RFC-0153: Ninja customization for Fuchsia

RFC-0153: Ninja customization for Fuchsia
  • Build

Proposal to use a customized version of the Ninja build tool for Fuchsia to speed up the build, improve status reporting and usability.

Gerrit change
Date submitted (year-month-day)2022-01-22
Date reviewed (year-month-day)2022-03-17


This RFC proposes to use a temporary customized version of the open-source Ninja tool, used by the Fuchsia platform build, in order to integrate several existing third-party patches that significantly improve its performance and usability, and which the upstream maintainer(s) have not been able to accept yet for a variety of reasons explained in the Motivation section.

In particular, doing this would allow to:

  • Reduce the total build time noticeably, by optimizing the scheduling algorithm used by Ninja.

  • Improve the status reporting and logging, as well as solving a serious usability issue that routinely triggers in our CI builds.

  • Improve the responsiveness and general performance of Ninja without changing its behaviour. For example by speeding up certain Ninja operations 22x.

  • As an outcome of the above, significantly speed up integration between the build system and IDEs such as Visual Studio Code and Vim, by making it possible to regenerate IDE bindings in a fraction of the time, and providing great interactive code-editing experience.

Customization would be achieved by maintaining a custom git branch on https://fuchsia.googlesource.com/third_party/ninja, which would be managed using a disciplined branch strategy that closely tracks upstream, as well as keep it expressible as a series of small patches on top of recent upstream history.

The situation of this experimental branch will be revised in Q3 2022, since there are plans to help upstream maintainers accept pull requests of interest, but these depend on many external factors that are out of scope for this RFC. It is expected though that by that time the custom branch will no longer be needed.


The Ninja build tool was developed by Evan Martin at Google as an experiment while he was working for Chrome 1. The experiment succeeded, turned into a very useful tool that became part of the official Chrome build and was open-sourced. It was then quickly picked up by a number of other build configuration tools or systems that are widely deployed, including today:

  • The GN build tool, now used by Chrome, Fuchsia and Pigweed projects.
  • The Blueprint and Kati tools, used by the Android project.
  • The CMake build generator, which supports Ninja as a backend.
  • The Meson build system, used by many Linux open-source projects.
  • LLVM, which developers can build with either CMake or GN, which includes Fuchsia team members who contribute to LLVM.

Ninja is now being used by thousands of developers worldwide, and is maintained as an open-source project on GitHub. Ninja's maintainers try to ensure that the project remains true to its original goal of being small, simple, reliable and extremely portable.

Due to a number of reasons, progress on the upstream Ninja project remains slow, with a large number of interesting pull requests that have been waiting for review for several months, if not years. The current maintainer has been contacted privately, and recognizes that regrettably, this is mostly due to lack of time to properly review and test non-trivial changes, and that this is also a consequence of the state of the regression test suite, which is currently very basic, requiring manual testing of pull requests in many cases. Said maintainer would also first appreciate any help to ramp up said test suite in the Github CI in order to facilite and speed up Ninja's maintenance and evolution. As an example, he is not opposed to switching the codebase from C++03 to C++11, as long as there is a guarantee that the code compiles and works correctly on older distributions (namely Centos 7, Debian 8, Ubuntu 18.04 and OSX 10.12) using their default toolchains, enforced by proper continuous integration verification.

There are unfortunately a large number of blocking issues like the one describe previously, and solving all of them requires a non-trivial amount of work that the Fuchsia build team currently doesn't have the capacity to achieve right now.

This RFC provides a technical proposition to temporarily work-around this limitation, even though it is strongly hoped that proper head-count could be dedicated soon to address the situation in cooperation with the upstream maintainer(s). Such head count could come from the Fuchsia project, or a joint effort with other teams at Google who have expressed in interest in Ninja's future evolution and improvements. However, funding such work is out-of-scope for this RFC.

In the meantime, a customized branch of Ninja would allow cherry-picking the most impactful pull requests, with the greatest benefits to Fuchsia developers. To ensure that this branch keeps as close as possible to upstream, a disciplined branch strategy will be enforced to ensure that the Fuchsia branch is always expressible as a series of patches on top of recent upstream releases.

Note that this situation is very different from the custom Android Ninja fork which has deviated from upstream significantly since its creation, and lacks features required by the Fuchsia build.

The most interesting pull requests, that motivate this RFC are the following:

Faster builds using better scheduling algorithms

Three open pull requests have the potential to reduce build time in important ways. They both modify the way Ninja picks up new commands to launch based on different conditions:

  • PR 2019: Assign priorities to commands to be launched, using the build log to estimate their duration. The author states that this reduces the build time of a large build project from 20 to 15 minutes!

  • PR 1949: Limits spawning new commands to avoid saturating the load limitation. This avoids overloading the build machine when spawning too many processes that fight for the same CPU resources. The author claims that reduces a test build from 22 to 15 minutes under certain conditions.

  • PR 1140: Adds support to Ninja for GNU Make jobserver, which is a way for Ninja and the commands it executes to coordinate their allocation of CPU processes. This comes from the Kitware branch of Ninja. Note the similarities with the previous PR. No timings were posted by the author, and is less likely to be useful, since it requires invoked programs to support this explicitly.

Fixing Ninja's build output for long commands.

During a build, Ninja only prints status lines for commands that have completed. In practice, when a long running command that stalls the build is running then:

  • If the command times out, as it happens on our CI bots during very long Rust link commands, there is absolutely nothing in the output or the Ninja log files to tell which target/command actually expired!

  • On a terminal, the single-line status seems frozen, and still displays the name of the last completed target, which is confusing (many developers assume it is the root cause for the delay). The only way to see what is really going on is to use "ps" in another terminal, but few people actually know that.

The first point is a problem for the Fuchsia build, that cannot be solved without modifying Ninja. The second one is a usability annoyance that has been bugging users for several years, to the point where there exist several pull requests to fix it in different ways:

And this is due to the fact that Ninja's output is text-based, with a pretty limited format, which also includes arbitrary command outputs, in case of error. This creates all kind of subtle issues, and makes the output difficult to parse reliably.

Indeed the current behaviour was introduced intentionally in 2015 to solve the issue that Ninja's build output was not machine parseable. Hence, any change in the output format is considered very risky by upstream.

However, in the context of Fuchsia, where the scripts that parse the Ninja output are controlled by the Fuchsia infra team, a small change to the output format makes sense if it can get rid of the problem completely.

Serialized status updates for better log filtering

Another interesting feature of the Android customization is a work-around for the output limitation, which instead sends status updates to an external "frontend" program, as a stream of structured binary data.

This has allowed the Android team to store multiple output streams with different levels of logging, collect errors messages into a separate file, as well as produce accurate build traces that can be loaded with chrome://tracing.

For the Fuchsia build, it should be possible to extract that feature from the Android branch to get the same benefits.

Note that this was proposed as a pull request, but was rejected after some long discussions where upstream concluded that a better way to solve the output parsing problem is to turn Ninja into a library, a considerably larger effort given the state of the code base (someone tried to do it with an experimental PR that requires 120 commits).

Improved developer experience

The Android branch has implemented dramatically faster (up to 22x) parsing of Ninja input files, through the use of threads to parse splits of the inputs, determined at proper token boundaries anad merged in a final pass.

This was motivated by the fact that the Android build tools generated around 1.2 GiB of Ninja build plans which took 12s to parse before doing anything (on a powerful workstation with hot filesystem caches).

The Fuchsia build is reaching a similar situation, unfortunately, generating around 800 MiB currently of Ninja build plans, which take up to 10s to parse, making the experience of incremental builds annoying. The Fuchsia build team thinks these improvements are significant and should be integrated into the Ninja version used by the build.

When writing code, compiler error messages and warnings are the first line of feedback to the developer. This is especially true in static languages and becoming more true in newer languages like Rust that are designed to check a wide range of conditions at compile time. Therefore the time to get feedback is critical for developer productivity. Every second spent waiting for compiler output is a second the developer is left guessing whether their code is correct or at risk of being distracted by something else, and fast feedback is especially critical for programmers who are still learning a language.

The most productive environments give feedback instantly in the development environment once a file is saved. This feeling of "flow" is so important that Fuchsia developers have created tooling to allow them to use an entirely different build system, cargo, to get quick feedback and testing cycles. This tooling is still in use by some developers despite being unsupported and breaking fairly regularly.

For Fuchsia, Ninja takes up to 10 seconds at the beginning of every invocation to parse its own build files (the build.ninja file and the other ones it includes). That's a very large lower bound on how quickly we can deliver feedback, and at least 25x slower than what we can provide with cargo today. By cutting parse time down by 25-100x, combined with other tooling workstreams rolling out this quarter, we can take the experience of writing code for Fuchsia from "snails on a chalkboard slow" to "snappy and responsive". The result will be faster development cycles, happier engineers, and higher quality software.

Below you can see a screen capture of the current experience of changing code in an IDE and reviewing compilation errors.

Screen capture showing slow IDE feedback

The delay in developer feedback is almost fully attributable to time that Ninja spends parsing its files. Compare to this demo below of the IDE experience that's possible with a patched Ninja.

Screen capture showing fast IDE feedback

Similarly, fx setup-go was created to use the Go build system exclusively during local development, precisely because incremental builds with Ninja are too slow.

Note that this exact feature was proposed in 2019 by the Android team, but was rejected by upstream maintainers at the time, due to its requirement for C++11, which was not considered acceptable yet. Time have passed since then and such a switch could be considered, granted the guarantees described in the motivation section are enforced.

Improved user interface

The following PR was submitted to implement a table-like status output in interactive terminals, inspired by the Buck build system (see animated example).

The PR was dropped by its author, but could be rebased to provide a very nice UI improvements when building Fuchsia locally.



pascallouis@google.com has been appointed by FEC to shepherd this RFC through the RFC process.


shayba@google.com, Build maruel@google.com, Fuchsia Platform EngProd abarth@google.com, Platform Lead

Other team members who actively participated in prior conversations on this topic:

brettw@google.com, (as Fuchsia Tools contributor). fangism@google.com, Build haowei@google.com, LLVM for Fuchsia jayzhuang@google.com, Build olivernewman@google.com, Fuchsia CI phosek@google.com, Fuchsia Toolchain


fangism@google.com, Build jayzhuang@google.com, Build tmandry@google.com, Rust on Fuchsia rudymathu@google.com, Fuchsia EngProd


The idea was first proposed by tmandry from Rust on Fuchsia team, then socialized on Google-internal email threads, and is now presented here in a publicly accessible forum for further discussion and approval.


A Fuchsia-controlled git repository will be used to fork the upstream Ninja master branch, in order to manage a branch containing Fuchsia-specific customizations, while tracking the upstream origin/master. Patches will be reviewed by the OWNERS of the branch and should be considered on a case-by-case basis in terms of:

  • Benefits provided
  • Maintainability of the patch
  • Divergence from upstream, which affects maintainability.

As well as other criteria the owners see as appropriate.

The list of local modifications will be tracked in a FUCHSIA.readme file as standard practice.

The maintainers of that branch will be the Fuchsia Build Team, which is responsible and accountable for the Fuchsia build system and its correctness, maintainability and performance.

It is the intent of this RFC to ensure that this branch does not diverge to the point of making upstream changes difficult to integrate. To achieve this, the following strategy will be applied:

Strategy to keep the Fuchsia branch close to upstream

The Fuchsia Ninja branch should be disciplined, which means that in order to keep it as close as possible to upstream, we will routinely create "usptream-sync" branches that express our changes as a series of clean patches on top of recent upstream releases. A few graphics should help understand what is meant here.

When creating a custom git fork, one typically creates a new branch on top of an existing upstream release, that adds new commits. The following graphics illustrates a situation when an 'upstream' history is branched into a 'fuchsia' branch which adds 3 commits on top of them:

upstream  ___U1__U2___
fuchsia                \__F1__F2__F3

Commits are added to the upstream project by its maintainers. The upstream and fuchsia history have now diverged, as in:

upstream  ___U1__U2______U3__U4__U5
fuchsia                \__F1__F2__F3

A simple way to get upstream changes into the fuchsia branch is to perform a merge operation, which may reveal conflicts between commits that need to be resolved manually, which can be seen as:

upstream  ___U1__U2______U3__U4___U5___
                      \                \
fuchsia                \__F1__F2__F3____\F4__

Where F4 denotes a merge commit. Now the 'fuchsia' branch has all the improvements from upstream, but it can no longer be expressed as a series of patches on top of the upstream's most recent release (i.e. U5).

To keep this goal, it is possible to avoid merging upstream directly into the fuchsia branch. Instead, create a duplicate of the fuchsia branch and rebase it on top of U5, solving any conflict on the way (this can be done locally first with a git rebase operation). Let's call this branch upstream-sync-U5 as in:

upstream  ___U1__U2______U3__U4__U5
                      \            \
upstream-sync-U5       \            \__F1'__F2'__F3'
fuchsia                  \__F1__F2__F3

Both fuchsia and upstream-sync-U5 should be functionally equivalent at this point (to be enforced by testing), the content of the new commits F1' to F3' may even be identical to F1 to F3, unless some conflicts had to be resolved.

It is now possible to merge the latter into the former (resolving any conflict by simply taking the changes from upstream-sync-U5) as in:

upstream  ___U1__U2______U3__U4__U5
                      \            \
upstream-sync-U5       \            \__F1'__F2'__F3'
                        \                           \
fuchsia                  \__F1__F2__F3_______________\F4

This can be repeated later when syncing with other upstream changes, as in:

upstream  ___U1__U2______U3__U4__U5__________________U6__U7
                      \            \                       \
upstream-sync-U5       \            \__F1'__F2'__F3'        \
                        \                           \        \
upstream-sync-U7         \                           \        \__F1"__F2"__F3"__F5"
                          \                           \                            \
fuchsia                    \__F1__F2__F3_______________\F4__F5______________________\F6

Each new upstream-sync-XXX branch expresses the customization state as a new series of patches on top of a recent upstream release. This makes the Fuchsia changes easier to understand, and improves the ability to send them back to upstream if needed.

Maintainers of the Fuchsia branch should create upstream-sync branches whenever possible.


The branch will be maintained on a Git-on-Borg instance at https://fuchsia.googlesource.com/third_party/ninja (which already exists), and its public Gerrit instance will be used to review and accept patches.

The LUCI recipe used to build Ninja prebuilt binaries for the Fuchsia platform build will switch its source GIT URL accordingly, since it currently points to a mirror of the upstream GitHub repository.


Reducing the Fuchsia platform build time is the main motivation for this RFC, along with improving the developer experience with much faster tool response times.


By lowering the time to developer feedback, we will improve developer throughput and reduce context switching.

This lower latency also opens up new use cases for Fuchsia's build system, like reporting compile-time errors inside of IDEs for source code that depends on generated files. This includes code using FIDL as well as all Rust and Go source code.

Backwards Compatibility

In-tree tooling is allowed to rely on new features exposed by the CLI of Fuchsia's fork of Ninja. Any tools intended to be used outside the Fuchsia tree must not assume the presence of Fuchsia's fork of Ninja.

However, these Fuchsia-specific features should be limited to the strict minimum as being able to rollback to upstream is an important goal for this RFC.

For example, for certain uses, a valid alternative is to write offline tools that parse Ninja build plans directly and perform computations over them (as in https://fuchsia-review.googlesource.com/c/fuchsia/+/644561).

Security considerations

This change should not have any significant impact on security. Ninja is already capable of running arbitrary commands on a developer's system. Any security vulnerabilities in Ninja could just as easily come from upstream, and our fork will involve Fuchsia engineers reviewing every change, which is an improvement over the status quo.

By using Google's standard suite of source code tooling, we mitigate any source code supply chain issues that could occur.

Privacy considerations

This proposal should not affect privacy in any way.


As is currently the case, the Ninja test suite will be run for any change that is submitted to Fuchsia's Ninja fork, including upstream-sync merges. Changes must adhere to Ninja's standard testing practices, which include unit testing.


A standard README.fuchsia file will be added at the top of the Fuchsia git branch explaining the differences between the current state of the branch and upstream.

It will contain a link to this RFC to explain the branch management strategy described here.

Drawbacks, alternatives, and unknowns

The main drawback of this proposal is the work required to keep the branch close to upstream, by performing upstream-sync branches as frequently as possible. Each such operation may trigger one or more rebase conflicts that need to be manually resolved by maintainers of the Fuchsia branch.

Note that a good way to make these considerably easier to fix is to decompose the changes in the Fuchsia branch in as many small patches as possible (each one resulting in a fully testable source tree).

Trying to use the Android Ninja branch as a starting point has been considered, but its divergence is very large. Just merging recent upstream changes into it would require resolving a huge amount of conflicts (e.g. they switched everything to C++17 and removed C++03 support code entirely), and would be extremely difficult to rebase into a set of clean patches of top of upstream.

Starting from upstream, and rebasing a few Android-specific features on top of it thus seems a better solution, capable of providing results in a few weeks.

Additionally, several other attempts have been made to get the same benefits as some of the upstream patches without changing upstream Ninja, but they've all fallen far short. Most notably, thanks to a multi-year investigation and a series of changes to GN we were able to reduce Ninja no-op build times. However we were disappointed to find that we've only achieved marginal improvements.

Note that Ninja is being used by several different teams at Google, and some people have expressed interest in a coordinated way to manage Ninja evolution across the company, or to find a way to help the upstream maintainers. This RFC does not prevent any of these things, but its priority is to rapidly bring benefits to the Fuchsia platform first.

Finally, Fuchsia now has a Bazel SDK and has demonstrated a Bazel build for Fuchsia components (see RFC-0139). In the long term Fuchsia should consider alternatives to Ninja as a platform build backend and explore potential multi-year migrations. For the purpose of this RFC we consider such migrations to be out of scope.