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Reading logs

Reading CI logs can be confusing if you are unfamiliar with how Nix builds packages. Don't worry though, it is very simple once you know the basics of how Nix works.

One of Nix' goals is to never build the same thing twice, in order to save us a ton of time and resources.

It works like this for each package:

  1. Create the derivation, which is a pure function from inputs (source code + dependencies) to an output (example: the composable-node binary).
  2. Get the hash of this derivation.
  3. Look at our cache servers. Have we already built a derivation with this hash?
  • If we have already built this, we copy the result from the cache server.
  • If we have not built this yet, we do the actual building.

This is why most of our log lines are copying path statments like these:

copying path '/nix/store/qrqwd1ji31vmas9gax819j11w5ickgz1-gnugrep-3.7' from ''...
copying path '/nix/store/xcs3ns14mddbjsr96cg7mzkqp7ml21qi-centauri' from ''...

The first one is copying gnugrep-3.7 with hash qrqwd1ji31vmas9gax819j11w5ickgz1 from the cache server This is a system dependency, so it is in the global cache server. The second one is copying centauri with the hash xcs3ns14mddbjsr96cg7mzkqp7ml21qi from the cache server This is one of our own pacakges, so it is cached in our own cache server.

The "Build all packages" job

Most of our building happens in the "Build all packages" job, which builts all of our packages. This happens on two machines, x64-monster and arm-monster. These are beefy machines with a ton of cores, capable of compiling all of our packages in parallel for both CPU architectures.

Most of the log lines will be copying path (as mentioned above). For the derivations that are not found in the cache servers (the things that have been altered by your PR) you will see log lines like the following:

zombienet> patching sources
zombienet> updateAutotoolsGnuConfigScriptsPhase
zombienet> configuring
zombienet> no configure script, doing nothing
zombienet> building

Where zombienet> is the package being built, succeeded by the unaltered log lines emitted by the build process of this package.

When all packages have been downloaded/built, you will see the following log lines at the end:

Waiting to finish: 1 pushing, 3 in queue

This is the process of Nix uploading our built derivations to our cache server, so that they do not need to be rebuilt next time.

The "List built results" step

Once the "Build all packages" step is completed, we list all of the things that we have built, such as this:


Which packages will be built?

All of the packages that are in the monorepo's /flake/all.nix list.

The "Build all packages" step is failing! Why?

Read through the logs and look for packages that are being built. You will see

a-package-you-worked-on> Some error emitted by the pacakge you have altered in your PR

If you want to easily reproduce this error locally you can then run

nix build .\#a-package-you-worked-on -L

in your local copy of the monorepo.

I want to build everything that CI is building locally, how do I do this?

Just run the following:

nix build .\#all -L

on your machine, inside of the monorepo directory. If you have installed Nix correctly and configred your machine to use our cache server, then this should be quick as most derivations will be on our cache server.