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
$OUT_PATHS
variable is a space-separated list of Nix store paths. In this case, we expect and want the shell to perform word splitting to make each output path its own argument tonix store sign
. Nix guarantees the paths will not contain any spaces, however a store path might contain glob characters. Theset -f
disables globbing in the shell. If you want to upload the.drv
file too, the$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.