[Nix-dev] Hydra: infinite CPU and RAM consumption in evaluator

[Christian Kauhaus]

Am 19.01.2017 um 11:12 schrieb Christian Theune:
> Nevertheless, it looks like we may have introduced poisonous code that sends
> the evaluator off into infinity. We’ll come back when we find out what it was.

I got it and I would like to share my findings.

We introduced a change that imported an item "nginxModules" into all-packages:

https://github.com/flyingcircusio/nixpkgs/blob/2926129b8940e880a7745e44df511a3d6adacfa9/nixos/modules/flyingcircus/packages/all-packages.nix#L59

The problem was that (1) this import returnes an attrset or attrsets (none
containing a derivation) and that our all-packages gets imported entirely into
our Hydra master job so that all of our custom packages are guaranteed to be
built:

https://github.com/flyingcircusio/nixpkgs/blob/fc-15.09-dev/nixos/release-flyingcircus.nix#L121

This construct seems to send Hydra's evaluator into nowhere land.

After removing the "nginxModules" attribute, everything seems to be fine again:

https://github.com/flyingcircusio/nixpkgs/blob/fc-15.09-dev/nixos/modules/flyingcircus/packages/all-packages.nix#L54

I don't understand the exact reason. My expectation would be that Hydra's
evaluator ignores attrsets defining no derivations (analogous to a Nix
expression like `collect isDerivation nixpkgs`. Obviously, it doesn't exactly
do so. But hey, now it's running again. :-)

Regards

Christian

-- 
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