|author||Bryan Henry <email@example.com>||Fri Jun 24 23:25:44 2022 -0700|
|committer||Brett Wilson <firstname.lastname@example.org>||Fri Aug 05 23:00:24 2022 +0000|
[gn] Prevent build.ninja deletion when regeneration is interrupted Prior to this change, the top-level build.ninja file generated by GN was configured in such a way that interrupting ninja file regeneration (e.g. with Ctrl-C) would cause ninja to delete build.ninja. This was done by ninja's Builder::Cleanup() function because the "build build.ninja" statement that GN produces uses a depfile argument. GN uses this depfile to capture implicit dependencies and automatically trigger ninja regeneration if, for example, any BUILD.gn source file is modified. If build.ninja is deleted for this reason, the user must manually run `gn gen` to recreate it; a poor user experience. Worse, command-line arguments that the user previously passed to `gn gen` are also lost, meaning that the user must be sure to pass the same arguments again. Because of this bug, developer tools layered on top of GN (like Pigweed's `pw watch` file watcher) have added handling to automatically run `gn gen` for the user in an attempt to provide a smoother user experience, but these attempts are still frustrated by the command-line argument loss. This change modifies the flow of ninja file generation slightly such that build.ninja should never be deleted, even if regeneration is interrupted, and thus ninja should always be able to restart it. The build rule for build.ninja has been split in two so that ninja will only delete the .tmp file if interrupted: build build.ninja.stamp: gn depfile = build.ninja.d build build.ninja: phony build.ninja.stamp generator = 1 `gn gen` now does the following: 1. If the '--regeneration' flag is present (i.e. if the caller is ninja rather than a user), the 'build.ninja' file is truncated to only the commands needed to initiate regeneration again. (This is the same operation `gn clean` applies to 'build.ninja'.) 2. Ninja file generation proceeds as before, writing 'build.ninja' and 'build.ninja.d', plus a new 'build.ninja.stamp' file. Essentially, this works by misleading ninja about when 'build.ninja' is actually written. Importantly, because of this gn takes on the responsibility for ensuring that 'build.ninja' is always valid, which it achieves through atomic writes. Fixed: https://bugs.chromium.org/p/gn/issues/detail?id=25 Change-Id: I4aef0317466b5e3430a9ef44eaa63c9515afefab Reviewed-on: https://gn-review.googlesource.com/c/gn/+/14200 Reviewed-by: Brett Wilson <email@example.com>
GN is a meta-build system that generates build files for Ninja.
GN is currently used as the build system for Chromium, Fuchsia, and related projects. Some strengths of GN are:
It is designed for large projects and large teams. It scales efficiently to many thousands of build files and tens of thousands of source files.
It has a readable, clean syntax. Once a build is set-up, it is generally easy for people with no backround in GN to make basic edits to the build.
It is designed for multi-platform projects. It can cleanly express many complicated build variants across different platforms. A single build invocation can target multiple platforms.
It supports multiple parallel output directories, each with their own configuration. This allows a developer to maintain builds targeting debug, release, or different platforms in parallel without forced rebuilds when switching.
It has a focus on correctness. GN checks for the correct dependencies, inputs, and outputs to the extent possible, and has a number of tools to allow developers to ensure the build evolves as desired (for example,
It has comprehensive build-in help available from the command-line.
Although small projects successfully use GN, the focus on large projects has some disadvanages:
GN has the goal of being minimally expressive. Although it can be quite flexible, a design goal is to direct members of a large team (who may not have much knowledge about the build) down an easy-to-understand, well-lit path. This isn't necessarily the correct trade-off for smaller projects.
The minimal build configuration is relatively heavyweight. There are several files required and the exact way all compilers and linkers are run must be specified in the configuration (see “Examples” below). There is no default compiler configuration.
It is not easily composable. GN is designed to compile a single large project with relatively uniform settings and rules. Projects like Chromium do bring together multiple repositories from multiple teams, but the projects must agree on some conventions in the build files to allow this to work.
GN is designed with the expectation that the developers building a project want to compile an identical configuration. So while builds can integrate with the user‘s environment like the CXX and CFLAGS variables if they want, this is not the default and most project’s builds do not do this. The result is that many GN projects do not integrate well with other systems like ebuild.
There is no simple release scheme (see “Versioning and distribution” below). Projects are expected to manage the version of GN they require. Getting an appropriate GN binary can be a hurdle for new contributors to a project. Since GN is relatively uncommon, it can be more difficult to find information and examples.
GN can generate Ninja build files for C, C++, Rust, Objective C, and Swift source on most popular platforms. Other languages can be compiled using the general “action” rules which are executed by Python or another scripting language (Google does this to compile Java and Go). But because this is not as clean, generally GN is only used when the bulk of the build is in one of the main built-in languages.
Alternatively, you can build GN from source with a C++17 compiler:
git clone https://gn.googlesource.com/gn cd gn python build/gen.py # --allow-warning if you want to build with warnings. ninja -C out # To run tests: out/gn_unittests
On Windows, it is expected that
lib.exe can be found in
PATH, so you'll want to run from a Visual Studio command prompt, or similar.
On Linux, Mac and z/OS, the default compiler is
clang++, a recent version is expected to be found in
PATH. This can be overridden by setting the
AR environment variables.
cd gn python build/gen.py --zoslib-dir /path/to/zoslib
By default, if you don't specify
gn/build/gen.py expects to find
zoslib directory under
There is a simple example in examples/simple_build directory that is a good place to get started with the minimal configuration.
To build and run the simple example with the default gcc compiler:
cd examples/simple_build ../../out/gn gen -C out ninja -C out ./out/hello
For a maximal configuration see the Chromium setup:
and the Fuchsia setup:
If you find a bug, you can see if it is known or report it in the bug database.
Register at https://gn-review.googlesource.com. ... edit code ... ninja -C out && out/gn_unittests
Then, to upload a change for review:
git commit git push origin HEAD:refs/for/main
The first time you do this you'll get an error from the server about a missing change-ID. Follow the directions in the error message to install the change-ID hook and run
git commit --amend to apply the hook to the current commit.
When revising a change, use:
git commit --amend git push origin HEAD:refs/for/main
which will add the new changes to the existing code review, rather than creating a new one.
We ask that all contributors sign Google's Contributor License Agreement (either individual or corporate as appropriate, select ‘any other Google project’).
You may ask questions and follow along with GN‘s development on Chromium’s gn-dev@ Google Group.
Most open-source projects are designed to use the developer‘s computer’s current toolchain such as compiler, linker, and build tool. But the large centrally controlled projects that GN is designed for typically want a more hermetic environment. They will ensure that developers are using a specific compatible toolchain that is versioned with the code.
As a result, GN expects that the project choose the appropriate version of GN that will work with each version of the project. There is no “current stable version” of GN that is expected to work for all projects.
As a result, the GN developers do not maintain any packages in any of the various packaging systems (Debian, RedHat, HomeBrew, etc.). Some of these systems to have GN packages, but they are maintained by third parties and you should use them at your own risk. Instead, we recommend you refer your checkout tooling to download binaries for a specific hash from Google's build infrastructure or compile your own.
GN does not guarantee the backwards-compatibility of new versions and has no branches or versioning scheme beyond the sequence of commits to the main git branch (which is expected to be stable).
In practice, however, GN is very backwards-compatible. The core functionality has been stable for many years and there is enough GN code at Google alone to make non-backwards-compatible changes very difficult, even if they were desirable.
There have been discussions about adding a versioning scheme with some guarantees about backwards-compatibility, but nothing has yet been implemented.