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# Using the `post-build-hook`

# Implementation Caveats

Here we use the post-build hook to upload to a binary cache. This is a simple and working example, but it is not suitable for all use cases.

The post build hook program runs after each executed build, and blocks the build loop. The build loop exits if the hook program fails.

Concretely, this implementation will make Nix slow or unusable when the internet is slow or unreliable.

A more advanced implementation might pass the store paths to a user-supplied daemon or queue for processing the store paths outside of the build loop.

# Prerequisites

This tutorial assumes you have configured an S3-compatible binary cache as a substituter, and that the `root` user's default AWS profile can upload to the bucket.

# Set up a Signing Key

Use `nix-store --generate-binary-cache-key` to create our public and private signing keys. We will sign paths with the private key, and distribute the public key for verifying the authenticity of the paths.

    # nix-store --generate-binary-cache-key example-nix-cache-1 /etc/nix/key.private /etc/nix/key.public
    # cat /etc/nix/key.public
    example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM=

Then update `nix.conf` on any machine that will access the cache. Add the cache URL to `substituters` and the public key to `trusted-public-keys`:

    substituters = https://cache.nixos.org/ s3://example-nix-cache
    trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM=

Machines that build for the cache must sign derivations using the private key. On those machines, add the path to the key file to the `secret-key-files` field in their `nix.conf`:

    secret-key-files = /etc/nix/key.private

We will restart the Nix daemon in a later step.

# Implementing the build hook

Write the following script to `/etc/nix/upload-to-cache.sh`:

    #!/bin/sh

    set -eu
    set -f # disable globbing
    export IFS=' '

    echo "Uploading paths" $OUT_PATHS
    exec nix copy --to "s3://example-nix-cache" $OUT_PATHS

> Note

> The `$OUT_PATHS` variable is a space-separated list of Nix store paths. In this case, we expect and want the shell to perform word splitting to make each output path its own argument to `nix store sign`. Nix guarantees the paths will not contain any spaces, however a store path might contain glob characters. The `set -f` disables globbing in the shell. If you want to upload the `.drv` file too, the `$DRV_PATH` variable is also defined for the script and works just like `$OUT_PATHS`.

Then make sure the hook program is executable by the `root` user:

    # chmod +x /etc/nix/upload-to-cache.sh

# Updating Nix Configuration

Edit `/etc/nix/nix.conf` to run our hook, by adding the following configuration snippet at the end:

    post-build-hook = /etc/nix/upload-to-cache.sh

Then, restart the `nix-daemon`.

# Testing

Build any derivation, for example:

    $ nix-build --expr '(import <nixpkgs> {}).writeText "example" (builtins.toString builtins.currentTime)'
    this derivation will be built:
      /nix/store/s4pnfbkalzy5qz57qs6yybna8wylkig6-example.drv
    building '/nix/store/s4pnfbkalzy5qz57qs6yybna8wylkig6-example.drv'...
    running post-build-hook '/home/grahamc/projects/github.com/NixOS/nix/post-hook.sh'...
    post-build-hook: Signing paths /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
    post-build-hook: Uploading paths /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
    /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example

Then delete the path from the store, and try substituting it from the binary cache:

    $ rm ./result
    $ nix-store --delete /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example

Now, copy the path back from the cache:

    $ nix-store --realise /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example
    copying path '/nix/store/m8bmqwrch6l3h8s0k3d673xpmipcdpsa-example from 's3://example-nix-cache'...
    warning: you did not specify '--add-root'; the result might be removed by the garbage collector
    /nix/store/m8bmqwrch6l3h8s0k3d673xpmipcdpsa-example

# Conclusion

We now have a Nix installation configured to automatically sign and upload every local build to a remote binary cache.

Before deploying this to production, be sure to consider the implementation caveats.