[Nix-dev] Binary cache substituter

[Michael Raskin]

>> Are binary diffs pregenerated - and only for a fixed set of second 
>> arguments?
>> 
>> If yes, narinfo could contain them. 
>
>Binary diffs are typically generated after the NAR / .narinfo has been produced,
>so adding them later requires modifying an existing .narinfo file.  That would
>be annoying because life is a lot simpler if .narinfo files are immutable.
>
>Moreover, since patches can be chained, you need to know the entire graph of
>available patches.  The manifest provides that graph, so it's efficient to
>compute the shortest path from the available paths to the target path.  But
>without the manifest, you'd have to recursively fetch all .narinfo files that
>*might* contribute to the graph, which would be pretty slow.

Hm. What about hash-of-name -> list-of-binary patches lists? It would be
just as bad as MANIFEST from mutability point of view, but without 
downloading a huge chunk of data in one sitting.