Lix contributors

Information you might want if you're thinking about contributing to Lix

Lix Beta Guide

Thank you for choosing to help us in our beta!

There is a lot of work-in-progress documentation and a lot of it is work in progress or awaiting move to the wiki. Our apologies for this state, let us know if there is something you need.

If you run into any friction, please let us know. We would like to hear all your complaints, and this beta is as much about testing our processes as it is about testing the software.

Getting yourself set up with an account (if desired)

Sign in with GitHub on https://identity.lix.systems.

Note that your email will be visible on Gerrit if you use it, so change it on https://identity.lix.systems if necessary.

A brief tour of the Lix systems

See Information Organisation for where information is.

The Lix sources are developed on Gerrit, built with Buildbot, and released on a Forgejo repo.

Contributor documentation for the project is maintained on this wiki. FIXME(jade): a lot of it is awaiting migration onto the wiki from the private pad system, see tracking issue.

Status

We are confident enough to run nightly builds on the machines we care about. We expect Lix to have, generally, fewer bugs than Nix 2.18, which is what you probably already have.

Notable changes:

We have an installer, but it is not easy to use for HEAD builds. We also have a binary cache but we need to do more work to make it actually hit for building HEAD.

On NixOS/nix-darwin

Use the overlay: https://git.lix.systems/lix-project/nixos-module

Please file bugs if this explodes the build of tooling you use, we can fix it in the overlay.

Flakes

Add Lix to your system configuration like so:

{
  inputs = {
    lix = {
      url = "https://git.lix.systems/lix-project/lix/archive/main.tar.gz";
      flake = false;
    };

    lix-module = {
      url = "https://git.lix.systems/lix-project/nixos-module/archive/main.tar.gz";
      inputs.nixpkgs.follows = "nixpkgs";
      inputs.lix.follows = "lix";
    };
  };
  
  outputs = {nixpkgs, lix-module, lix, ...}: {
    # or equivalent for darwin
    nixosConfigurations.your-box = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ./machines/your-box
        lix-module.nixosModules.default
      ];
    };
  };
}

You can then update it with nix flake update lix; nix flake update lix-module.

Not flakes

Also supported.

Add inputs for git+https://git.lix.systems/lix-project/lix and git+https://git.lix.systems/lix-project/nixos-module to your preferred pinning tool.

Use in a NixOS module: e.g. imports = [ (import "${your-pinning-thingy.lix-nixos-module}/module.nix" { lix = your-pinning-thingy.lix; }) ];

Niv

Add the sources for the module and Lix itself, using ssh:// after registering your keys with git.lix.systems:

$ niv add git -n lix-nixos-module --repo 'https://git.lix.systems/lix-project/nixos-module'
$ niv add git -n lix-lix --repo 'https://git.lix.systems/lix-project/lix'

Then, import the Lix NixOS module:

  imports = [
    (import "${sources.lix-nixos-module}/module.nix"
      (let lix = sources.lix-lix.outPath;
      in {
        inherit lix;
        versionSuffix =
          "pre${builtins.substring 0 8 lix.lastModifiedDate}-${lix.shortRev}";
      }))
  ];

On other Linux or on macOS

Currently we are still working on the installer (see tracking project). It is possible to convert an existing Nix install to Lix.

flakey-profile

This is experimental. We believe that people have successfully used it on macOS. We have tested it on an Arch Linux system installed a long time ago with the shell-based installer, and it works fine. This method works by replacing your system profile with one that is built by simple Nix code with flakey-profile.

You can rollback if it blows up by /nix/var/nix/profiles/default-{SECOND-HIGHEST-NUMBER}/bin/nix-env --rollback --profile /nix/var/nix/profiles/default.

Clone https://git.lix.systems/lix-project/nixos-module.git, then, inside it, run sudo nix run --extra-experimental-features 'nix-command flakes' .#system-profile.switch.

Finally, run sudo systemctl daemon-reload && sudo systemctl restart nix-daemon, or, for macOS (FIXME someone please test these; they won't break your computer but I haven't tested them myself):

sudo launchctl stop system/org.nixos.nix-daemon
sudo launchctl enable system/org.nixos.nix-daemon
# the below command has been known to crash macos occasionally. don't run it unless you are sure of what you are doing.
# sudo launchctl load /Library/LaunchDaemons/org.nixos.nix-daemon.plist
sudo launchctl kickstart -k system/org.nixos.nix-daemon

Manually, with nix profile

We::Qyriad have used these steps on macOS has it has seemed to work, but we would recommend flakey-profile over it.

$ sudo -H --preserve-env=SSH_AUTH_SOCK nix --experimental-features 'nix-command flakes' profile install --profile /nix/var/nix/profiles/default git+ssh://git@git.lix.systems/lix-project/lix --priority 3

If you then run sudo nix --experimental-features 'nix-command flakes' profile list --profile /nix/var/nix/profiles/default, you should get output similar to this:

Index:              0
Store paths:        /nix/store/8ma7xas2nb0i3lq8mm7fpgalv94s8pzh-nss-cacert-3.92

Index:              1
Store paths:        /nix/store/53r8ay20mygy2sifn7j2p8wjqlx2kxik-nix-2.19.2

Index:              2
Flake attribute:    packages.aarch64-darwin.default
Original flake URL: git+ssh://git@git.lix.systems/lix-project/lix
Locked flake URL:   git+ssh://git@git.lix.systems/lix-project/lix?ref=refs/heads/main&rev=98b497a1a43a4ff39263ed5259f12c5d00b4d8c0
Store paths:        /nix/store/8040hxr4rr8bpb5yp4b48709x3qs4bwb-nix-2.90.0

You may then use sudo nix --experimental-features 'nix-command flakes' profile remove --profile /nix/var/nix/profiles/default 1 to remove your original installation of Nix. This is (probably) optional.

Verification

You should now get something like the following:

~ » nix --version
nix (Nix) 2.90.0-lixpre20240324-f86b965

Information organisation

Lix has a lot of information as a project, and we want to make it accessible in a way that it can be found later if necessary.

There are various tools for keeping information in the project, and they have different purposes

Chat

Chat is good for:

The chat is expected to move way too fast to follow. As such:

The log pad (https://pad.lix.systems/lix-event-log [private])

The log pad is intended as a tool to communicate what is going on in general, without having to have everyone pay attention to chat too much.

It should be used for:

It should not be used for:

Pad (https://pad.lix.systems [private])

We anticipate that the pad service will be semi-permanently private by default, since it doesn't support ACLs.

The pad is good for:

The pad is not good for:

N.B. For users who aren't in the Lix core team, the service returns 500 when you attempt to login. This is a known issue that can't be fixed.

Wiki (https://wiki.lix.systems)

The wiki is good for:

The wiki is not good for:

Markdown files in the Lix repo

Markdown files in the Lix repo are good for:

Markdown files in the Lix repo are bad for:

Forgejo issues (https://git.lix.systems)

Our primary issue tracker is Forgejo issues.

We are currently attempting to use the Forgejo project boards feature to communicate what people are working on; it may be replaced with better Kanban software in the future. When making project boards on Forgejo, make them on the lix-project organisation unless they are strictly contained within one project.

The issue tracker is good for:

The issue tracker is not good for:

Where to put an issue

Gerrit (https://gerrit.lix.systems)

Gerrit is good for:

Gerrit is not good for:

Why Lix?

(page under construction. editor's note: parts of https://pad.lix.systems/lix-manifesto (PRIVATE) are ported, parts need review before posting here)

(editor's note (ii): this page wants to be a contributor facing page, as opposed to the website page that maybe will have more general info?)

We should introduce ourselves! We are the Lix team, and we are working on a fork of CppNix focused on stability and user experience over features.

Core team members

FAQ

What is Lix anyway?

Lix is a fork of CppNix 2.18, focused on stability and the user experience of both users and contributors. We want to create a safe platform to move Nix technology forward, as a piece of critical infrastructure.

To this end, we have instituted a freeze on the core, where we apply high standards to changes to the core of the system and pursue testing and stability as our first priority on the core. Our long term vision is to shrink and decouple parts of the core, and move features like Flakes to the periphery of the system.

To achieve our goals in user experience, we are allowing significantly more contributions, still with tests, to the user facing surface of the system where there are fewer stability guarantees, and explicitly define what is expected to be stable and what can change.

Part of our work on the interface of Lix is in Qyriad's project Xil, which is an experiment in an alternate CLI for Nix implementations, which will potentially slowly merge with the Lix CLI.

Technical differences from CppNix

Views on flakes

The Lix project acknowledges that flakes are the way that the majority of people use Nix today, and does not intend to remove support for them. However, as part of our overall focus on stability and dependability, some features of Flakes will be changed to be stricter.

Flakes are not the only way to write Nix language code in Lix, and we intend to provide a good experience to flakes users, while also improving the experience for those not using flakes, by evolving a compatible but more flexible flake-like abstraction in the periphery of the Lix system.

Why is Lix different from tvix?

tvix is a Nix implementation from the ground up in Rust, aiming to be compatible with CppNix, by building a system from the ground up. It is developed by some of the same people. tvix also aims to improve the stability of Nix technology, but with the approach of starting from the beginning.

Lix is intended to evolve CppNix into a stable foundation for future evolution, without breaking clients along the way. Its goals are to aggressively pursue technical debt and remove the skeletons from the closet, while remaining deliberate about behavioural changes through testing. Lix will contain Rust components in the near future.

The two projects have similar goals but different approaches, and there will likely be cross-pollination between them; though cross-pollination of code is difficult due to licensing.

Style Guide

Not just about code, a style guide is a list of decisions we've made, that we want to be consistent about going forwards. It does not need to be comprehensive of all possible issues, nor does it need to confine itself to trivial topics such as formatting. It's a tool for ourselves so we don't forget where we've been, and can avoid solving the same problems again.

Don't be shy about adding to it. Things written here do impose some burden, but the hope is that they lessen other burdens in the long run. Use your judgement about what's worth it.

Please fix style issues in existing code as you encounter them. Style is aspirational, a journey not a destination. :)

Style Guide

Language and terminology

Language

Most existing Lix documentation is written in British English. We intend to continue with that.

Terminology

(FIXME: unsure if this should be in the style guide but ... it kinda should be -jade)

Style Guide

Code

Code changes

Tests

If at all practicable, all new code should be tested to some extent. If writing a test is hard, we need to prioritize making it easier, and potentially block features if that is the case.

Documentation

Reference documentation should be added, in addition to release notes (doc/manual/rl-next-dev), for user visible changes.

For notable dev facing changes, consider adding release notes in doc/manual/rl-next-dev. This is not critical for all changes; in some cases it may make more sense to write it up in dev documentation instead, and indeed it may be ok to defer writing that dev documentation (it's helpful to create an issue to not forget).

Benchmarking

Changes that touch the core of the evaluator or other performance critical code in Lix should be benchmarked.

See bench/README.md for instructions.

Changelist size

If a CL is too long to review, it should be split up into smaller pieces with tests. The exact length varies but passing the 1000 line mark should give significant thought to splitting.

Commit messages

Include at least a sentence or two as to why you are making a commit. For example, it can be nice to have the reproduction of a bug in the commit message. The commit message is the message for your review.

There's no particular format or specific style for commit messages; just make sure they're descriptive and informative.

C++

While we hope to migrate the lix interpreter from C++ to Rust eventually, C++ is a language that is likely to exist for a long time, and we may end up having to use it in other contexts.

Lix is a C++20 codebase. Features of C++20 that compile on all supported platforms can be used.

Static vs anonymous namespace

Prefer anonymous namespace, both currently exist in the codebase (jade: any other opinions?).

Type Aliases with typedef vs using

Prefer using declarations, as they can be used in more places, can be templatized, and have clearer syntax. Both currently exist in the codebase. (Qyriad)

TODO/FIXME/XXX Comments

jade: this is not consistent with the conventions I use, needs further discussion imo (TODO: block in pre-commit hook, used in local tree but should never pass code review, FIXME(name||feature): its busted, someone should go fix it later, XXX: this is bad, we are writing down that it is ugly but leaving it as-is as we didn't figure out a better way)

Something along the lines of:

Header files

Filenames

Headers should end with .hh. This reduces the likelihood anyone will try to include them from C files, which would require following the rules of both languages and is easy to get wrong.

The implementation of the functions declared in a .hh file should be in a .cc file of the same name, absent reasons to do otherwise.

Order-independence

Headers should not care what order they're loaded in.

The exception, for now, is config.h in the lix repo. This must always come before all other headers. This observation should not be taken to imply it must always be that way, but at the moment it's helpful to be aware of.

Idempotence

Use #pragma once, it helps. You can see this in most existing header files.

///@file and header documentation

///@file should be at the top of all nix headers - Doxygen and other tools use it to decide whether a header should have documentation generated for definitions in it. See the relevant Doxygen documentation for more details.

Strongly consider adding a description of the purpose of a header file at the top of it in with @brief A sentence saying what it is for.

Examples:

/**
 * @file
 * @brief This header is for meow meow cat noises.
 */

/// @file
/// @brief meow meow meow

Source files

Filenames

Source files should end with .cc.

Nix language

Unsurprisingly Nix contains Nix code. Some amount is tests and a lot is packaging.

We use the nixfmt formatter on files outside the test suite. It's run through treefmt with pre-commit hooks. Nix code outside the test suite is expected to be formatted.

Test suite files need not be formatted with the formatter at this time, but please consider doing so with new tests that don't rely on formatting.

with

Prefer not to use with to bring things into scope as it obscures the source of variables and degrades language server diagnostics.

Use let inherit (attrset) attrs instead.

Meson

Generally based on the style in Meson's docs made consistent and with a couple tweaks; notably multiline function calls are done in "block style" (think like rustfmt does it), rather than aligned, e.g.:

executable('sdlprog', 'sdlprog.c',
  win_subsystem : 'windows',
  dependencies : sdl2_dep,
)

rather than:

executable('sdlprog', 'sdlprog.c',
           win_subsystem : 'windows',
           dependencies : sdl2_dep)

Meson's docs go back and forth on this, but we also put a space before and after the colon for keyword arguments (so win_subsystem : 'windows', rather than win_subsystem: 'windows').

Style Guide

Operations

Operational Conventions

Code Review

Self Stamping and Merging

On our Gerrit, core members have permissions to +2 any arbitrary CL, because sometimes we should be able to get something in quickly, and talk about it afterwards. In almost every case, the author of a change should not +2 their own CL, however Lix members may use their best judgement so long as they talk about it with the team when they can. Some cases where skipping synchronous review is a good idea:

Just make sure to talk about what you do :)

Working in the Lix codebase

See also: See also: https://git.lix.systems/lix-project/lix/src/branch/main/doc/manual/src/contributing/hacking.md

Working in the Lix codebase

Codebase overview

The Lix system is constituted of two broad parts, the evaluator and the store daemon. The two pieces may run on the same machine or on different machines.

For example, in a remote build setup like https://hydra.nixos.org, one node is running several evaluators in parallel, and builds are dispatched to several builder nodes.

(fyi to anyone editing this, double click the image in the preview to edit it)

Evaluator

The evaluator is an AST tree-walking evaluator with lazy semantics.

Notable files:

Known design flaws

Lix team plans

Store protocol

The store protocol is a hand rolled binary protocol with monotonically increasing versioning. It runs over a few different transports such as ssh (src/libstore/ssh-store.cc) or Unix sockets (src/libstore/uds-remote-store.cc).

Known design flaws

Lix team plans

Store daemon

The store daemon takes derivations (≈ execve args and dependencies) and realises (builds or substitutes) them. It also implements store path garbage collection.

Lix's local store implementation currently uses a SQLite database as the source of truth for mapping derivation outputs to derivations as well as maintaining derivation metadata.

Notable files:

Known design flaws

Lix team plans

Working in the Lix codebase

Freezes and recommended contributions

Suggested contributions

Consider taking an issue marked E-help wanted: assign it to yourself and have a go. Feel free to ask for help in the development channel. The Lix team wants these issues fixed, but they are not high on our agenda to fix ourselves.

When in doubt, please ask the Lix team before beginning work, to make sure it is in line with our current priorities.

Freezes

This document describes the state of freeze that Lix is in.

We do expect to have main always be in a state to run on machines you care about, unconditionally. Nightly builds should not be a problem to run in production in any freeze state.

The purpose of this policy is to set expectations of what we are looking for in contributions, rather than to set hard rules.

Lix is currently in status "ice cube" until public release.

ice cube

No major features or code changes are accepted that touch the core (e.g. the hot paths of the evaluator, the store), absent a good justification, good test coverage and a strong belief that they will not cause regressions. In this state, we don't recommend external contributors do substantive work outside the roadmap without speaking with the Lix team first. However, this is a guideline, and such work can be done if discussed and planned carefully beforehand.

Simple surface features with low impact are likely to be accepted with tests, assuming that they do not impact reliability.

New tests are gleefully accepted.

Bug fixes (with tests) are gleefully accepted.

For example, the following would require discussion with the Lix team before work begins, as it is likely to not fit our goals:

For example, the following would likely be accepted assuming it has tests, without needing prior discussion:

hard ice cream

Changes that improve maintainability of the core are accepted, with careful review depending on their significance. Changes that add more complexity to the core need to pass scrutiny.

Features at the edge are accepted if they have low impact, assuming that they have tests.

soft serve

FIXME

milkshake

FIXME

Working in the Lix codebase

Bug tracker organisation

We have a repo of directly imported nix bugs at https://git.lix.systems/nixos/nix. Please don't file bugs in there, we want the IDs to match. When we import a bug, we might put notes on there as we triage it, and potentially close it.

Bug labels on NixOS/nix

Dispositions:

Closed, marginal

Working in the Lix codebase

Gerrit

What is Gerrit and why do people like it?

Gerrit is a code review system from Google in a similar style to Google's internal Critique tool, but based on Git, and publicly available as open source. It hosts a Git repo with the ability to submit changes for review and offers mirroring to other repos (like https://git.lix.systems/lix-project/lix). It has an entirely different review model to GitHub (and Forgejo, GitLab, etc that copy GitHub's review model), where, instead of pull requests, you have changelists (CLs): reviews on individual commits, with each revision of a commit being a different "patchset", rather than reviewing an entire branch at a time. CLs may be merged one by one or in a batch.

Although this has some learning curve, we expect that you will find it pleasant to work with after figuring it out. It has some rough edges and strong opinions that take some getting used to, but it has served us well and saved us an inordinate amount of time both as reviewers and change authors. The rest of this document gives some pointers on the workflows we use with Gerrit.

People like Gerrit because it makes the following things trivial or easy, all of which are somewhere between annoying and impossible on GitHub modeled systems:

Gerrit produces better code:

Gerrit makes reviewers' lives easier and reduces review round trips:

Gerrit makes your life easier as a contributor:

That being said, there are some downsides:

Learning materials

Our installation

Gerrit is at https://gerrit.lix.systems

The Gerrit SSH server is running on port 2022. The repo URLs are:

Hit the d key on any change to download it, which will give you the right URLs.

Basic workflow for a change

The unit of code review is a "change", which yields a single commit when "submitted" (merged). The commit message is taken from the change description in Gerrit; in our experience this tends to lead to more comprehensive commit messages.

For a change to be merged, it must have the following four "votes", in Gerrit's terminology:

If you're newly part of the core team you will need to add yourself to the Gerrit lix group, otherwise you can't set the Has-Release-Notes or Has-Tests labels. If you're not, this doesn't affect you.

When all of those labels are set, a change becomes Ready to submit, in Gerrit's termology, and Gerrit will give you a Submit button in the top right: Screenshot_20240507_165352.png

By convention, the change author has the final say on clicking the Submit button (note: this is the opposite of the Github convention), and there is no special permission to merge a change once it has been fully reviewed (the permissions are in the reviewer +2'ing it). This gives you a last chance to have a look at your change before merging it.

Workflow tips

Local branches and commits

Gerrit is very mean to you if you don't have your local commit history in a linear presentable state, which takes getting used to but it is very low overhead once you get used to it. In short, amended commits become "patchsets", new commits become changes, and multiple commits help link your changes together as a "relation chain".

Note: if you're coming from Chromium, this is different to how they use Gerrit, where multiple commits become patchsets, and only the first commit on a local branch creates a new change.

Gerrit’s commit-msg hook generates a new Change-Id for each commit you make, which in turn creates a new change that gets reviewed separately. To update an existing change after review feedback, amend or squash your changes into your old commit, keeping its Change-Id unchanged, then push.

Consider not pushing for review before it is clean, or split commits up with git-revise (good) or jj (better) after the fact, amending as you work. If you want a backup of your changes, you can fork it on Forgejo and push to that fork.

Basic Pushing

If you cloned the repo from Forgejo, be sure to change your remote URL to point to Gerrit before continuing. Assuming your remote is called origin (which is the default):

git remote set-url origin ssh://{username}@gerrit.lix.systems:2022/lix

Then you can push to Gerrit with:

git push origin HEAD:refs/for/main

If you get tired of doing this every time, you can automate it by setting the .git/config as follows:

git config remote.origin.push HEAD:refs/for/main

You will have to do that in each fresh check-out. Once it's done, git push will work without additional options.

If you get a “remote unpack failed” error while pushing, run git fetch then try again.

If you wish to push a change and immediately mark it as WIP, you can push with -o wip, or make that the default behavior by checking Set new changes to "work in progress" by default in Gerrit's user settings, under "Preferences".

Topics & Push Arguments

A Gerrit topic may be set on push with:

git push origin HEAD:refs/for/main%topic=foo

Which will create all pushed changes with the topic "foo". Topics are helpful for grouping long series of related changes.

A change may also be marked as "work in progress" on push:

git push origin HEAD:refs/for/main%wip

Gerrit has documentation on other push arguments you can use here, but it also takes a help argument whose output is more canonical and might be easier to understand, which you can view with:

git push origin HEAD:refs/for/main%help

At the time of this writing (2024/06/26), that output looks like this:

$ git push origin @:refs/for/main%help
Total 0 (delta 0), reused 0 (delta 0), pack-reused 0 (from 0)
remote: Processing changes: refs: 1, done
remote:
remote: Help for refs/for/branch:
remote:
remote:  --base BASE                            : merge base of changes
remote:  --cc CC                                : add CC to changes
remote:  --create-cod-token                     : create a token for consistency-on-dema
remote:                                           nd (default: false)
remote:  --deadline NAME                        : deadline after which the push should
remote:                                           be aborted
remote:  --edit (-e)                            : upload as change edit (default: false)
remote:  --hashtag (-t) HASHTAG                 : add hashtag to changes
remote:  --help (-h)                            : display this help text (default: true)
remote:  --ignore-automatic-attention-set-rules : do not change the attention set on
remote:  (-ias, -ignore-attention-set)            this push (default: false)
remote:  --label (-l) LABEL+VALUE               : label(s) to assign (defaults to +1 if
remote:                                           no value provided)
remote:  --merged                               : create single change for a merged
remote:                                           commit (default: false)
remote:  --message (-m) MESSAGE                 : Comment message to apply to the review
remote:  --no-publish-comments (--np)           : do not publish draft comments
remote:                                           (default: false)
remote:  --notify [NONE | OWNER |               : Notify handling that defines to whom
remote:  OWNER_REVIEWERS | ALL]                   email notifications should be sent.
remote:                                           Allowed values are NONE, OWNER,
remote:                                           OWNER_REVIEWERS, ALL. If not set, the
remote:                                           default is ALL.
remote:  --notify-bcc USER                      : user that should be BCC'd one time by
remote:                                           email
remote:  --notify-cc USER                       : user that should be CC'd one time by
remote:                                           email
remote:  --notify-to USER                       : user that should be notified one time
remote:                                           by email
remote:  --private                              : mark new/updated change as private
remote:                                           (default: false)
remote:  --publish-comments                     : publish all draft comments on updated
remote:                                           changes (default: false)
remote:  --ready                                : mark change as ready (default: false)
remote:  --remove-private                       : remove privacy flag from updated
remote:                                           change (default: false)
remote:  --reviewer (-r) REVIEWER               : add reviewer to changes
remote:  --skip-validation                      : skips commit validation (default:
remote:                                           false)
remote:  --submit                               : immediately submit the change
remote:                                           (default: false)
remote:  --topic NAME                           : attach topic to changes
remote:  --trace NAME                           : enable tracing
remote:  --wip (-work-in-progress)              : mark change as work in progress
remote:                                           (default: false)
remote:
To ssh://gerrit.lix.systems:2022/lix
 ! [remote rejected]     HEAD -> refs/for/main%help (see help)
error: failed to push some refs to 'ssh://gerrit.lix.systems:2022/lix'

Pulling

Pulling from Gerrit will work normally. It's worth keeping in mind that sometimes a CL you're working on has been edited in the web UI or by another contributor, so the commit in your repo isn't the latest. Rebasing will usually make the duplicate go away; this is part of the normal rebase semantics, not Gerrit magic. You might consider making rebase-on-pull your default.

Sandbox branches

This feature has some notable ways to shoot yourself in the foot. We still support it, since it allows for running CI builds on things before they become proper CLs. If you don't need that and don't want to worry about the footguns, consider using a branch on a Forgejo fork for sharing WIP code.

In particular, if a commit is in any branch already including a sb/ branch, it will be rejected with the error "no new changes" if it is later pushed to refs/for/main. This can be worked around by amending all the commits so they are distinct, or by git push origin HEAD:refs/for/main%base=$(git rev-parse origin/main), which forces the merge-base

Use refs/heads/sb/USERNAME/*.

CI rerun

Push the CL again with a no-changes commit amendment if you want to force CI to rerun.

Finding CLs to review

Consider bookmarking: https://gerrit.lix.systems/q/status:open+-is:wip+-author:me+label:Code-Review%3C2

Working in the Lix codebase

Improving build times

Setup

Use a clang stdenv:

nix develop .#native-clangStdenvPackages

Then delete build/ if you were using gcc before. Enable build-time profiling with:

just setup; meson configure build -Dprofile-build=enabled

Then run the build: just compile.

Enabling build-time profiling itself costs about 10% of compile time but has no other disadvantage.

Build time reports

Use maintainers/buildtime_report.sh build/ to generate a build time report. This will tell you where all our build time went by looking at the trace files and producing a badness summary.

Sample report

Build-time report sample 
lix/lix2 » ClangBuildAnalyzer --analyze buildtimeold.bin
Analyzing build trace from 'buildtimeold.bin'...
**** Time summary:
Compilation (551 times):
  Parsing (frontend):         1465.3 s
  Codegen & opts (backend):   1110.9 s

**** Files that took longest to parse (compiler frontend):
 10478 ms: build/src/libstore/liblixstore.so.p/build_local-derivation-goal.cc.o
 10319 ms: build/src/libexpr/liblixexpr.so.p/primops.cc.o
  9947 ms: build/src/nix/nix.p/flake.cc.o
  9850 ms: build/src/libexpr/liblixexpr.so.p/eval.cc.o
  9751 ms: build/src/nix/nix.p/profile.cc.o
  9643 ms: build/src/nix/nix.p/develop.cc.o
  9296 ms: build/src/libcmd/liblixcmd.so.p/installable-attr-path.cc.o
  9286 ms: build/src/libstore/liblixstore.so.p/build_derivation-goal.cc.o
  9208 ms: build/src/libcmd/liblixcmd.so.p/installables.cc.o
  9007 ms: build/src/nix/nix.p/.._nix-env_nix-env.cc.o

**** Files that took longest to codegen (compiler backend):
 24226 ms: build/src/libexpr/liblixexpr.so.p/primops_fromTOML.cc.o
 24019 ms: build/src/libexpr/liblixexpr.so.p/primops.cc.o
 21102 ms: build/src/libstore/liblixstore.so.p/build_local-derivation-goal.cc.o
 16246 ms: build/src/libstore/liblixstore.so.p/store-api.cc.o
 14586 ms: build/src/nix/nix.p/.._nix-build_nix-build.cc.o
 13746 ms: build/src/libexpr/liblixexpr.so.p/eval.cc.o
 13287 ms: build/src/libstore/liblixstore.so.p/binary-cache-store.cc.o
 13263 ms: build/src/nix/nix.p/profile.cc.o
 12970 ms: build/src/nix/nix.p/develop.cc.o
 12621 ms: build/src/libfetchers/liblixfetchers.so.p/github.cc.o

**** Templates that took longest to instantiate:
 42922 ms: nlohmann::basic_json<>::parse<const char *> (69 times, avg 622 ms)
 32180 ms: nlohmann::detail::parser<nlohmann::basic_json<>, nlohmann::detail::i... (69 times, avg 466 ms)
 27337 ms: nix::HintFmt::HintFmt<nix::Uncolored<std::basic_string<char>>> (246 times, avg 111 ms)
 25338 ms: nlohmann::basic_json<>::basic_json (293 times, avg 86 ms)
 23641 ms: nlohmann::detail::parser<nlohmann::basic_json<>, nlohmann::detail::i... (69 times, avg 342 ms)
 20203 ms: boost::basic_format<char>::basic_format (492 times, avg 41 ms)
 17174 ms: nlohmann::basic_json<>::json_value::json_value (368 times, avg 46 ms)
 15603 ms: boost::basic_format<char>::parse (246 times, avg 63 ms)
 13268 ms: std::basic_regex<char>::_M_compile (28 times, avg 473 ms)
 12757 ms: std::__detail::_Compiler<std::regex_traits<char>>::_Compiler (28 times, avg 455 ms)
 10813 ms: std::__detail::_Compiler<std::regex_traits<char>>::_M_disjunction (28 times, avg 386 ms)
 10719 ms: std::__detail::_Compiler<std::regex_traits<char>>::_M_alternative (28 times, avg 382 ms)
 10508 ms: std::__detail::_Compiler<std::regex_traits<char>>::_M_term (28 times, avg 375 ms)
  9516 ms: nlohmann::detail::json_sax_dom_callback_parser<nlohmann::basic_json<... (69 times, avg 137 ms)
  9112 ms: std::__detail::_Compiler<std::regex_traits<char>>::_M_atom (28 times, avg 325 ms)
  8683 ms: std::basic_regex<char>::basic_regex (18 times, avg 482 ms)
  8241 ms: std::operator<=> (438 times, avg 18 ms)
  7561 ms: std::vector<boost::io::detail::format_item<char, std::char_traits<ch... (246 times, avg 30 ms)
  7475 ms: std::vector<boost::io::detail::format_item<char, std::char_traits<ch... (246 times, avg 30 ms)
  7309 ms: std::reverse_iterator<std::_Bit_iterator> (268 times, avg 27 ms)
  7131 ms: boost::stacktrace::basic_stacktrace<>::basic_stacktrace (246 times, avg 28 ms)
  6868 ms: boost::stacktrace::basic_stacktrace<>::init (246 times, avg 27 ms)
  6518 ms: std::reverse_iterator<std::_Bit_const_iterator> (268 times, avg 24 ms)
  5716 ms: std::__detail::_Synth3way::operator()<std::variant<nix::OutputsSpec:... (182 times, avg 31 ms)
  5303 ms: nix::make_ref<nix::SingleDerivedPath, nix::DerivedPathOpaque> (178 times, avg 29 ms)
  5244 ms: std::__uninitialized_move_a<boost::io::detail::format_item<char, std... (246 times, avg 21 ms)
  4857 ms: std::make_shared<nix::SingleDerivedPath, nix::DerivedPathOpaque> (178 times, avg 27 ms)
  4813 ms: std::__detail::_Synth3way::operator()<std::variant<nix::TextIngestio... (158 times, avg 30 ms)
  4648 ms: nlohmann::detail::json_sax_dom_callback_parser<nlohmann::basic_json<... (69 times, avg 67 ms)
  4597 ms: std::basic_regex<char>::basic_regex<std::char_traits<char>, std::all... (10 times, avg 459 ms)

**** Template sets that took longest to instantiate:
 55715 ms: std::__do_visit<$> (3603 times, avg 15 ms)
 47739 ms: std::__detail::__variant::__gen_vtable_impl<$>::__visit_invoke (11132 times, avg 4 ms)
 43338 ms: nlohmann::basic_json<$>::parse<$> (85 times, avg 509 ms)
 43097 ms: std::__detail::__variant::__raw_idx_visit<$> (2435 times, avg 17 ms)
 32390 ms: nlohmann::detail::parser<$>::parse (83 times, avg 390 ms)
 30986 ms: nix::HintFmt::HintFmt<$> (1261 times, avg 24 ms)
 30255 ms: std::__and_<$> (25661 times, avg 1 ms)
 29762 ms: std::unique_ptr<$> (2116 times, avg 14 ms)
 28609 ms: std::__tuple_compare<$>::__eq (2978 times, avg 9 ms)
 27560 ms: nlohmann::detail::parser<$>::sax_parse_internal<$> (167 times, avg 165 ms)
 27239 ms: std::variant<$> (1959 times, avg 13 ms)
 26837 ms: std::__invoke_result<$> (10782 times, avg 2 ms)
 25972 ms: std::tuple<$> (5714 times, avg 4 ms)
 24247 ms: std::__uniq_ptr_data<$> (2116 times, avg 11 ms)
 24061 ms: std::__result_of_impl<$> (9029 times, avg 2 ms)
 23949 ms: std::__uniq_ptr_impl<$> (2116 times, avg 11 ms)
 21185 ms: std::optional<$> (2502 times, avg 8 ms)
 21044 ms: std::pair<$> (4989 times, avg 4 ms)
 20852 ms: std::__or_<$> (24005 times, avg 0 ms)
 20203 ms: boost::basic_format<$>::basic_format (492 times, avg 41 ms)
 20184 ms: std::tie<$> (2895 times, avg 6 ms)
 19938 ms: nlohmann::basic_json<$>::create<$> (668 times, avg 29 ms)
 19798 ms: std::allocator_traits<$>::construct<$> (5720 times, avg 3 ms)
 19182 ms: std::__detail::__variant::_Variant_base<$> (1959 times, avg 9 ms)
 19151 ms: std::_Rb_tree<$>::_M_erase (2320 times, avg 8 ms)
 19094 ms: std::_Rb_tree<$>::~_Rb_tree (2022 times, avg 9 ms)
 18735 ms: nlohmann::basic_json<$>::basic_json (243 times, avg 77 ms)
 18546 ms: std::__detail::_Synth3way::_S_noexcept<$> (2542 times, avg 7 ms)
 17174 ms: nlohmann::basic_json<$>::json_value::json_value (368 times, avg 46 ms)
 17111 ms: nlohmann::detail::conjunction<$> (907 times, avg 18 ms)

**** Functions that took longest to compile:
  2091 ms: _GLOBAL__sub_I_primops.cc (../src/libexpr/primops.cc)
  1799 ms: nix::fetchers::GitInputScheme::fetch(nix::ref<nix::Store>, nix::fetc... (../src/libfetchers/git.cc)
  1388 ms: nix::Settings::Settings() (../src/libstore/globals.cc)
  1244 ms: main_nix_build(int, char**) (../src/nix-build/nix-build.cc)
  1021 ms: nix::LocalDerivationGoal::startBuilder() (../src/libstore/build/local-derivation-goal.cc)
   918 ms: nix::LocalStore::LocalStore(std::map<std::__cxx11::basic_string<char... (../src/libstore/local-store.cc)
   835 ms: opQuery(Globals&, std::__cxx11::list<std::__cxx11::basic_string<char... (../src/nix-env/nix-env.cc)
   733 ms: nix::daemon::performOp(nix::daemon::TunnelLogger*, nix::ref<nix::Sto... (../src/libstore/daemon.cc)
   589 ms: _GLOBAL__sub_I_tests.cc (../tests/unit/libutil/tests.cc)
   578 ms: main_build_remote(int, char**) (../src/build-remote/build-remote.cc)
   522 ms: nix::fetchers::MercurialInputScheme::fetch(nix::ref<nix::Store>, nix... (../src/libfetchers/mercurial.cc)
   521 ms: nix::LocalDerivationGoal::registerOutputs[abi:cxx11]() (../src/libstore/build/local-derivation-goal.cc)
   461 ms: nix::getNameFromURL_getNameFromURL_Test::TestBody() (../tests/unit/libutil/url-name.cc)
   440 ms: nix::Installable::build2(nix::ref<nix::Store>, nix::ref<nix::Store>,... (../src/libcmd/installables.cc)
   392 ms: nix::prim_fetchClosure(nix::EvalState&, nix::PosIdx, nix::Value**, n... (../src/libexpr/primops/fetchClosure.cc)
   390 ms: nix::NixArgs::NixArgs() (../src/nix/main.cc)
   388 ms: update(std::set<std::__cxx11::basic_string<char, std::char_traits<ch... (../src/nix-channel/nix-channel.cc)
   340 ms: _GLOBAL__sub_I_primops.cc (../tests/unit/libexpr/primops.cc)
   332 ms: nix::flake::lockFlake(nix::EvalState&, nix::FlakeRef const&, nix::fl... (../src/libexpr/flake/flake.cc)
   305 ms: _GLOBAL__sub_I_lockfile.cc (../src/libexpr/flake/lockfile.cc)
   300 ms: nix_store::opQuery(std::__cxx11::list<std::__cxx11::basic_string<cha... (../src/nix-store/nix-store.cc)
   296 ms: nix::parseFlakeRefWithFragment(std::__cxx11::basic_string<char, std:... (../src/libexpr/flake/flakeref.cc)
   289 ms: _GLOBAL__sub_I_error_traces.cc (../tests/unit/libexpr/error_traces.cc)
   278 ms: nix::ErrorTraceTest_genericClosure_Test::TestBody() (../tests/unit/libexpr/error_traces.cc)
   274 ms: CmdDevelop::run(nix::ref<nix::Store>, nix::ref<nix::Installable>) (../src/nix/develop.cc)
   269 ms: nix::flake::lockFlake(nix::EvalState&, nix::FlakeRef const&, nix::fl... (../src/libexpr/flake/flake.cc)
   257 ms: nix::NixRepl::processLine(std::__cxx11::basic_string<char, std::char... (../src/libcmd/repl.cc)
   251 ms: nix::derivationStrictInternal(nix::EvalState&, std::__cxx11::basic_s... (../src/libexpr/primops.cc)
   249 ms: toml::result<toml::basic_value<toml::discard_comments, std::unordere... (../src/libexpr/primops/fromTOML.cc)
   238 ms: nix::LocalDerivationGoal::runChild() (../src/libstore/build/local-derivation-goal.cc)

**** Function sets that took longest to compile / optimize:
 10243 ms: std::vector<$>::_M_fill_insert(__gnu_cxx::__normal_iterator<$>, unsi... (190 times, avg 53 ms)
  9752 ms: bool boost::io::detail::parse_printf_directive<$>(__gnu_cxx::__norma... (190 times, avg 51 ms)
  8377 ms: void boost::io::detail::put<$>(boost::io::detail::put_holder<$> cons... (191 times, avg 43 ms)
  5863 ms: boost::basic_format<$>::parse(std::__cxx11::basic_string<$> const&) (190 times, avg 30 ms)
  5660 ms: std::vector<$>::_M_fill_insert(std::_Bit_iterator, unsigned long, bo... (190 times, avg 29 ms)
  4264 ms: non-virtual thunk to boost::wrapexcept<$>::~wrapexcept() (549 times, avg 7 ms)
  4023 ms: std::_Rb_tree<$>::_M_erase(std::_Rb_tree_node<$>*) (1238 times, avg 3 ms)
  3715 ms: boost::stacktrace::detail::to_string_impl_base<boost::stacktrace::de... (166 times, avg 22 ms)
  3705 ms: std::vector<$>::_M_fill_assign(unsigned long, boost::io::detail::for... (190 times, avg 19 ms)
  3326 ms: boost::basic_format<$>::str[abi:cxx11]() const (144 times, avg 23 ms)
  3070 ms: void boost::io::detail::mk_str<$>(std::__cxx11::basic_string<$>&, ch... (191 times, avg 16 ms)
  2839 ms: boost::basic_format<$>::make_or_reuse_data(unsigned long) (190 times, avg 14 ms)
  2321 ms: std::__cxx11::basic_string<$>::_M_replace(unsigned long, unsigned lo... (239 times, avg 9 ms)
  2213 ms: std::_Rb_tree<$>::_M_get_insert_hint_unique_pos(std::_Rb_tree_const_... (203 times, avg 10 ms)
  2200 ms: boost::wrapexcept<$>::~wrapexcept() (549 times, avg 4 ms)
  2093 ms: std::vector<$>::~vector() (574 times, avg 3 ms)
  1894 ms: bool std::__detail::_Compiler<$>::_M_expression_term<$>(std::__detai... (112 times, avg 16 ms)
  1871 ms: int boost::io::detail::upper_bound_from_fstring<$>(std::__cxx11::bas... (190 times, avg 9 ms)
  1867 ms: boost::wrapexcept<$>::clone() const (549 times, avg 3 ms)
  1824 ms: std::_Rb_tree_iterator<$> std::_Rb_tree<$>::_M_emplace_hint_unique<$... (244 times, avg 7 ms)
  1821 ms: toml::result<$> toml::detail::sequence<$>::invoke<$>(toml::detail::l... (93 times, avg 19 ms)
  1814 ms: nlohmann::json_abi_v3_11_2::detail::serializer<$>::dump(nlohmann::js... (39 times, avg 46 ms)
  1799 ms: nix::fetchers::GitInputScheme::fetch(nix::ref<$>, nix::fetchers::Inp... (1 times, avg 1799 ms)
  1771 ms: boost::io::detail::format_item<char, std::char_traits<char>, std::al... (190 times, avg 9 ms)
  1762 ms: std::__detail::_BracketMatcher<$>::_BracketMatcher(std::__detail::_B... (112 times, avg 15 ms)
  1760 ms: std::_Function_handler<$>::_M_manager(std::_Any_data&, std::_Any_dat... (981 times, avg 1 ms)
  1733 ms: std::__detail::_Compiler<$>::_M_quantifier() (28 times, avg 61 ms)
  1694 ms: std::__cxx11::basic_string<$>::_M_mutate(unsigned long, unsigned lon... (251 times, avg 6 ms)
  1650 ms: std::vector<$>::vector(std::vector<$> const&) (210 times, avg 7 ms)
  1650 ms: boost::io::basic_altstringbuf<$>::overflow(int) (190 times, avg 8 ms)

**** Expensive headers:
178153 ms: ../src/libcmd/installable-value.hh (included 52 times, avg 3426 ms), included via:
  40x: command.hh 
  5x: command-installable-value.hh 
  3x: installable-flake.hh 
  2x: <direct include>
  2x: installable-attr-path.hh 

176217 ms: ../src/libutil/error.hh (included 246 times, avg 716 ms), included via:
  36x: command.hh installable-value.hh installables.hh derived-path.hh config.hh experimental-features.hh 
  12x: globals.hh config.hh experimental-features.hh 
  11x: file-system.hh file-descriptor.hh 
  6x: serialise.hh strings.hh 
  6x: <direct include>
  6x: archive.hh serialise.hh strings.hh 
  ...

173243 ms: ../src/libstore/store-api.hh (included 152 times, avg 1139 ms), included via:
  55x: <direct include>
  39x: command.hh installable-value.hh installables.hh 
  7x: libexpr.hh 
  4x: local-store.hh 
  4x: command-installable-value.hh installable-value.hh installables.hh 
  3x: binary-cache-store.hh 
  ...

170482 ms: ../src/libutil/serialise.hh (included 201 times, avg 848 ms), included via:
  37x: command.hh installable-value.hh installables.hh built-path.hh realisation.hh hash.hh 
  14x: store-api.hh nar-info.hh hash.hh 
  11x: <direct include>
  7x: primops.hh eval.hh attr-set.hh nixexpr.hh value.hh source-path.hh archive.hh 
  7x: libexpr.hh value.hh source-path.hh archive.hh 
  6x: fetchers.hh hash.hh 
  ...

169397 ms: ../src/libcmd/installables.hh (included 53 times, avg 3196 ms), included via:
  40x: command.hh installable-value.hh 
  5x: command-installable-value.hh installable-value.hh 
  3x: installable-flake.hh installable-value.hh 
  2x: <direct include>
  1x: installable-derived-path.hh 
  1x: installable-value.hh 
  ...

159740 ms: ../src/libutil/strings.hh (included 221 times, avg 722 ms), included via:
  37x: command.hh installable-value.hh installables.hh built-path.hh realisation.hh hash.hh serialise.hh 
  19x: <direct include>
  14x: store-api.hh nar-info.hh hash.hh serialise.hh 
  11x: serialise.hh 
  7x: primops.hh eval.hh attr-set.hh nixexpr.hh value.hh source-path.hh archive.hh serialise.hh 
  7x: libexpr.hh value.hh source-path.hh archive.hh serialise.hh 
  ...

156796 ms: ../src/libcmd/command.hh (included 51 times, avg 3074 ms), included via:
  42x: <direct include>
  7x: command-installable-value.hh 
  2x: installable-attr-path.hh 

150392 ms: ../src/libutil/types.hh (included 251 times, avg 599 ms), included via:
  36x: command.hh installable-value.hh installables.hh path.hh 
  11x: file-system.hh 
  10x: globals.hh 
  6x: fetchers.hh 
  6x: serialise.hh strings.hh error.hh 
  5x: archive.hh 
  ...

133101 ms: /nix/store/644b90j1vms44nr18yw3520pzkrg4dd1-boost-1.81.0-dev/include/boost/lexical_cast.hpp (included 226 times, avg 588 ms), included via
:
  37x: command.hh installable-value.hh installables.hh built-path.hh realisation.hh hash.hh serialise.hh strings.hh 
  19x: file-system.hh 
  11x: store-api.hh nar-info.hh hash.hh serialise.hh strings.hh 
  7x: primops.hh eval.hh attr-set.hh nixexpr.hh value.hh source-path.hh archive.hh serialise.hh strings.hh 
  7x: libexpr.hh value.hh source-path.hh archive.hh serialise.hh strings.hh 
  6x: eval.hh attr-set.hh nixexpr.hh value.hh source-path.hh archive.hh serialise.hh strings.hh 
  ...

132887 ms: /nix/store/h2abv2l8irqj942i5rq9wbrj42kbsh5y-gcc-12.3.0/include/c++/12.3.0/memory (included 262 times, avg 507 ms), included via:
  36x: command.hh installable-value.hh installables.hh path.hh types.hh ref.hh 
  16x: gtest.h 
  11x: file-system.hh types.hh ref.hh 
  10x: globals.hh types.hh ref.hh 
  10x: json.hpp 
  6x: serialise.hh 
  ...

  done in 0.6s.

Manually looking at traces

Note that the summary in the report can miss details like why one particular header is bad; to find that out, use a trace viewer to inspect the JSON trace file; we suggest rg -t json -uu error\.hh build/ | less to find some .cc trace that the bad header (in this example, error.hh) appears in.

You can look at individual file traces by opening some file like build/src/libcmd/liblixcmd.so.p/command.cc.json in https://ui.perfetto.dev or another Chrome-trace-json compatible trace viewer like Speedscope.

This will produce a flamegraph of the trace (screenshot shows Perfetto):

The most general spans of compile time are at the top, and the constituent spans are shown as you go down.

Successful build time reduction CLs

See the build-time-optimisation Gerrit topic for more related things.

Working in the Lix codebase

Backport guide

Don't forget, using Gerrit is a bit different than other systems.

single commits

try git cherry-pick -x first. if this works, excellent. if not, apply the usual cherry-picking procedures:

full prs

single-commit prs were mostly picked using cherry-pick -x -m1 to keep the association with the upstream pr number for clarity. this implicitly squashes the pr into a single commit so it's only useful for single-commit prs. (some prs that have broken intermediate commits also benefit from this, but see above for that)

when pushing these to gerrit please set a topic like backport-<pr-number> using push options (-o topic=backport-<pr-number> in git push) to delineate one picked pr from a pr that depends on it

Working in the Lix codebase

Misc tips

buildbot user style to make the pulsing pills bearable

@keyframes pulse_animation {
  0% { transform:scale(.9) }
  50% { transform:scale(1) }
  to { transform:scale(.9) }
}

.pulse {
    animation-duration: 10s !important;
}

FIXME: someone should PR this, now that we have the ability to patch buildbot

run all lix vm tests locally

tests=$(
    nix eval --json --impure \
        --apply '
            let f = n: t:
                if __isAttrs t
                then (if t.type or "" == "derivation"
                    then (if t.system == __currentSystem
                            then [ n ]
                            else [])
                    else __concatMap (m: f "${n}.${m}" t.${m}) (__attrNames t))
                else [];
            in f ".#hydraJobs.tests"
        ' \
        .#hydraJobs.tests \
        | jq -r '.[]'
)

nix build --no-link -L ${tests[@]}

check out current patchset of a cl by git alias

put this in a gitconfig that can configure aliases:

[alias]
	cocl = !\
	 ps=$(\
	   ssh $(git config remote.origin.gerriturl) \
	     gerrit query --format=json --current-patch-set $1 \
        | jq -sr .[0].currentPatchSet.ref \
	 ) && git fetch origin $ps && git checkout FETCH_HEAD && true

then run as git cocl <cl-number>. needs a git config remote.origin.gerriturl gerrit.lix.systems, or some other url that ssh can connect to. (could've extracted it from the remote url but we didn't want to do that much shell)

git stuff

git-revise

git-revise is a cool tool for splitting and shuffling commits in-memory without breaking your working tree. it's great.

It also has some broken stuff with respect to gerrit commit-msg hooks. However, this can be fixed (this is opt-in because some commit-msg hooks make unsound assumptions but the gerrit one should be fine):

# allow gerrit git hooks to run on git-revise
[revise "run-hooks"]
        commit-msg = true

Making git clean clean the stuff that isn't removed by make clean

If you don't want to use git clean -x to remove all git-ignored stuff, but want to remove things that are generated in Lix's build process but aren't removed by make clean, apply this patch: no-ignore-not-cleaned.patch

Working in the Lix codebase

Building Locally

See hacking.md in the Lix repo for the main documentation. Extra tips can go here.

Working in the Lix codebase

RISC-V support

Goal: install lix on a riscv64-linux system

The target is a DevTerm R-01, so it's an AllWinner D1 RISC-V processor @ 1GHz, with 1GB of memory and 32GB of microSD.

We can't run the Lix installer without building it, because there's no canned build for it. So let's try building it natively:

$ rustup
$ git clone https://git.lix.systems/lix-project/lix-installer
$ cd lix-installer
$ RUSTFLAGS="--cfg tokio_unstable" cargo install --path .

This doesn't work because there's some conditional complication^Wcompilation that doesn't cover riscv64. So we need to open self_test.rs and add an entry:

    #[cfg(all(target_os = "linux", target_arch = "riscv64"))]
    const SYSTEM: &str = "riscv64-linux";

At this point, it will, in principle, build. In practice, however, 1GB is just not enough RAM. If you add some swap it'll make it to the last step, but then it wants 1.5GB+ for that. I wouldn't try it on a system with less than 2GB, and ideally more.

Ok, native build is a bust unless I want to let it thrash all night. So let's cross-compile it on ancilla, which is, conveniently, already running nixos.

The nix-installer flake doesn't come with riscv64 cross support, and rather than try to figure it out I just winged it with nix-shell. I am skipping over a lot of false starts and blind alleys here as I ran into things like dependency crates needing a cross-compiling gcc, or rust not having a stdlib on riscv64-musl.

$ git clone https://git.lix.systems/lix-project/lix-installer
$ cd lix-installer
$ $EDITOR shell.nix
with import <nixpkgs> {
  crossSystem.config = "riscv64-unknown-linux-gnu";
};
mkShell {
  nativeBuildInputs = with import <unstable> {}; [ cargo rustup ];
}

$ nix-shell
[long wait for gcc to compile]

$ export RUSTUP_HOME=$PWD/.rustup-home
$ export CARGO_HOME=$PWD/.cargo-home
$ rustup default stable
$ rustup target add riscv64gc-unknown-linux-gnu
$ edit src/self_test.rs
[apply that same patch to SYSTEM]

The build invocation is a bit more complicated here, because we need to tell it where to find the linker:

$ RUSTFLAGS="--cfg tokio_unstable" cargo build \
    --target riscv64gc-unknown-linux-gnu \
    --config target.riscv64gc-unknown-linux-gnu.linker='"riscv64-unknown-linux-gnu-gcc"'
[another long wait]

$ file target/riscv64gc-unknown-linux-gnu/debug/lix-installer
target/riscv64gc-unknown-linux-gnu/debug/lix-installer:
  ELF 64-bit LSB pie executable, UCB RISC-V, RVC, double-float ABI,
  version 1 (SYSV), dynamically linked,
  interpreter /nix/store/g4xam7gr35sziib1zc033xvn1vy9gg8m-glibc-riscv64-unknown-linux-gnu-2.38-44/lib/ld-linux-riscv64-lp64d.so.1,
  for GNU/Linux 4.15.0, with debug_info, not stripped

Since we couldn't do a static musl build it needs the nix ld.so, but we can get around that!

$ scp target/riscv64gc-unknown-linux-gnu/debug/lix-installer root@riscv:.
$ ssh root@riscv
# ./lix-installer
-bash: ./lix-installer: no such file or directory

# ldd ./lix-installer
/nix/store/.../ld-linux-riscv64-lp64d.so.1 => /lib/ld-linux-riscv64-lp64d.so.1
[other output elided]

# /lib/ld-linux-riscv64-lp64d.so.1 ./lix-installer
The Determinate Nix installer (lix variant)
[...]

Sadly we can't actually use it to install, because nix_package_url needs a default value, and on RISC-V, it doesn't have one! It's self_test.rs all over again except it doesn't manifest until runtime.

So, off to src/settings.rs we go. It doesn't need to be a valid URL, just something URL-shaped.

/// Default [`nix_package_url`](CommonSettings::nix_package_url) for unknown platforms
pub const NIX_UNKNOWN_PLATFORM_URL: &str =
    "https://releases.lix.systems/unknown-platform";

    #[cfg_attr(
        all(target_os = "linux", target_arch = "riscv64", feature = "cli"),
        clap(
            default_value = NIX_UNKNOWN_PLATFORM_URL,
        )
    )]

Rebuild, re-push, re-run:

# /lib/ld-linux-riscv64-lp64d.so.1 /opt/lix-installer install linux
Error: 
   0: Planner error
   1: `nix-installer` does not support the `riscv64gc-unknown-linux-gnu` architecture right now

Ok, missed a few places in settings.rs, let's put a quick and dirty hack in there:

            #[cfg(target_os = "linux")]
            (_, OperatingSystem::Linux) => {
                url = NIX_UNKNOWN_PLATFORM_URL;
                nix_build_user_prefix = "nixbld";
                nix_build_user_id_base = 30000;
                nix_build_user_count = 32;
            },

            #[cfg(target_os = "linux")]
            (_, OperatingSystem::Linux) => {
                (InitSystem::Systemd, linux_detect_systemd_started().await)
            },

It also needs a tarball to install; jade_ kindly updated the flake for it to support riscv64, so we just check it out (or, well, check out review branch 1444) and then nix build -L .#nix-riscv64-linux.binaryTarball and away we go.

This, it turns out, also doesn't work, because the installer is hardcoded to expect the directory the tarball contains to start with nix-*. You can either unpack and repack the tarball to meet that requirement, or find all the places in lix-installer that assume that and edit them -- they're in src/action/base/move_unpacked_nix.rs and src/action/base/setup_default_profile.rs.

Finally, this particular kernel lacks seccomp support -- in order to get it working, I had to edit the lix (not lix-installer) package.nix and add (lib.mesonEnable "seccomp-sandboxing" false) to the meson flags.

And with that done, it works!

root@devterm-R01:~# uname -a && nix --version
Linux devterm-R01 5.4.61 #12 PREEMPT Wed Mar 30 14:44:22 CST 2022 riscv64 riscv64 riscv64 GNU/Linux
nix (Lix, like Nix) 2.90.0pre20240613_dirty
Working in the Lix codebase

Branches

The Lix repository contains multiple releases in parallel. The branches work as follows:

Version types

Git tags

Git tags are created with the format 2.90.0.

Docker tags

Design documents

This category contains design documents written by the Lix team, which may or may not be implemented.

Design documents

regexp engine investigation

nix uses libstdc++'s std::regex. it uses whatever version of libstdc++ the host system has.

which it invokes in both std::regex_replace std::regex_match modes.

nix occasionally uses the flags std::regex::extended and std::regex::icase which determine the available features - it's always either no flags, or both of these together. there's also a couple things that use the flag std::regex::ECMAScript. when the constructor is called without a flags parameter, the flags default to std::regex::ECMAScript (see method signature in C++23 32.7.2), so really we have only two cases.

std::cregex_iterator and std::sregex_iterator are used.

there's a header regex-combinators.hh which defines regex::group and regex::list.... and a couple others that are unused. but those are just trivial textual things, not extensions, so we can ignore the file.

getting the C++ standard

someday when C++23 is official you will be able to pirate the PDF. otherwise, you can clone https://github.com/cplusplus/draft and check out the tag n4950 which is the current formally adopted working draft as of 2024-03-14 and is intended to have the same technical content as the final standard. you can then invoke make in the source subdirectory which will produce std.pdf. you will need LaTeX installed. if you're ever not sure which working draft is the one that became a particular version of the standard, Wikipedia will probably tell you...

(personally I install texlive.combined.scheme-full from nixpkgs on all my machines that have room for it, but this is surely more than necessary, it just makes me feel warm and fuzzy -- Irenes)

chapter 32 is the one that documents regular expressions.

open questions that require reading the standard

required functionality

the extended flag, per the C++ standard, "Specifies that the grammar recognized by the regular expression engine shall be that used by extended regular expressions in POSIX.". it references POSIX, Base Definitions and Headers, Section 9.4.

the ECMAScript flag "Specifies that the grammar recognized by the regular expression engine shall be that used by ECMAScript in ECMA-262, as modified in [section 32.12 of the C++ standard]." it references ECMA-262 15.10. the changes in 32.12 are important and probably do create real compatibility issues for us, though fortunately it's only a single page.

if we complete this chart we can use it to assess which existing engines would meet our needs, or how much of a pain in the ass it would be to make a new one

the columns are the two ways it gets invoked

extended + icase ECMAScript
Syntactic constructs -- --
(TODO: fill in every construct here)
Semantics -- --
Case-insensitivity yes ?
(TODO: fill in other behaviors here)
Design documents

Dreams

This page documents the dreams of the Lix team. These are features which we have generally not roadmapped yet, and which may not have complete and thoroughly thought-through plans, and which we would like to think about more completely before implementing. We are writing them down publicly so that others can dream with us.

fixing ux

slaying the hydra

these are problems that make hydra sad

Design documents

Language versioning

This document is extremely a draft. It needs some editing and discussion before it can be made into a useful thing. It's been simply copy pasted out of the pad in its current form.

See also

musings

puck: honestly, having language version as part of a scopedImport-style primop would be funny horrors: we're shitposting about setting language version from the source accessor

horrific writeup

basic mechanism

add a new syntactic element that is only valid at the head of a file and used only to declare language requirements. nix versions that cannot satisfy all requirements must reject this element to situations in which two nix versions parse the same file differently, or even evaluating the same file to different derivation hashes. any kind of comment as used by eg GHC is not viable for nix for this reason.

proposed syntax for the first implementation: use $( $feature: ident )+;

anything ahead of this directive could be either unversioned nix code or versioned nix code (see below for details), but since the directive is only valid at the head of a file or expression this "code" can only be comments. this kind of locks us into supporting the current comment syntax forever, but the comment syntax is rather fine so this won't be a problem.

each feature may declare a syntactical requirement for the file, a semantic requirement, or possible both (cf rust editions, or perl use v<something>).

features may be global, namespaced to their implementations, or live in a reserved experimental namespace an implementation can add to and remove from as it wishes with absolutely no guarantee of future evaluatility.

syntactic features

syntax is entirely local to the file itself and has few to no intercompatibility constraints with other code. a very useful syntax requirement would something like no-url-literals, which might strip the syntactic ability to parse url-like sequences of characters into strings and, rather than nix currently does the experimental feature of the same name simply throwing a parse error, parse them as eg a lambda with a sequence of divisions in its body.

(realistically no-url-literals would not appear in practice, instead it should be implied by use itself since url literals are such an obvious misfeature)

semantic features

semantic features produce evaluation changes that could be achieved any other way. examples of this are:

semantic changes may escape the expression that requires them and usually some of amount of cross-compatibility with other semantic versions must be given. using the same examples as above, considerations can include:

this is in fact a full classification of cross-compatibility issues: side-effects changing, evaluation outputs changing, and evaluation inputs changing. side-effects need not be considered very much since nixlang is supposed to be pure and all side-effects that are not part of the store interface must already be considered incidental. evaluation outputs changing can be handled by optional lint or runtime warnings when a versioned evaluation structure passes a semantic version boundary without being annotated as an intentional behavioral leak. evaluation inputs changing is a non-issue because nix plugins and the ExternalValue infrastructure already make it impossible to rely on the type system being fully specified at the time an expression is written

inter-file inter-actions

by default language features must not be propagated across an unadorned import boundary to retain compatibility with existing nix code (eg nixpkgs, which will not be able to switch for quite some time). in some circumanstances it is however required to propagate language features across imports to provide a consistent and meaningful interface, eg in the case of a hypothetical requiredLanguageFeatures attribute for a flake. to allow for both of these requirements to peacefully coexist we add a new primop:

scopedImportUsing
:: { features ? <current language features> :: AttrSetOf bool
     ## ^ language features as would be specified by `use ...;`.
     ## selecting a default-off feature is achieved by setting its key to `true`,
     ## deselecting a default-on feature is achieve by setting its key to `false`.
     ## nesting is not needed because features are identifiers. future changes to
     ## the use interface may extend the type of this set.
   , newGlobals ? env: env :: AttrSetOf Any -> AttrSetOf Any
     ## ^ function to produce the new global environment. it receives the default globals
     ## set for the target expression language features (as calculated form `features` and
     ## the target `use` clause) and produces a new set.
     ## `scopedImport` behavior is recovered by setting this to `const newEnv`.
   }
-> PathLike
## ^ imported path as in `scopedImport`
-> Any
## ^ import result. may be cached, most immediately using the intransparent internal
## object id of the provided features and the globals set. this mimics the beavior
## or `import` in cppnix

if the imported expression selects a different set of language features the features specified by scopedImportUsing are ignored.

scopedImportUsing is available in the builtins set and crucially, can be replaced. this allows a hypothetical flake implementation to replace both scopedImportUsing and import with its own versions that provide propagation behaviors that might be expected from such a library:

additionally the current language features might be made available through a builtin value languageFeatures by such a replacement of scopedImportUsing.

builtins versioning, global versions

a language feature may add or remove elements of builtins or the global environment. as mentioned earlier this does not pose a large hazard since evaluation is sufficiently unespecified that this must already be expected to happen.

interactions with eg nixpkgs lib

nixpkgs lib (and other libraries) will have to cater to the smallest common denominator when exposing library functions/constants as they do now. if we change a function to have a different prototype and a library reexports it from builtins to its own namespace the language features used by the code importing the library do not matter. to make this problem less unbearable we may want to introduce a concept of library objects and a "use library" directive like eg python from ... import ... that can pass language features down to the library being imported in some way.

as a first approximation is would be sufficient to encourage libraries to version their namespaces in such a way that accessing a namespace that relies on language features not present in the current evaluator will fail to evaluate (eg by providing the library itself as a plain set and each version as an attribute that (lazily) imports the specific version of the library needed to fulfill the requested version).

bad ideas for features to remove/change in the first langver

Feature detection

jade: I think we might want to be able to feature detect certain features, e.g. new builtin args, which can be done without, but we would like to know if they are there.

builtins.nixVersion has been defanged, which means that an alternate cross impl compatible mechanism needs to be created.

Minimally thought-through proposal

builtins.features is an attribute set, where individual attribute names are exposed with the value true if they are implemented by a given implementation.

Attribute names are of the format:

"domainname.feature", for example, "systems.lix.somefeature".

Design documents

Docs rewrite plans

Here, for now (public edit link): https://pad.lix.systems/lix-docs-planning

Release names

Release names are the names of frozen desserts. There's a list on Wikipedia of frozen desserts, but of course, others can be added. The purpose of release names is that they are cute, and they are not necessarily picked for any reason.

Used release names:

Here are some ideas of release names: