Materialization

What is materialization?

Capturing and storing the Nix files for a project so that they do not need to be built (or checked). This allows us to cache the input of an IFD (import from derivation).

Why use materialization?

Using functions like project, cabalProject, stackProject and hackage-package results in a lot of dependencies (all the dependencies of nix-tools for instance).

  • They can be slow to calculate (even if no work needs to be done it is not unusual for it to take 5 seconds per project).

  • They can be slow to build (or download) on machines that do not yet have them in the Nix store.

  • Hydra does not show progress because it does not provide feedback until it has a list of jobs and the list of jobs cannot depend on the Nix expressions being present (although this is often blamed on IFD it would be the same if it wrote out JSON files and read them in)

When is it OK to materialize?

  • The Nix expressions are unlikely to change frequently (and when it does you are happy to manually update it).

  • You are happy to script something to update the materialized Nix files automatically.

  • You are certain that the IFD you materialize is not system-dependent. If it was you'd obtain different Nix expressions depending on which system the IFD was evaluated.

How can we materialize the Nix files?

Lets say we want to build hlint. We might start with an hlint.nix file that looks like this:

let inherit (import ./. {}) sources nixpkgsArgs;
    pkgs = import sources.nixpkgs nixpkgsArgs;
    hlint = pkgs.haskell-nix.hackage-package {
      compiler-nix-name = "ghc8102";
      name = "hlint";
      version = "2.2.11";
    };
in hlint

Building this may result in a lot of output, but if you build it again it should give just:

$ nix-build hlint.nix -A components.exes.hlint
trace: No index state specified for hlint, using the latest index state that we know about (2021-01-04T00:00:00Z)!
/nix/store/2ybrfmcp79gg75ad4pr1cbxjak70yg8b-hlint-exe-hlint-2.2.11

To materialize the Nix files we need to take care to pin down the inputs. Stack projects have their inputs pinned through specifying the snapshot. For cabal projects this means we must specify the index-state of hackage we want to use:

let inherit (import ./. {}) sources nixpkgsArgs;
    pkgs = import sources.nixpkgs nixpkgsArgs;
    hlint = pkgs.haskell-nix.hackage-package {
      compiler-nix-name = "ghc8102";
      name = "hlint";
      version = "2.2.11";
      index-state = "2021-01-04T00:00:00Z";
    };
in hlint

Now if we build again we get a hint telling use how to calculate a suitable sha256 hash to turn the derivation containing the Nix files into a fixed-output derivation:

$ nix-build hlint.nix -A components.exes.hlint
trace: To make project.plan-nix for hlint a fixed-output derivation but not materialized, set `plan-sha256` to the output of the 'calculateMaterializedSha' script in 'passthru'.
trace: To materialize project.plan-nix for hlint entirely, pass a writable path as the `materialized` argument and run the 'updateMaterialized' script in 'passthru'.
/nix/store/2ybrfmcp79gg75ad4pr1cbxjak70yg8b-hlint-exe-hlint-2.2.11

$ nix-build hlint.nix -A project.plan-nix.passthru.calculateMaterializedSha | bash
trace: To make project.plan-nix for hlint a fixed-output derivation but not materialized, set `plan-sha256` to the output of the 'calculateMaterializedSha' script in 'passthru'.
trace: To materialize project.plan-nix for hlint entirely, pass a writable path as the `materialized` argument and run the 'updateMaterialized' script in 'passthru'.
04hdgqwpaswmyb0ili7fwi6czzihd6x0jlvivw52d1i7wv4gaqy7

For a Stack project all occurences of plan-nix and plan-sha256 are replaced by stack-nix and stack-sha256, respectively. We can add the hash as plan-sha256:

let inherit (import ./. {}) sources nixpkgsArgs;
    pkgs = import sources.nixpkgs nixpkgsArgs;
    hlint = pkgs.haskell-nix.hackage-package {
      compiler-nix-name = "ghc8102";
      name = "hlint";
      version = "2.2.11";
      index-state = "2021-01-04T00:00:00Z";
      plan-sha256 = "04hdgqwpaswmyb0ili7fwi6czzihd6x0jlvivw52d1i7wv4gaqy7";
    };
in hlint

Just adding the hash might help reuse of the cached Nix expressions, but Nix will still calculate all the dependencies (which can add seconds to nix-build and nix-shell commands when no other work is needed) and users who do not yet have the dependencies in their store will have to wait while they are built or downloaded.

Running nix-build again gives us a hint on what we can do next:

$ nix-build hlint.nix -A components.exes.hlint
trace: To materialize project.plan-nix for hlint entirely, pass a writable path as the `materialized` argument and run the 'updateMaterialized' script in 'passthru'.
/nix/store/2ybrfmcp79gg75ad4pr1cbxjak70yg8b-hlint-exe-hlint-2.2.11

To capture the Nix expressions we can do something like:

let inherit (import ./. {}) sources nixpkgsArgs;
    pkgs = import sources.nixpkgs nixpkgsArgs;
    hlint = pkgs.haskell-nix.hackage-package {
      compiler-nix-name = "ghc8102";
      name = "hlint";
      version = "2.2.11";
      index-state = "2021-01-04T00:00:00Z";
      plan-sha256 = "04hdgqwpaswmyb0ili7fwi6czzihd6x0jlvivw52d1i7wv4gaqy7";
      materialized = ./hlint.materialized;
    };
in hlint

Now we can copy the Nix files needed and build with:

$ nix-build hlint.nix 2>&1 | grep -om1 '/nix/store/.*-updateMaterialized' | bash
$ nix-build hlint.nix -A components.exes.hlint
building '/nix/store/wpxsgzl1z4jnhfqzmzg3xxv3ljpmzr5h-hlint-plan-to-nix-pkgs.drv'...
/nix/store/2ybrfmcp79gg75ad4pr1cbxjak70yg8b-hlint-exe-hlint-2.2.11

How can we check sha256 and materialized are up to date?

Let's pretend we had to go back to hlint version 2.2.10. We can tell haskell.nix to check the materialization either by:

  • Removing the materialization files with rm -rf hlint.materialized

  • Temporarily adding checkMaterialization = true;

If we choose to add the checkMaterialization flag you would have:

let inherit (import ./. {}) sources nixpkgsArgs;
    pkgs = import sources.nixpkgs nixpkgsArgs;
    hlint = pkgs.haskell-nix.hackage-package {
      compiler-nix-name = "ghc8102";
      name = "hlint";
      version = "2.2.10";
      index-state = "2021-01-04T00:00:00Z";
      plan-sha256 = "04hdgqwpaswmyb0ili7fwi6czzihd6x0jlvivw52d1i7wv4gaqy7";
      materialized = ./hlint.materialized;
      checkMaterialization = true;
    };
in hlint

This will fail and report the details of what is wrong and how to fix it:

$ nix-build hlint.nix -A components.exes.hlint

...

Calculated hash for hlint-plan-to-nix-pkgs was not 04hdgqwpaswmyb0ili7fwi6czzihd6x0jlvivw52d1i7wv4gaqy7. New hash is :
    plan-sha256 = "0jsgdmii0a6b35sd42cpbc83s4sp4fbx8slphzvamq8n9x49i5b6";
Materialized nix used for hlint-plan-to-nix-pkgs incorrect. To fix run: /nix/store/6wp0zzal40ls874f5ddpaac7qmii9y4z-updateMaterialized
builder for '/nix/store/61a0vginv76w4p9ycyd628pjanav06pl-hlint-plan-to-nix-pkgs.drv' failed with exit code 1
error: build of '/nix/store/61a0vginv76w4p9ycyd628pjanav06pl-hlint-plan-to-nix-pkgs.drv' failed
(use '--show-trace' to show detailed location information)

Checking the materialization requires Nix to do all the work that materialization avoids. So while it might be tempting to leave checkMaterialization = true all the time, we would be better off just removing materialized and plan-sha256.

How can we update the Nix files with a script?

We can simply put the commands we used earlier in a script:

#!/bin/sh

# Output new plan-sha256
nix-build hlint.nix -A project.plan-nix.passthru.calculateMaterializedSha | bash

# Update materialized Nix expressions
nix-build hlint.nix 2>&1 | grep -om1 '/nix/store/.*-updateMaterialized' | bash

Can we skip making a copy and use materialized = /nix/store/...?

Yes and it gives us the same speed improvement, however:

  • It does not help at all in restricted-eval mode (Hydra).

  • Users will still wind up building or downloading the dependencies needed to build the Nix files (if they do not have them).

For those reasons it might be best to make a copy instead of using the /nix/store/... path directly.

If you really want to use the /nix/store/... path directly you should guard against the path not existing as passing in a non-existing path is now an error:

let inherit (import ./. {}) sources nixpkgsArgs;
    pkgs = import sources.nixpkgs nixpkgsArgs;
    hlintPlan = /nix/store/63k3f8bvsnag7v36vb3149208jyx61rk-hlint-plan-to-nix-pkgs;
    hlint = pkgs.haskell-nix.hackage-package {
      compiler-nix-name = "ghc8102";
      name = "hlint";
      version = "2.2.11";
      index-state = "2021-01-04T00:00:00Z";
      plan-sha256 = "04hdgqwpaswmyb0ili7fwi6czzihd6x0jlvivw52d1i7wv4gaqy7";
      materialized = if __pathExists hlintPlan then hlintPlan else null;
    };
in hlint

Running when no building is needed is still slow in restricted evaluation mode.

$ time nix-build --option restrict-eval true -I . --option allowed-uris "https://github.com/NixOS https://github.com/input-output-hk" hlint.nix -A components.exes.hlint --show-trace
/nix/store/2ybrfmcp79gg75ad4pr1cbxjak70yg8b-hlint-exe-hlint-2.2.11

real    0m4.463s
user    0m4.440s
sys 0m0.461s
$ time nix-build hlint.nix -A components.exes.hlint
/nix/store/2ybrfmcp79gg75ad4pr1cbxjak70yg8b-hlint-exe-hlint-2.2.11

real    0m2.206s
user    0m1.665s
sys 0m0.332s