[Rpm-maint] [rpm] te: Don't reload header for JUSTDB transactions (#52)

[Colin Walters]

Ideally, we just let `mmap()` handle that.  That's what OSTree does for metadata by default.  It'd probably work for packed RPMs as well...except in practice we byteswap in `regionSwab()` so require a writable mapping =/  For OSTree, GVariant handles a lot of this automatically for interior data, for "exterior" integers we just byteswap on access.

Anyways, one option I looked at would be for OSTree to store *everything* from the original RPM (lead, signature, header), then attempt to re-synthesize it from the ostree commit when RPM asks for it.  

Downsides:
  - A pipe-to-self is just ugly and inefficient
  - I think I'd have to parse the RPM myself to accurately extract the start of the cpio stream

Positives:
 - We'd also store the original gpg sigs

I guess regardless, it'd be nice if RPM supported (somehow) a mode where the calling application could optionally provide just a `Header` reference, not a full fd to an RPM.


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https://github.com/rpm-software-management/rpm/pull/52#issuecomment-173925491
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