[gentoo-dev] RFC: optinal run time dependencies

[Peter Volkov]

Hi guys. We've had discussion on optional runtime dependencies in bug
361255, but I think it's worth to have broader discussion of this issue.

=================================================================
Abstract

Optional runtime dependencies are dependent packages that are not
required to run program but when installed enhance program run-time
features. Below new use flag prefix is suggested to handle such rdeps.

Motivation

Optional run time dependencies are packages that are not required at
runtime
but when installed provide additional features for program. For example,
app-text/asciidoc works fine without media-gfx/graphviz, but to generate
diagrams from a textual specification media-gfx/graphviz have to be
installed.

Currently there are two common approaches for this problem:

1. add a use flag to control runtime dependency
2. add elog message into pkg_postinst to notify users that some features
depend
on installing package A, B, etc.

Both of this approaches are suboptimal.

Use-flags (1) will require user to rebuild package just to install
optional runtime
dependency.

elog messages do not present possible features during emerge --preted.
So 
after reading elog message this requires user to run another emerge just
to
have all asciidoc features installed.

elog-message approach complicates developement. Let's consider case
where
package foo depends on asciidoc with graph support. Without use flags 
foo-ver.ebuild have to contain something like:

RDEPEND="
	app-text/asciidoc
	media-gfx/graphviz"

Now it is possible that graph provider for app-text/asciidoc changes
(e.g. on
graphviz-ng). In such case all this packages that require asciidoc with
graph support
will have to be updated (controrary to use-flag approach where
asciidoc[graphviz]
dependency is sufficient).

Also sometimes more then one package is required to provide feature.
net-im/psi it requires both net-im/psimedia and app-crypt/qca-ossl:2 for
voice calls (jingle) support:

PDEPEND="
	jingle? ( net-im/psimedia
	app-crypt/qca-ossl:2 )"

net-im/psimedia builds fine without app-crypt/qca-ossl:2 but it is still
required
to have working jingle in psi. Such dependencies are not very evident
for
non-maintainer and may cause problems during upgrade if, say, another
bar
dependency will became required for jingle support, but user/developer
will be
not very attentive rereading yet another time ewarn messages.

Also elog approach adds text into anyway overloaded elog output.

Specification

Starting with EAPI=X new prefix ~ is allowed in IUSE use flag
definition. Use
flags prefixed with ~ are not allowed to be used anywhere but only
inside
PDEPEND dependency specification. This USE flags are used during
dependency
resolution as normal. Package manager is allowed to skip re-installation
of the
package due to this USE flag change but still it should record such USE
change
into package database.

Rationale

Since use-flag approach has only one drawback it's good idea to fix it.

Backwards Compatibility

This should be inside some future EAPI thus is backward compatible.
Already existing prefixes (+ and -) are allowed to be in any part of
prefix, i.e. IUSE="~foo ~+bar +~baz" should work.
=================================================================

Comments?

May be instead of ~ introduce three additional prefixes (~ and another
two for +~ and -~ cases)?

--
Peter.