[gentoo-dev] Unification of variables used within SCM eclasses

[Michał Górny]

As suggested by ssuominen on bug #311101, I am posting the issue
to the mailing list.

Currently, various SCM eclasses differ very much in the subset
of features and control variables implemented. The idea is to establish
a single subset of such variables and rules for all SCM eclasses
to follow, and maybe even develop a common scm.eclass which would be
sourced by other SCM eclasses.

Variables suggested by me:

a) Common variables - the variables which would have to be used by
various SCM eclasses as default/fallback values.

1. ESCM_DISTDIR (defaulting to PORTAGE_ACTUAL_DISTDIR/PORTDIR)
    - an alternate parent dir to all SCM stores. It would be useful
	if user would like to use an small file-inefficient filesystem
	for main DISTDIR or rsync it with other machine (where SCM
	files are not as important as the tarballs are).

2. ESCM_OFFLINE (most eclasses use it already)
    - a common switch to easily switch off all network interaction.

3. ESCM_LIVE_FAIL_IF_REPO_NOT_UPDATED (similar to the one in git.eclass)
    - a common switch to force unpack() phase to fail if no updates
	were found during the pull/update.

b) Common eclass-specific variables - these ones should allow user to
override above variables for single SCM.

1. E*_STORE_DIR (defaulting to ${ESCM_DISTDIR}/*-src)
    - already used by few eclasses, allowing user to change
	the location where SCM-specific clones are stored.

2. E*_OFFLINE (defaulting to ${ESCM_OFFLINE})
    - allowing user to override global 'offline switch'. Thus, it
	should also support setting 'false' value to enable network
	interaction for single SCM.

3. E*_LIVE_FAIL_...
    - another override for the global one.

4. E*_REPO_URI
    - the URI to the main repository. It might be extended to support
	multiple URIs.

5. E*_REVISION
    - explicit expected-revision/tag specification, preferably along
	with implicit one (e.g. in ESVN_REPO_URI) deprecation.
	This would allow applications to easily distinguish
	between 'real' live ebuilds and snapshot ones fetching directly
	from the repo.

c) Common export variables - these ones should be exported by SCM eclass
and stored in environment.bz2 after successful emerge.

1. E*_VERSION (or _REVISION, or ...)
    - the version/revision to which the package was updated. This would
	be useful to determine whether the current repo is newer
	than one used when merging package.

2. E*_WC_PATH
    - the absolute path to the last-used clone dir (i.e.
	${E*_STORE_DIR}/sth) and thus the most probable location
	to perform further updates in.

d) Other:

1. ESCM_CUSTOM_FETCH
    - this one is not directly related to eclasses but for use of ebuild
	authors. Setting this in an ebuild should notice applications
	that the ebuild does use custom fetching procedures
	(i.e. fetches from multiple repositories in a manner
	unsupported directly by the eclass) and thus external
	applications should not try to update the repository themselves.

-- 
Best regards,
Michał Górny

<http://mgorny.alt.pl>
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