragile
(recent pushes for eclass versioning could be interpretted as proof of this).

Beyond that, eclasses were originally intended as a method to allow for ebuilds
to use a pre-existing block of code, rather then having to duplicate the code in
each ebuild.  This is a good thing, but there are ill effects that result from
the current design. Eclasses inherit other eclasses to get a single function- in
doing so, modifying the the exported 'template' (default src_compile, default
src_unpack, various vars, etc). All the eclass designer was after was reusing a
function, not making their eclass sensitive to changes in the template of the
eclass it's inheriting.  The eclass designer -should- be aware of changes in the
function they're using, but shouldn't have to worry about their default src_*
and pkg_* functions being overwritten, let alone the env changes.

Addressing up front why a collection of eclass refinements are being rolled into
a single set of changes, parts of this proposal -could- be split into multiple
phases.  Why do it though?  It's simpler for developers to know that the first
eclass specification was this, and that the second specification is that, 
rather then requiring them to be aware of what phase of eclass changes is in 
progress.

By rolling all changes into one large change, a line is intentionally drawn in
the sand.  Old eclasses allowed for this, behaved this way.  New eclasses allow
for that, and behave this way. This should reduce misconceptions about what is
allowed/possible with eclasses, thus reducing bugs that result from said
misconceptions.


Specification.
==============

The various parts of this proposal are broken down into a set of changes and
elaborations on why a proposed change is preferable.  It's advisable to the
reader that this be read serially, rather then jumping around.


Ebuild Libraries (elibs for short)
----------------------------------

As briefly touched upon in Motivation and Rationale, the original eclass design
allowed for the eclass to modify the metadata of an ebuild, metadata being the
DEPENDS, RDEPENDS, SRC_URI, IUSE, etc, vars that are required to be constant,
and used by portage for dep resolution, fetching, etc.  Using the earlier
example, if you're after a single function from an eclass (say epatch from
eutils), you -don't- want the metadata modifications the eclass you're
inheriting might do.  You want to treat the eclass you're pulling from as a
library, pure and simple.

A new directory named elib should be added to the top level of the tree to serve
as a repository of ebuild function libraries.  Rather then relying on using the
source command, an 'elib' function should be added to portage to import that
libraries functionality.  The reason for the indirection via the function is 
mostly related to portage internals, but it does serve as an abstraction such 
that (for example) zsh compatability hacks could be hidden in the elib function.

Elib's will be collections of bash functions- they're not allowed to do anything
in the global scope aside from function definition, and any -minimal-
initialization of the library that is absolutely needed.  Additionally, they 
cannot modify any ebuild functions- src_compile, src_unpack fex.  Since they are
required to not modify the metadata keys, nor in any way affect the ebuild aside
from providing functionality, they can be conditionally pulled in.  They also
are allowed to pull in other elibs, but strictly just elibs- no eclasses, just
other elibs. A realworld example would be the eutils eclass.

Portage, since the elib's don't modify metadata, isn't required to track elibs
as it tracks eclasses.  Thus a change in an elib doesn't result in half the tree
forced to be regenerated/marked stale when changed (this is more of an infra
benefit, although regen's that take too long due to eclass changes have been
known to cause rsync issues due to missing timestamps).  The only thing portage
will do for elibs, aside from provide the elib function, is track what elibs
have been loaded thus far, and load an elib only if it hasn't been loaded once
already. An implication of this (if it wasn't clear from the elib description)
is that elibs cannot change their exported api dependant on the api (as some
eclass do for example).

Regarding maintainability of elibs, it should be a less of a load then old
eclasses.  One of the major issues with old eclasses is that their functions are
quite incestuous- they're bound tightly to the env they're defined in. This
makes eclass functions a bit fragile- the restrictions on what can, and cannot
be done in elibs will address this, making functionality less fragile (thus a
bit more maintainable).

There is no need for backwards compatability with elibs- they just must work
against the current tree.  Thus elibs can be removed when the tree no longer
needs them.  The reasons for this are explained below.

Structuring of the elibs directory will be exactly the same as that of the new
eclass directory (detailed below), sans a different extension.


The reduced role of Eclasses, and a clarification of existing Eclass requirements
---------------------------------------------------------------------------------

Since elibs are now intended on holding common bash functionality, the focus of
eclasses should be in defining an appropriate template for ebuilds. For example,
defining common DEPENDS, RDEPENDS, src_compile functions, src_unpack, etc.
Additionally, eclasses should pull in any elibs they need for functionality.

Eclass functionality that isn't directly related to the metadata, or src_* and
pkg_* funcs should be shifted into elibs to allow for maximal code reuse.  This
however isn't a hard requirement, merely a strongly worded suggestion.

Previously, it was 'strongly' suggested by developers to avoid having any code
executed in the global scope that wasn't required.  This suggestion is now a
requirement.  Execute only what must be executed in the global scope.  Any code
executed in the global scope that is related to configuring/building the package
must be placed in pkg_setup.  Metadata keys (already a rule, but now stated as
an absolute requirement to clarify it) *must* be constant.  The results of
metadata keys exported from an ebuild on system A, must be *exactly* the same as
the keys exported on system B.

If an eclass (or ebuild for that matter) violates this constant requirement, it
leads to portage doing the wrong thing for rsync users- for example, wrong deps
pulled in, leading to compilation failure.

If the existing metadata isn't flexible enough for what is required for a
package, the parsing of the metadata is changed to address that.  Cases where
the constant requirement is violated are known, and a select few are allowed-
these are exceptions to the rule that are required due to inadequacies in
portage.  In other words, those *few* exceptions are allowed because it's the
only way to do it at this time.  Any case where it's determined the constant
requirement may need to be violated the dev must make it aware to the majority
of devs, and the portage devs- violation of the constant rule has far reaching
effects.

It's quite likely there is a way to allow what you're attempting- if you just go
and do it, the rsync users (our userbase) suffer the results of compilation
failures and unneeded deps being pulled in.

After that stern reminder, back to new eclasses.  Defining INHERITED and ECLASS
within the eclass is no longer required.  Portage already handles those vars if
they aren't defined.

As with elibs, it's no longer required backwards compatability be maintained
indefinitely- compatability must be maintained against the current tree, but
just that.  As such new eclasses (the true distinction of new vs old is
elaborated in the next section) can be removed from the tree once they're no
longer in use.


The end of backwards compatability...
-------------------------------------

With current eclasses, once the eclass is in use, it's api can no longer be
changed, nor can the eclass ever be removed from the tree.  This is why we still
have *ancient* eclasses that are completely unused sitting in the tree, for
example inherit.eclass .  The reason for this, not surprisingly is a portage 
deficiency- on unmerging an installed ebuild, portage used the eclass from the
current tree.

For a real world example of this, if you merged a glibc 2 years back, whatever
eclasses it used must still be compatible, or you may not be able to unmerge the
older glibc version during an upgrade to a newer version.  So either the glibc
maintainer is left with the option of leaving people using ancient versions out
in the rain, or maintaining an ever increasing load of backwards compatability
cruft in any used eclasses.

Binpkgs suffer a similar fate.  Merging of a binpkg pulls needed eclasses from
the tree, so you may not be able to even merge a binpkg if the eclasses api has
changed.  If the eclass was removed, you can't even merge the binpkg, period.

The next major release of portage will address this- the environment that the
ebuild was built in already contains the eclasses functions, as such the env can
be re-used rather then relying on the eclass.  In other words, binpkgs and
installed ebuilds will no longer go and pull needed eclasses from the tree,
they'll use the 'saved' version of the eclass they were built/merged with.

So the backwards compatability requirement for users of the next major portage
version (and beyond) isn't required.  All the cruft can be dropped.

The problem is that there will be users using older versions of portage that
don't support this functionality.  So backwards compatability must be maintained
for them.  Additionally, earlier versions of portage haven't always handled the
env correctly- for broken saved envs, the eclasses backwards compatability is
still required.  Waiting N months preserving backwards compatability in current
eclasses, then dropping the support isn't much of an option.  There always are
stragglers who don't upgrade, beyond that, there is the possibility of cases
where users -will- upgrade, but still be bitten (broken saved envs from earlier
portage installations).  More importantly, it doesn't provide a route to
upgrade/fix things if a user lags behind, exempting trying to find a compatabile
version of the eclass in viewcvs (assuming it hasn't been sent to the attic
already).  Obviously, that isn't acceptable.

With the next major portage release, it will be possible to drop backwards
compatability for eclasses, and all lingering cruft.  What is needed is a way to
take full advantage of this functionality, without completely screwing over the
unfortunates and those who don't upgrade.

Unfortunately, the creation of new eclasses within the tree has an additional
snag due to portage.  The existing inherit function that is used to pull in old
eclasses- basically, whatever it's passed (inherit kernel or inherit
kernel/kernel) it will pull in (kernel.eclass, and kernel/kernel.eclass
respectively).  So even if the new eclasses were implemented within a
subdirectory of the eclass dir in the tree, all current portage versions would
still be able to access them.

In other words, these new eclasses would in effect, be old eclasses since older
portage versions could still access them.


Tree restructuring.
-------------------

There are only two way to block the existing (as of this writing) inherit
functionality from accessing the new eclasses- either change the extension of
eclasses to something other then 'eclass', or to have them stored in a seperate
subdirectory of the tree then eclass.

The latter is preferable, and the proposed solution.  Reasons are- the current
eclass directory is already overgrown.  Structuring of the new eclass dir
(clarified below) will allow for easier signing, ChangeLogs, and grouping of
eclasses.  New eclasses allow for something akin to a clean break and have new
capabilities/requirements, thus it's advisable to start with a clean directory, 
devoid of all cruft from the old eclass implementation.

If it's unclear as to why the old inherit function *cannot* access the new
eclasses, please reread the previous section.  It's unfortunately a requirement
to take advantage of all that the next major portage release will allow.

The proposed directory sructure is ${PORTDIR}/include/{eclass,elib}.
Something like ${PORTDIR}/new-eclass, or ${PORTDIR}/eclass-ng could be used
(although many would cringe at the -ng), but such a name is unwise. Consider the
possibility (likely a fact) that new eclasses someday may be found lacking, and
refined further (version three as it were). Or perhaps we want to add yet more
functionality with direct relation to sourcing new files, and we would then need
to further populate ${PORTDIR}.

The new-eclass directory will be (at least) 2 levels deep- for example:

::
        kernel/
        kernel/linux-info.eclass
        kernel/linux-mod.eclass
        kernel/kernel-2.6.eclass
        kernel/kernel-2.4.eclass
        kernel/ChangeLog
        kernel/Manifest

No eclasses will be allowed in the base directory- grouping of new eclasses will
be required to help keep things tidy, and for the following reasons.  Grouping
of eclasses allows for the addition of ChangeLogs that are specific to that
group of eclasses, grouping of files/patches as needed, and allows for
saner/easier signing of eclasses- basically, you can just stick a signed
Manifest file w/in that grouping, thus providing the information portage needs
to ensure no files are missing, and that nothing has been tainted.

The elib directory will be structured in the same way, for the same reasons.

Repoman will have to be extended to work within new eclass and elib groups, and
to handle signing and commiting. This is intentional, and a good thing. This
gives repoman the possibility of doing sanity checks on elibs/new eclasses.
It won't solve developers doing dumb things with eclasses (no technological
solution would, exempting a tazering), but it will give us a way to automate 
checks to try and prevent honest mistakes from slipping through and breaking
things for our users.


The start of a different phase of backwards compatability
---------------------------------------------------------

As clarified above, new eclasses will exist in a seperate directory that will be
intentionally inaccessible to the inherit function.  As such, users of older
portage versions *will* have to upgrade to merge any ebuild that uses elibs/new
eclasses. A depend on the next major portage version would address
transparently handle this for rsync users.

There still is the issue of users who haven't upgraded to the required portage
version. This is a minor concern frankly- portage releases include new
functionality, and bug fixes.  If they won't upgrade, it's assumed they have
their reasons and are big boys, thus able to handle the complications themselves.

The real issue is broken envs, whether in binpkgs, or for installed packages.
Two options exist- either the old eclasses are left in the tree indefinitely, or
they're left for N months, then shifted out of the tree, and into a tarball that
can be merged.

Shifting them out of the tree is advisable for several reasons- less cruft in
the tree, but more importantly the fact that they are not signed (thus an angle
for attack).  Note that the proposed method of eclass signing doesn't even try
to address them.  Frankly, it's not worth the effort supporting two variations
of eclass signing, when the old eclass setup isn't designed to allow for easy
signing.

If this approach is taken, then either the old eclasses would have to be merged
to an overlay directory's eclass directory (ugly), or to a safe location that
portage's inherit function knows to look for (less ugly).

For users who do not upgrade within the window of N months while the old
eclasses are in the tree, as stated, it's assumed they know what they are doing.
If they specifically block the new portage version, as the ebuilds in the tree
migrate to the new eclasses, they will have less and less ebuilds available to
them.  If they tried injecting the new portage version (lieing to portage,
essentially), portage would bail since it cannot find the new eclass.  Note that
for them to even get to this point, they'd have to somehow disable the DEPEND on
a new version of portage- either hack up the ebuild, or do an injection.
Essentially they'd have to actively try to sidestep sanity checks implemented to
make the shift over from old to new transparent.  If they've
disabled/sidestepped our attempt at a transparent migration, they can deal with
the repercussions of it.

What is a bit more annoying is that once the old eclasses are out of the tree,
users will lose the ability to unmerge any installed ebuild that used an old
eclass, further users will lose the ability to merge any tbz2 that uses old
eclasses.

They however will *not* be left out in the rain.  For merging old eclass
binpkgs, and unmerging installed packages, they can merge the old eclass compat
ebuild.  The compat ebuild provides the missing eclasses, thus providing that
lost functionality.  

The intention isn't to force them to upgrade, hence the ability to restore the
lost functionality. The intention is to clean up the existing mess, and allow us
to move forward. The saying "you've got to break a few eggs to make an omelete"
is akin, exempting the fact we're providing a way to make the eggs whole again
(the king's men would've loved such an option).

It's advisable that once all old eclasses are no longer in use in the tree, the
old eclass package is added to system default.  Remember that even those who
have upgraded to a portage version that handles the env correctly, may run into
instances where an installed packages env is corrupt.  For new bootstraps (which
automatically upgrade portage right off the bat), an injection of the compat
package would be advisable- unless they downgrade portage, they will never need
the old eclasses.


Migrating to the new setup
--------------------------

As has been done in the past whenever a change in the tree results in ebuilds
requiring a specific version of portage, as ebuilds migrate to the new eclasses,
they should depend on a version of portage that supports it.  From the users
viewpoint, this transparently handles the migration.

This isn't so transparent for devs or a particular infrastructure server however.
Devs, due to them using cvs for their tree, lack the pregenerated cache rsync
users have.   Devs will have to be early adopters of the new portage.  Older
portage versions won't be able to access the new eclasses, thus the local cache
generation for that ebuild will fail, ergo the depends on a newer portage
version won't transparently handle it for them.

Additionally, prior to any ebuilds in the tree using the new eclasses, the
infrastructure server that generates the cache for rsync users will have to
either be upgraded to a version of portage supporting new eclasses, or patched.
The former being much more preferable then the latter for the portage devs.

Beyond that, an appropriate window for old eclasses to exist in the tree must be
determined, and prior to that window passing an ebuild must be added to the tree
so users can get the old eclasses if needed.

For eclass devs to migrate from old to new, it is possible for them to just
transfer the old eclass into an appropriate grouping in the new eclass directory,
although it's advisable they cleanse all cruft out of the eclass. You can
migrate ebuilds gradually over to the new eclass, and don't have to worry about
having to support ebuilds from X years back.

Essentially, you have a chance to nail the design perfectly/cleanly, and have a
window in which to redesign it.  It's humbly suggested eclass devs take
advantage of it. :)


Backwards Compatibility
=======================

All backwards compatability issues are addressed inline, but a recap is offered-
it's suggested that if the a particular compatability issue is
questioned/worried over, the reader read the relevant section.  There should be
a more in depth discussion of the issue, along with a more extensive explanation
of the potential solutions, and reasons for the choosen solution.

To recap:
::

        New eclasses and elib functionality will be tied to a specific portage
        version. A DEPENDs on said portage version should address this for rsync
        users who refuse to upgrade to a portage version that supports the new
        eclasses/elibs and will gradually be unable to merge ebuilds that use said
        functionality.  It is their choice to upgrade, as such, the gradual
        'thinning' of available ebuilds should they block the portage upgrade is
        their responsibility.
        
        Old eclasses at some point in the future should be removed from the tree,
        and released in a tarball/ebuild. This will cause installed ebuilds that
        rely on the old eclass to be unable to unmerge to behave as expected, with
        the same applying for merging of binpkgs.
                
        This eclass ebuild should be a system depends target to make the transition
        transparent. Future portage ebuilds, and the old eclass compat ebuild should
        not inherit any eclasses. The reason for this is that in doing so, it may
        block upgrade paths. At least for portage, this already is something of a
        known issue for ebuild functionality- due to what it is/provides, it must
        essentially be standalone, and cannot benefit from any eclass/elib
        functionality.


Copyright
=========

This document has been placed in the public domain.