# Immutable header regions in rpm-4.0.1 and later

The header data structure has changed in rpm-4.0.[12] to preserve the original header from a package. The goal is to keep the original header intact so that metadata can be verified separately from the payload by the RHN up2date client and by the rpm commmand line verify mode using signatures saved in the rpm database. I believe the change is entirely forward and backward compatible, and will not require any artifacts like changing the version number of packaging or adding an "rpmlib(...)" tracking dependendency. We'll see ...

Here's a short description of the change. An rpm header has three sections:

    	1) intro		(# entries in index, # bytes of data)
    	2) index		16 byte entries, one per tag, big endian
    	3) data			tag values, properly aligned, big endian

Representing sections in the header (ignoring the intro) with

    	A,B,C			index entries sorted by tag number
    	a,b,c			variable length entry data
    	| 			boundary between index/data

a header with 3 tag/value pairs (A,a) can be represented something like

    	ABC|abc

The change is to introduce a new tag that keeps track of a contiguous region (i.e. the original header). Representing the boundaries with square/angle brackets, an "immutable region" in the header thus becomes

    	[ABC|abc]

or more generally (spaces added for clarity)

    	[ABC> QRS | <abc] qrs

or with concatenated regions (not implemented yet)

    	[ABC> [DEF> QRS | <abc] <def] qrs

or with nested regions (not implemented yet)

    	[ABC [DEF>> QRS | <<abc] def] qrs

Todo:
  Either concatenated/nested regions may be used to implement a metarpm, aka a package of packages, dunno how, when, or even if, yet.

What complicates the above is legacy issues, as various versions of rpm have added/deleted/modified entries in the header freely. Thus, representing altered tag entries/data with a '.', there is a need to preserve deleted information something like

    	[A.C> QRS XYZ | <a.c] qrs xyz

Note:
  This is basically the change that replaces the filename with a {dirname,basename,dirindex} triple between rpm-3.x and rpm-4.x.

or

    	[AB.> QRS D | <ab.] qrs d

Note:
  The header is no longer sorted because of replacing Cc with Dd.

and yet permit retrieval of the original

    	[ABC|abc]

region. PITA, really.

What made header regions trickier yet is the desire to have an implementation that is both backward and forward compatible. I won't bore you with the tedious details.

However, even after doing regressions against supported Red Hat releases, there's a Great Big Universe of rpm packages out there, and I'm *very* interested in hearing (as bug reports against rpm at http://bugzilla.redhat.com) about any and all problems with header regions in rpm-4.0.1.