[Swift-devel] execute side md5sum

[Ben Clifford]

Its pretty straightforward to modify the wrapper to take a hash (eg 
md5sum) of input files before and output files after execution (I made a 
prototype yesterday afternoon) and log those hashes.

This gives a convenient summary of the content of the inputs and outputs 
that is automated and hard to break through lack of attention; and so is 
probably useful for questions like "was this run with the same version or 
a different version of a particular input file".

(this is not some universal versioning solution that will solve the 
unsolvable; instead it provides an answer to 'was this file the same or 
different than some other file?' over which can be laid other exciting 
versioning systems)

I think having something like this is probably useful optional 
functionality (enabled in the same was as kickstart, perhaps).

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