how hard is it to rewrite code in a .beam file?

[Matthias Lang]

Hi,

I'd appreciate some wisdom about writing a .beam -> .beam transform
which lets me load a few known (i.e. not every possible) .beam files
compiled with R10B into an R13B04 VM. 

I've looked at beam_load.c and beam_disasm.erl, they seem like
reasonable starting points. Anything else I should look at? Am
I doomed to fail, or do I have a sporting chance? Kostis? Björn? Tony?

---

I want to do this because upgrades on my embedded systems work roughly
like this:

    1. Unpack a release file
    2. Run gth_install.beam on the freshly unpacked release
    3. Reboot

This works fine until someone tries to downgrade from a modern release,
which uses R13B04, to an old release, which uses R10B:

  =ERROR REPORT==== 27-Aug-2010::15:07:03 ===
  beam/beam_load.c(1730): Error loading function gth_install:make_nand_symlinks/1: op m_plus p y i r:
  please re-compile this module with an R13B04 compiler 

I should have heeded warnings about efforts being made to preserve .beam
compatibility for two major OTP versions only. But I didn't. Duh.

I have a few ideas about how to extricate myself from this,
e.g. running multiple VMs at the same time or providing new versions
of the old .beams, but the .beam->.beam seems fun and neat, given
that I only have to make it work for a handful of versions of one module.

Matt