PSA: Chromium's build system is switching from Ninja to Siso for external developers

[Junji Watanabe]

Hi everyone,

This PSA is for all external Chromium developers regarding an upcoming change to the Chromium build system.

What's changing?

Chromium's build system is switching from Ninja 🥷 to Siso 🌿. This migration has already been completed for developers and CI/CQ within Google, and we are now extending it to our external contributors.

Siso is developed by the Chrome Build Infra Team at Google as a drop-in replacement for Ninja to support remote execution natively.

What do you need to do?

Just keep building Chromium with autoninja. autoninja will automatically use Siso after you run gn clean the next time.

How to opt out? 

If you encounter problems with Siso, you can switch back by setting use_siso=false in your args.gn.

About Ninja/Reclient/GN

• GN will stay the same as before.

• Reclient will be removed from Chromium at the end of September.

• Ninja will be unsupported after the end of September.

Please prepare to switch to Siso before the deadline.

Questions or Issues?

If you have any questions or run into any problems with this change, please don't hesitate to reach out to us:

• Slack: #build

• Chromium Issue Tracker: File a Siso bug.

• Email: bu...@chromium.org

Thanks,

Junji - on behalf of the Chrome Build Infra Team

[Dirk Pranke]

Hi Junji,

Presumably GN is still generating .ninja files that work with Ninja, so what are the implications of this for people that aren't using remote execution to build? Are we going to be making changes that aren't compatible with Ninja? 

Or is this more of a support thing, in that you only want to support one build tool?

Also, does Siso support building with other RBE backends?

-- Dirk

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[Junji Watanabe]

Hi Dirk,

Thank you for your questions. Let me clarify.

Regarding your first question, there are a few reasons for moving away from Ninja:

Maintenance Cost: Currently, we have limited testing for Ninja and Reclient builds on our CI builders. While we are still addressing and fixing any issues that arise, our goal is to discontinue support for Ninja after the upcoming deadline.

Build Improvements: We're planning to improve our build processes by integrating GN and Siso more closely. This includes features like mtime-less builds and more efficient build graph serialization. It's difficult to predict how challenging it will be to maintain compatibility with Ninja as we implement these improvements, and it's possible that Ninja builds may eventually stop working. So, we encourage switching to Ninja soon. If you encounter any workflow issues with Siso, please provide feedback. 

Regarding your question about Siso's support for other RBE backends, Siso is designed to send Remote Execution API (REAPI) requests to a custom API address. I am currently in contact with various remote build service providers to confirm full compatibility with Siso's REAPI support.

Thank you,

Junji

[Raphael Kubo da Costa]

+chromium-packagers

Hey Junji,

Given the "Ninja builds may eventually stop working" bit, can you clarify how this impacts downstreams like Linux distros that build Chromium from the published tarballs, or embedders like Electron, CEF, perhaps even Node due to V8?

Specifically, I'm wondering:
* Will there be releases like Ninja does?
* If not, I'm guessing everyone will be expected to fetch and build their own Siso copy?
* Are Siso versions supposed to be pinned to specific Chromium releases like GN?

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[Raphael Kubo da Costa]

+chromium-packagers

Hey Junji,

Given the "Ninja builds may eventually stop working" bit, can you clarify how this impacts downstreams like Linux distros that build Chromium from the published tarballs, or embedders like Electron, CEF, perhaps even Node due to V8?

Specifically, I'm wondering:

* Will there be releases like Ninja does?

* If not, I'm guessing everyone will be expected to fetch and build their own Siso copy?

* Are Siso versions supposed to be pinned to specific Chromium releases like GN?

On 6/17/25 11:17, 'Junji Watanabe' via Chromium-dev wrote:

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[Liviu Laslau]

In simple terms, please, will Siso work exactly the same for local builds under win/mac ?
And also ccache support will stay ?

[Junji Watanabe]

Yes, Siso can do local builds without remote execution on Windows and Mac, too.

Regarding ccache, there should be no difference. Please let us know if you encounter any issue.

[Junji Watanabe]

(bcc chromium-dev@)

Let me fork the thread since I got responses from different people.

Chromium for Linux distros:

How do they build Chromium now? They are supposed to use the GN/Ninja binaries fetched via DEPS.

Siso also should be fetched via DEPS, as well.

Electron:

I got a message from Electron, and am discussing it with them.

CEF:

Looking at the repo, there are GN config files, Bazel files, CMake files. So, it's not clear how they build the Chromium part. 

Does anyone know who I should notify the PSA?

Node:

I'm also not familiar with how the official Node releases are built. Although there are GN configs, they don't seem to be used for Node official builds. 

I guess they still use GYP presumably with Make? Note that V8 has migrated to Siso already.

Thank you,

Junji

[Raphael Kubo da Costa]

On 6/19/25 03:31, 'Junji Watanabe' via Chromium-dev wrote:

(bcc chromium-dev@)

Let me fork the thread since I got responses from different people.

Chromium for Linux distros:

How do they build Chromium now? They are supposed to use the GN/Ninja binaries fetched via DEPS.

Siso also should be fetched via DEPS, as well.

Distros usually download the tarballs we publish via https://ci.chromium.org/ui/p/infra/builders/cron/publish_tarball, but generally speaking pulling pre-built binaries to use as part of the build is either frowned upon or simply not allowed (and I believe this applies to other parties besides distros).

We bundle GN's sources into the tarball for this reason, and distros (and possibly others) use Ninja as provided by the distribution rather than pulling a binary via DEPS (hence my question about releases, pinned versions and whether one would be expected to build Siso, which would then require a Go toolchain before even beginning to build Chromium).

[Tor Arne Vestbø]

Cool stuff! A few questions:

1. Are there prebuilt binaries somewhere (CIPD) or should we build from source?

2. It seems a `build/config/siso/main.star` is needed to `siso ninja`. What is the minimal main.star that makes `siso ninja` a drop in replacement for `ninja` in an existing (CMake-based) project (that's not Chromium)?

3. reclient has `RBE_remote_headers=x-buildbuddy-api-key=mykey`, does siso respect that or is there another way to do header-based auth?

Cheers,

Tor Arne

[Levi Zim]

Hi, I am encountering an error that requires me to login when using siso. Is this expected?
0.00s Error: need to run `siso login`: failed to get token : exec: "gcloud": executable file not found in $PATH

[Walter Rader]

> Hi, I am encountering an error that requires me to login when using siso. Is this expected?

It's not documented in the build instructions - at least for Mac (https://chromium.googlesource.com/chromium/src/+/main/docs/mac_build_instructions.md ).

Further, following the instructions to run `siso login` and following its instructions results in failure.

/!\

This app is blocked

This app tried to access sensitive info in your Google Account. To keep your account safe, Google blocked this access.

Walter

[Hans Wennborg]

The "Error: need to run `siso login`" problem is https://crbug.com/429024909

(See also https://groups.google.com/a/chromium.org/g/build/c/yS_Z2a3kuJM/m/K2khrcxmAgAJ)

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[Fumitoshi Ukai (鵜飼文敏)]

Siso v1.3.5 is rolled out for the fix.

https://chromium-review.googlesource.com/c/chromium/src/+/6702692

On Fri, Jul 4, 2025 at 9:17 AM Fumitoshi Ukai (鵜飼文敏) <uk...@google.com> wrote:

On Fri, Jul 4, 2025 at 9:15 AM Fumitoshi Ukai (鵜飼文敏) <uk...@google.com> wrote:

On Fri, Jul 4, 2025 at 9:06 AM Fumitoshi Ukai (鵜飼文敏) <uk...@google.com> wrote:

I think you could use `--offline --reapi_insecure` to mitigate at the moment

ah, it doesn't work....

`--project=  --reapi_insecure` work?

 

 

or use siso latest (i.e. add `"siso_version": "latest"` in "custom_vars" section in .gclient, and run `gclient runhooks`)

Please use siso latest at the moment.

 

?

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[Fumitoshi Ukai (鵜飼文敏)]

I think you could use `--offline --reapi_insecure` to mitigate at the moment

or use siso latest (i.e. add `"siso_version": "latest"` in "custom_vars" section in .gclient, and run `gclient runhooks`)

?

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[Fumitoshi Ukai (鵜飼文敏)]

On Fri, Jul 4, 2025 at 9:15 AM Fumitoshi Ukai (鵜飼文敏) <uk...@google.com> wrote:

On Fri, Jul 4, 2025 at 9:06 AM Fumitoshi Ukai (鵜飼文敏) <uk...@google.com> wrote:

I think you could use `--offline --reapi_insecure` to mitigate at the moment

ah, it doesn't work....

`--project=  --reapi_insecure` work?

 

 

or use siso latest (i.e. add `"siso_version": "latest"` in "custom_vars" section in .gclient, and run `gclient runhooks`)

Please use siso latest at the moment.

 

?

[Fumitoshi Ukai (鵜飼文敏)]

On Fri, Jul 4, 2025 at 9:06 AM Fumitoshi Ukai (鵜飼文敏) <uk...@google.com> wrote:

I think you could use `--offline --reapi_insecure` to mitigate at the moment

ah, it doesn't work....

 

or use siso latest (i.e. add `"siso_version": "latest"` in "custom_vars" section in .gclient, and run `gclient runhooks`)

Please use siso latest at the moment.

 

?

[Levi Zim]

Thanks for the quick fix! I could confirm that it fixes the issue.

[Levi Zim]

Thanks for the pointer. I have received several emails from Fumitoshi Ukai explaining the error

but it appears that they didn't appear on the google group website for some reason.

The problem has been fixed.

Here are Fumitoshi Ukai's explanation, for reference:

I think you could use `--offline --reapi_insecure` to mitigate at the moment

ah, it doesn't work....

or use siso latest (i.e. add `"siso_version": "latest"` in "custom_vars" section in .gclient, and run `gclient runhooks`)

Please use siso latest at the moment.

Siso v1.3.5 is rolled out for the fix.

https://chromium-review.googlesource.com/c/chromium/src/+/6702692

[Shezan Baig]

Hi, is there an equivalent of NINJA_STATUS for siso?  (i.e. https://ninja-build.org/manual.html#_environment_variables)
Thanks, -shez-

[Junji Watanabe]

Hi Shezan, 

Siso doesn't support NINJA_STATUS. But it displays more status than Ninja by default. e.g. number of completed steps per second.  Also, we got a feature request for ETA (%W) specifically.

[Shezan Baig]

Thanks Junji!
Would it be possible to add an option to put the number of steps completed per second on the same line as the number of steps completed/total steps?  Currently, the number of steps per second only shows when it is connected to a tty, so we cannot see it when the build is running on a buildbot, for example.

Thanks!
-shez-

[Junji Watanabe]

Hi Shezan, 

Sorry for the late reply. I filed https://crbug.com/432148544 to track it. We got other UI related feature requests. So, we will try to handle them together.

Thank you,

Junji