Non-deterministic package IDs are really bad in 7.10

[Mateusz Kowalczyk]

On 05/14/2015 05:43 PM, Joachim Breitner wrote:
> Hi,
> 
> Am Sonntag, den 10.05.2015, 19:39 +0100 schrieb Mateusz Kowalczyk:
>> I'd like to bring some attention to ticket #4012 about non-determinism.
>> As many of you may know, the nix package manager distributes binaries
>> throughout its binary caches. The binaries are shared as long as the
>> hash of some of their inputs matches: this means that we can end up with
>> two of the same hashes of inputs but thanks to #4012 means that the
>> actual contents can differ. You end up with machines with some packages
>> built locally and some elsewhere and due to non-determinism, the GHC
>> package IDs don't line up and everything is broken.
> 
> is NixOS sensitive to changes in the build directory. Debian is, and
> since 7.8 the build path creeps into the interface files and affects the
> hash: https://bugs.debian.org/785282
> 
> But you probably are not, otherwise you’d have complained when 7.8 was
> out, and not now :-)
> 
> Greetings,
> Joachim
> 

Not a problem for nix, the paths are are calculated based on the
derivation inputs. I don't want to go into details but if same
expression goes in, same path comes out. If the path that comes out is
different, you simply don't get to use it. More or less nix does
‘calculate the store path from this expression, check if it exists in
binary caches and fetch it if it does, build if it does not’.

So for us the store paths in the interface files would always be the
same for the same inputs hence no problem there.

-- 
Mateusz K.