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

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

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.

New builtins

One thing that is a little bit tricky and can get contributions canned late in the process is new builtins. We should write up a more full document on this, but the gist is that the API of any builtins needs to stand the test of time as they have pretty strong compatibility and usefulness expectations.

As such, it would be greatly appreciated if work on builtins starts with an issue on lix-project/lix tagged RFD discussing the use case for the builtin, then, once there is rough agreement on the use case, write a simple document giving examples and design of the proposed API. Then, we can gather feedback before too much time is put into implementations that might not see the light of day.

I don't wanna write C++. How can I help?

This is totally reasonable, C++ is not super fun, but this isn't to say there isn't help if you do choose to. Nonetheless, there are still many issues on our tracker that do not require writing any C++ that would help out immensely.

Here's some possible places to look:

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 "milkshake".

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

All kinds of changes are acceptable, but we still strive to keep main always as stable as possible and a safe decision to daily-drive for all your nixy needs. Please don't jam the ice cream machine!

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

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.

SSH config

You might like to add the following configuration to your ~/.ssh/config:

Host gerrit.lix.systems
    User YOUR_GERRIT_USERNAME
    Port 2022
    # Keep sessions open for a bit in the background to make connections faster:
    ControlMaster auto
    ControlPath /tmp/ssh-%r@%h:%p
    ControlPersist 120

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

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.

Commit porting (cherry-picking) guide

This guide is primarily about CppNix->Lix commit ports and was written when we were doing a lot of those before 2.90. It also applies to Lix->Lix ports, though, those are much easier, often just taking clicking "cherry pick" in the gerrit UI.

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

Lix -> Lix ports

We generally only backport fixes from main if there are severe bugs that users will hit in production, or if we had to patch the derivation in nixpkgs. The only real thing to remember about this is to use git cherry-pick -x. Gerrit will match up the Change-Ids and link the CLs on release branches to the other CLs on release branches or main. Then, push as if any other change: git push origin HEAD:refs/for/release-2.92 (e.g.).

You can also use the cherry-pick button in the Gerrit UI.

CppNix -> Lix ports

Commit style:

GitHub encourages messy branch history, so the cherry-picking procedure can be rather annoying in a codebase like Lix which mandates that every commit passes CI. However, this document describes how we have generally done it.

Also note that we have renamed src/ to lix/, which may or may not confuse git sometimes.

Procedure (focused mostly on CppNix -> Lix)

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

Misc tips

pyright is not dealing with import paths correctly for functional2

I don't know why it's doing this but it seems to be assuming that the functional2 test suite is relative to the workspace root.

You can jam the following into :CocLocalConfig for vim, and probably the workspace config for VSCode and it should fix it.

{
    "python.analysis.include": ["tests"],
    "python.analysis.extraPaths": ["tests"],
}

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

Building Locally

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

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

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