Adding dependency syntax for a per-package tag probe

(Note: N == name, A == arch, V == version, R == release, E
== epoch
shorthand in what follows).

OK, I've been asked to implement
     Requires: N.A
one time too often.

So its time for a pre-emptive implementation so that I don't
have to  
waste
any more time explaining why a cross-arch dependency is
never going to
work.

aside: consider the aliasing problems with
x86_64/amd64/ia32e.

So I'm going to do an implementation and let packagers
discover all by
themsleves why cross-arch dependency are as silly and
useless as
%{?disttag} and Disttag: are.

Rather than creating a compound dependency string as in
    Requires: N.A
which will force all applications that use rpmlib to start
parsing the
data returned from header queries, I'm going to implement a
header
metadata probe.

Basically, the compound "N.A" is equivalent to
    Requires: N
    Requires: A
because all dependencies in rpm except triggers are
implicitly AND,  
as in
the logical statement
     condition1 AND condition2

The next part of the implementation (and fundamental reason
why  
packagers
are asking for the N.A compound) is that no package atm has
the matching
     Provides: A
Nor is anyone willing to add additional arch-qualified
strings to  
existing
packages even though the current rpm dependency mechanism
permits
everyone to add strings like (literally) N.A freely.
Rebuilding (and  
re-releasing
packages) is plain and simply too costly.

Since RPMTAG_ARCH is included in every existing package, I'm
going
to add a probe dependency on the header to synthesize
    Provides: A
by using headerGetEntry() to turn the value of RPMTAG_ARCH
into a  
Provides:

The syntax to accomplish this will be done by adding
existing  
RPMTAG_* names
as a class of probes.

An example using RPMTAG_ARCH probably clarifies the
gobbeldygook above:

Let's say you want to add a requirement on N.A of another
package. So
you would add (as always)
    Requires: N
In order to express the dependency on the value of
RPMTAG_ARCH from the
other package, one would write a probe dependency as
    Requires: Arch(N) = i386
(I'm leaving out E:V-R qualifiers for simplicity)

The Arch(...) wrapper is a hint to extract RPMTAG_ARCH from
package N  
using
headerGetEntry(), thereby augmenting the existing Provides:
in  
package N.

That syntax should generalize beyond RPMTAG_ARCH, and
further extensions
to handle package versioning, tag arrays and patterns seem
likely to  
be able to
be accomodated within the Foo(...) syntax.

The harder implementation problem is that a probe assumes
that an added
header is present when needed for probing. That is not true
atm,  
added header
references are not persistent in rpmlib to reduce memory
footprint,  
but -- at least
for RPMTAG_ARCH -- can be dealt with by synthesizing
    Provides: Arch(N) = i386
on the fly. Probing is always possible against rpmdb
headers. And  
implementing
a per-transaction backing store of added package headers is
not  
impossibly
hard down the road.

Opinions?

73 de Jeff
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Adding dependency syntax for a per-package tag probe

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