[gentoo-dev] RFD: EAPI specification in ebuilds

[Ulrich Mueller]

Hi all,

The way how we currently specify the EAPI in ebuilds has some
problems. For example, there is no sane way to allow usage of features
of a new bash version in a new EAPI. So we are currently stuck with
bash 3.2. Also changes of global scope behaviour, like addition of new
global scope functions (similar to "inherit") are not possible.

These flaws are outlined in GLEP 55 [1]:
| In order to get the EAPI the package manager needs to source the
| ebuild, which itself needs the EAPI in the first place. Otherwise it
| imposes a serious limitation, namely every ebuild, using any of the
| future EAPIs, will have to be source'able by old package managers
| [...]

The council has voted down GLEP 55 more than a year ago, but at the
same time requested that a solution for the mentioned issues should be
found. [2] However, there was no progress since then.

The issue arose again in bug 402167 [3] where several solutions have
been discussed. Below, I try to summarise the possible options
resulting from that discussion.


*** Proposal 1: "Parse the EAPI assignment statement" ***

This first proposal would require that the syntax of the EAPI
assignment statement in ebuilds matches a well defined regular
expression. A scan of the Portage tree shows that the statement only
occurs in the following variations (using EAPI 4 as example):

   EAPI=4
   EAPI="4"
   EAPI='4'

Sometimes this is followed by whitespace or a comment (starting with
a # sign). Also, with very few exceptions the EAPI assignment occurs
within the first few lines of the ebuild. For the vast majority of
ebuilds it is in line 5.

Written in a more formal way, appropriate for a specification:
- Ebuilds must contain at most one EAPI assignment statement.
- It must occur within the first N lines of the ebuild (N=10 and N=30
  have been suggested).
- The statement must match the following regular expression (extended
  regexp syntax):
  ^[ \t]*EAPI=(['"]?)([A-Za-z0-9._+-]*)\1[ \t]*(#.*)?$

Note: The first and the third point are already fulfilled by all
ebuilds in the Portage tree. The second point will require very few
ebuilds to be changed (9 packages for N=10, or 2 packages for N=30).

The package manager would determine the EAPI by parsing the assignment
with above regular expression. A sanity check would be added. Citing
Zac Medico in [3]: "The fact that we can compare the probed EAPI to
the actual EAPI variable after the ebuild is sourced seems like a
perfect sanity check. We could easily detect inconsistencies and flag
such ebuilds as invalid, providing a reliable feedback mechanism to
ebuild developers."

This proposal comes in two variants:
1a) The change is applied retroactively for all EAPIs.
1b) It is only applied for EAPI 5 and later (which means that the
    result of the EAPI parsing would be discarded for earlier EAPIs).


*** Proposal 2: "EAPI in header comment" ***

A different approach would be to specify the EAPI in a specially
formatted comment in the ebuild's header. No syntax has been suggested
yet, but I believe that the following would work as a specification:
- The EAPI must be declared in a special comment in the first line of
  the ebuild's header, as follows:
- The first line of the ebuild must contain the word "ebuild",
  followed by whitespace, followed by the EAPI, followed by
  end-of-line or whitespace.

Again, the proposal comes in two variants:
2a) It is combined with a one time change of the file extension, like
    .ebuild -> .eb.
2b) The usual EAPI assignment statement in the ebuild is still
    required, at least for a transition period.

In the 2a case, the EAPI variable could be made read-only in bash
before sourcing the ebuild. In the 2b case, a sanity check similar to
the one mentioned above would be added.


What do you think?

(I really hope for a constructive discussion here. So, if you want
to comment that all of the above proposals suck and GLEP 55 is much
superior, then please open a new thread for it.)

Ulrich


[1] <http://www.gentoo.org/proj/en/glep/glep-0055.html#problem>
[2] <http://www.gentoo.org/proj/en/council/meeting-logs/20100823-summary.txt>
[3] <https://bugs.gentoo.org/show_bug.cgi?id=402167>