Impure derivations are derivations that can produce a different result
every time they're built. Example:
stdenv.mkDerivation {
name = "impure";
__impure = true; # marks this derivation as impure
outputHashAlgo = "sha256";
outputHashMode = "recursive";
buildCommand = "date > $out";
};
Some important characteristics:
* This requires the 'impure-derivations' experimental feature.
* Impure derivations are not "cached". Thus, running "nix-build" on
the example above multiple times will cause a rebuild every time.
* They are implemented similar to CA derivations, i.e. the output is
moved to a content-addressed path in the store. The difference is
that we don't register a realisation in the Nix database.
* Pure derivations are not allowed to depend on impure derivations. In
the future fixed-output derivations will be allowed to depend on
impure derivations, thus forming an "impurity barrier" in the
dependency graph.
* When sandboxing is enabled, impure derivations can access the
network in the same way as fixed-output derivations. In relaxed
sandboxing mode, they can access the local filesystem.
Rather than having four different but very similar types of hashes, make
only one, with a tag indicating whether it corresponds to a regular of
deferred derivation.
This implies a slight logical change: The original Nix+multiple-outputs
model assumed only one hash-modulo per derivation. Adding
multiple-outputs CA derivations changed this as these have one
hash-modulo per output. This change is now treating each derivation as
having one hash modulo per output.
This obviously means that we internally loose the guaranty that
all the outputs of input-addressed derivations have the same hash
modulo. But it turns out that it doesn’t matter because there’s nothing
in the code taking advantage of that fact (and it probably shouldn’t
anyways).
The upside is that it is now much easier to work with these hashes, and
we can get rid of a lot of useless `std::visit{ overloaded`.
Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>
1. `DerivationOutput` now as the `std::variant` as a base class. And the
variants are given hierarchical names under `DerivationOutput`.
In 8e0d0689be @matthewbauer and I
didn't know a better idiom, and so we made it a field. But this sort
of "newtype" is anoying for literals downstream.
Since then we leaned the base class, inherit the constructors trick,
e.g. used in `DerivedPath`. Switching to use that makes this more
ergonomic, and consistent.
2. `store-api.hh` and `derivations.hh` are now independent.
In bcde5456cc I swapped the dependency,
but I now know it is better to just keep on using incomplete types as
much as possible for faster compilation and good separation of
concerns.
This changes was taken from dynamic derivation (#4628). It` somewhat
undoes the refactors I first did for floating CA derivations, as the
benefit of hindsight + requirements of dynamic derivations made me
reconsider some things.
They aren't to consequential, but I figured they might be good to land
first, before the more profound changes @thufschmitt has in the works.
PRs #4370 and #4348 had a bad interaction in that the second broke the fist
one in a not trivial way.
The issue was that since #4348 the logic for detecting whether a
derivation output is already built requires some logic that was specific
to the `LocalStore`.
It happens though that most of this logic could be upstreamed to any `Store`,
which is what this commit does.
Rather than storing the derivation outputs as `drvPath!outputName` internally,
store them as `drvHashModulo!outputName` (or `outputHash!outputName` for
fixed-output derivations).
This makes the storage slightly more opaque, but enables an earlier
cutoff in cases where a fixed-output dependency changes (but keeps the
same output hash) − same as what we already do for input-addressed
derivations.
If we resolve using the known path of a derivation whose output we
didn't have, we previously blew up. Now we just fail gracefully,
returning the map of all outputs unknown.
The new interface we offer provides a way of getting all the
DerivationOutputs with the storePaths directly, based on the observation
that it's the most common usecase.
It's a tiny function which is:
- hardly worth abstrating over, and also only used once.
- doesn't work once we get CA drvs
I rewrote the one callsite to be forwards compatable with CA
derivations, and also potentially more performant: instead of reading in
the derivation it can ust consult the SQLite DB in the common case.
